Flight Weather Elements Chapter 2 PDF

AFH15-101 5 NOVEMBER 2019 91 Chapter 2 FLIGHT WEATHER ELEMENTS 2.1. Clouds. Clouds form when water vapor changes to either liquid droplets (condensation) or ice crystals (deposition). This happens when air is cooled below its saturation point either directly (radiational cooling or advection) or by...

AFH15-101 5 NOVEMBER 2019 91 Chapter 2 FLIGHT WEATHER ELEMENTS 2.1. Clouds. Clouds form when water vapor changes to either liquid droplets (condensation) or ice crystals (deposition). This happens when air is cooled below its saturation point either directly (radiational cooling or advection) or by being raised higher in the atmosphere (adiabatic cooling). 2.1.1. Cloud development - adiabatic temperature change. Since air pressure decreases with altitude, adiabatic temperature change is the fundamental determinant of whether air will rise under its own energy. When air rises, it expands due to the lower ambient pressure. Similarly, when air sinks, it compresses. These pressure changes are associated with a change in temperature: rising air cools and sinking air warms. These are “adiabatic” temperature changes, resulting from processes that don’t involve true heat transfer. The amount of temperature change with altitude is virtually static in non-saturated air; unsaturated air warms and cools at approximately 3°C for every 1000 feet, or around 9.8°C per kilometer (this is the dry adiabatic lapse rate). Saturated air cools less rapidly as it rises; water vapor condenses into liquid droplets when the air parcel is cooled further, and latent heat from this change of state keeps the air parcel warmer than a dry parcel would be at the corresponding height. In addition, warm air can hold much more water vapor than cold air. 2.1.2. Cloud development – stability and instability. Objects float in water due to the buoyant force caused by density differences. In the atmosphere, the density of an air parcel compared to its environment determines whether it will sink or rise. Generally, the temperature of an air parcel is what determines its density for a given pressure level. If a particular parcel is less dense (warmer) than the surrounding air, it will rise until it reaches the “equilibrium level,” where it stops rising. The equilibrium level has the same temperature as the air parcel, and will no longer allow it to rise. If, however, the parcel is denser (colder) than the surrounding air, it will be forced to sink, creating a downdraft. Clouds are fundamentally formed by moist, rising air that cools beyond its dew point. The altitude where this occurs is the “lifted condensation level.” Above this point, if the atmosphere is unstable, the cloud will continue to grow vertically and potentially produce precipitation. This does not always happen, however, and the structure of a cloud can give clues as to the atmospheric conditions that produced it. These clues are invaluable for aviators in determining where potentially dangerous updrafts and downdrafts are occurring. 2.1.3. Cloud Types and States of the Sky. Clouds are classified by their appearance and how they form: cumuliform clouds are produced by rising air in an unstable atmosphere, while stratiform clouds occur when a layer of air is cooled below its saturation point without extensive vertical motion. Although stratiform clouds produce less spectacular weather, they may often cause persistent low ceilings and poor visibilities, which may be critical to Air Force and Army operations. Clouds are further classified by the altitude at which their bases form: low, middle and high cloud layers. For details on the cloud types described in the sections below, refer to the International Cloud Atlas (https://cloudatlas.wmo.int/coding-ofclouds.html). Keeping these basics in mind will help in understanding the various techniques and rules available for forecasting clouds. 2.1.3.1. Low Clouds. Low clouds are classified as clouds forming from the surface to 6500 feet above ground level. 92 AFH15-101 5 NOVEMBER 2019 2.1.3.1.1. Cumulus (CU). Cumulus clouds are cottony in appearance, with an internal structure of updrafts and downdrafts. Cumulus clouds develop by atmospheric lifting, especially convection. The L1-variety cumulus cloud is defined by little vertical extent, and may appear flattened or ragged, and is indicative of good weather. The L2 towering cumulus cloud shows moderate-to-strong vertical development. 2.1.3.1.2. Stratocumulus (SC). Stratocumulus clouds are formed by the spreading out of cumuliform clouds, or the lifting and mixing of stratiform clouds. Precipitation from stratocumulus is usually light and intermittent. L4-type stratocumulus is formed by the spreading out of cumulus, while L5 are not formed by the spreading out of cumulus. The L8 variety are combined with cumulus, and have bases at different levels. 2.1.3.1.3. Stratus (ST). Stratus clouds are sheet-like in appearance, with diffuse or fibrous edges. They usually produce light continuous or intermittent precipitation, but not showery. The L6 type is a more-or-less continuous layer, ragged sheets, or a combination of both, but no stratus fractus. The L7 variety has the stratus fractus or cumulus fractus present, indicative of wet weather. 2.1.3.1.4. Cumulonimbus (CB). Massive in appearance with great vertical extent, cumulonimbus clouds are indicative of the most intense weather on Earth – heavy rain, hail, lightning, damaging winds, and tornadoes. The L3 variety are younger or weaker CB, where there is no anvil top or cirriform development, while the L9 type represents a mature CB with a cirriform anvil. 2.1.3.2. Middle Clouds. Middle clouds are classified as clouds forming between 6500 feet and 20,000 feet above ground level. 2.1.3.2.1. Altostratus (AS). Similar in appearance to stratus, but occurring at higher altitudes, altostratus clouds are dense enough to prevent objects from casting shadows, and do not create the “halo” phenomenon associated with lower clouds. Altostratus is mainly categorized by the M1 type, a grayish-bluish mid-level cloud layer. 2.1.3.2.2. Nimbostratus (NS). Thicker and darker than altostratus clouds, nimbostratus clouds usually produce light to moderate precipitation. Although classified as a middle cloud by definition, its base usually builds downward into the low cloud height range. Nimbostratus are categorized by the M2 designation. 2.1.3.2.3. Altocumulus (AC). The appearance of altocumulus is similar to stratocumulus, but consists of smaller elements. There are several different types of AC, covered by the classifications M3 through M9. Two important variations of AC are altocumulus standing lenticular (ACSL, M4), and altocumulus castellanus (ACC, M8). ACSL clouds are caused by the lifting action inherent in mountain waves, and indicate turbulence. ACC has greater vertical extent than regular AC, implying midlevel instability. 2.1.3.3. High Clouds. High clouds are classified as clouds forming above 20,000 feet above ground level. AFH15-101 5 NOVEMBER 2019 93 2.1.3.3.1. Cirrus (CI). Cirrus clouds consist entirely of ice crystals. A partial halo around the sun or moon occasionally accompanies cirrus clouds; the presence of a complete halo indicates cirrostratus instead of cirrus. Cirrus are categorized as H1 through H4. 2.1.3.3.2. Cirrostratus (CS). Cirrostratus appear more “sheet-like” than cirrus clouds, and will produce complete halos if they are thin enough. CS is distinguishable from haze by its whiter and brighter appearance (haze is more yellowish-brown). CS is categorized as H5 through H8. 2.1.3.3.3. Cirrocumulus (CC). Cirrocumulus clouds appear similar to AC or ACC, but with smaller individual elements. Individual cloud elements of CC can be covered by your little finger when extended at arm’s length; AC and ACC cannot. The elements can be so small that they are often difficult to see by the unaided eye. Some cirrocumulus clouds may resemble fish scales and are sometimes referred to as a “mackerel sky.” CC are classified as H9. 2.1.4. General Cloud Forecasting Tools. 2.1.4.1. Worldwide Merged Cloud Analysis (WWMCA). The WWMCA provides an hourly analysis of cloud distribution based on information from five geostationary satellites and ten polar orbiting satellites; the satellite images are processed and merged into a single global cloud analysis. There are currently four WWMCA products available on AFWWEBS: Cloud Mask, Total Cloud Cover, Cloud Top Height, and Cloud Free Plot. Figure 2.1 is an example of the Cloud Free Plot over China and East Asia. Figure 2.1. WWMCA Cloud Free Plot over China and East Asia. 94 AFH15-101 5 NOVEMBER 2019 2.1.4.2. Climatology. AFW-WEBS and the 14th Weather Squadron provide a detailed suite of cloud climatology products available – some of the most useful cloud products are detailed below. 2.1.4.2.1. Surface climograms. As a refresher, climograms are two-dimensional views of the likelihood of an event occurring at a given time of day and month of the year. Combined ceiling and visibility climograms are available for a variety of parameters, ranging from probability of ceilings less than 3000 feet and visibility less than three statute miles, down to probability of ceilings less than 100 feet and visibility less than one-quarter statute mile. Climograms can also produce red/yellow/green “stoplight” charts for ceiling thresholds from 100 to 25,000 feet, showing the annual probability of ceilings at those thresholds. 2.1.4.2.2. Operational Climatic Data Summaries (OCDS-II). The OCDS-II web application enables users to generate tables of the same ceiling and visibility probabilities as the surface climograms. 2.1.4.2.3. Wind Stratified Conditional Climatologies (WSCC). Given a set of initial conditions (month, time of days, wind direction, ceiling height, and visibility), the WSCCs indicate the likelihood that a particular ceiling category will be observed at a future time. 2.1.4.2.4. Surface Climatology maps. Geographical surface climatology maps can be produced for several combined ceiling and visibility categories (3000/3, 1500/3, and 1000/2); the maps are available for numerous regions around the world and can be stratified by month and time of day. 2.1.4.2.5. GIS Cloud Climatology. Multiple cloud-related parameters can be plotted using the GIS climate service tool; the maps can be stratified by month and time of day, and looped and zoomed to specific areas of interest. Figure 2.2 shows an example product, displaying a global plot of mean cloud amount for the month of May, from 1800-2100 UTC. Figure 2.2. Global plot of mean total cloud amount, May, 1800-2100 UTC. AFH15-101 5 NOVEMBER 2019 95 2.1.4.2.6. WWMCA Climatology. The WWMCA climatology incorporates geostationary and polar orbiting satellite data as well as surface observed clouds. The latest 10 years of data are summarized to produce geographical maps, which can be generated for all regions around the world. Four parameters can be plotted: mean total cloud amount, standard deviation total cloud amount, mean cloud frequency by 20% band, and frequency of any ceiling. 2.1.4.3. Air Force NWP cloud forecasts. The Air Force produces two dedicated cloud forecast models, as well as several ensemble products; all of them are available on AFWWEBS. 2.1.4.3.1. The Short Range Cloud Forecast (SRCF) model. The SRCF initializes with the most recent WWMCA and advects the analysis forward in time using several model variables (winds, temperatures, and relative humidity). The model produces a 12-hour forecast at 24 km resolution, with hourly updates. Total Cloud Cover and Cloud Free plots are available. 2.1.4.3.2. The Diagnostic Cloud Forecast (DCF) model. The DCF is calibrated using a 0-hour SRCF forecast (based on the WWMCA), and then run forward in time using predictor variables (e.g., pressure, relativity humidity, temperature, vertical velocity). Cloud Top, Cloud Base, and Total Cloud Cover products are available. 2.1.4.3.3. Ensemble products. Cloud forecasts are also available from the Air Force GEPS and MEPS ensembles; ceiling forecasts are calculated using an algorithm accounting for the impacts of precipitation, relative humidity, wind speed, precipitable water, and dust. The 20 km and 4 km MEPS also produce forecasts of the probability of cloud cover less than or equal to 20% and greater than or equal to 80%. Stamp charts of simulated cloud and cloud ceiling height are also available from the 20 km and 4 km MEPS; the stamp charts are unique in that they show nine ensemble members as small panels on the right-hand side of the image, with the control member as a large stamp on the left-hand side of the image. The user is therefore able to gauge forecast certainty, while still being able to see physically-relevant detail. 2.1.5. Determining cloud heights using a Skew-T. 2.1.5.1. Bases of stratus and stratocumulus – the Mixing Condensation Level (MCL). The MCL is the lowest height at which saturation occurs after the complete mixing of the layer; use the MCL as base of stratus and cold-air stratocumulus decks. To find the MCL on a Skew-T, use the following procedures: 2.1.5.1.1. Determine the top of the layer height to be mixed (a subjective estimate based on winds, terrain roughness, original sounding, etc.). Stations in the cold air should have a pronounced low-level (but elevated) inversion. Use this as the top of the mixing layer. 2.1.5.1.2. Determine the average temperature and dew point within the layer. 2.1.5.1.3. Trace the average temperature up the dry adiabat and the average dew point up the mixing ratio line until they intersect; this is the MCL, and provides a good approximation of stratus or stratocumulus base heights, if they form. Figure 2.3 illustrates this process. 96 AFH15-101 5 NOVEMBER 2019 Figure 2.3. Calculation of the MCL. 2.1.5.2. Bases of non-precipitating cumuliform clouds – the Convective Condensation Level (CCL). The CCL is the height to which a parcel of air, if heated sufficiently from below, will rise adiabatically until it is saturated and condensation begins. In most cases, the bases of cumuliform clouds will form about 25 millibars above the CCL; the CCL serves as the generation height of cumuliform clouds produced solely from convection. There are two methods of finding the CCL – the simplest technique (the “parcel method”) uses only the surface dew point. In cases of high variations in surface layer moisture content, an average moisture value of the lowest layer is more representative (the “moist layer method”). 2.1.5.2.1. CCL Parcel Method (Figure 2.4). From the surface dew point, proceed up the Skew-T parallel to the saturation mixing ratio line until it intersects the temperature curve. The intersection point is the CCL. Figure 2.4. CCL Parcel Method. AFH15-101 5 NOVEMBER 2019 97 2.1.5.2.2. CCL Moist Layer Method. A layer is defined as “moist” if it has an RH of 65% or greater at all levels. In practice, the moist layer does not extend past the lowest 150 mb of the sounding. After finding the depth of the moist layer (or lowest 150 mb of the sounding, whichever is smaller), find the mean mixing ratio of this layer. Follow the mean mixing ratio line of the moist layer to the point where it crosses the temperature curve of the sounding. The intersection point is the CCL. 2.1.5.3. Bases of convective clouds using dew point depressions. Table 2.1 provides a quick reference of expected cumulus cloud bases based on current or forecast surface dew point depression; the table is not suitable for use at locations in mountainous or hilly terrain, and should be used only when clouds are formed by active surface convection in the vicinity. Use with caution when the surface temperature is below freezing, due to the difficulties in accurately determining dew points at low temperatures. Table 2.1. Expected bases of convective clouds from surface dew point depression. 2.1.6. Cloud Forecasting Using 700 mb Features. The location and coverage of mid-level clouds can be estimated by using the following guidelines on the 700 mb chart. 2.1.6.1. If 700 mb height contours and isotherms are parallel to the front, expect an extensive cloud band. If the height contours and isotherms are perpendicular to the front, expect a narrow cloud band. 2.1.6.2. If 700 mb streamlines are cyclonic, extensive cloud cover will occur. If the streamlines are anticyclonic, cloud cover will be sparse. 2.1.6.3. A 700 mb ridge passing ahead of a cold front generally coincides with low and middle cloud formation. 98 AFH15-101 5 NOVEMBER 2019 2.1.6.4. A 700 mb trough passing after a cold front generally coincides with low and middle cloud clearing. 2.1.7. Formation, advection and dissipation of low stratus. Air cooled by contact with a colder surface may be transferred upwards by turbulent mixing caused by the wind. The height to which the cooling is diffused upwards depends on the stability of the atmosphere, the wind speed, and the roughness of the surface. One study found the mean depth of the turbulent layer to be 60 meters (200 feet) for each knot of wind at ground level up to a surface wind speed of 16 knots. With stronger winds, the depth was independent of wind speed, averaging 1066 meters (3500 feet) in the early morning, rising during the day to 1200 meters (4000 feet). When the air is cloud-free but initially stable in the lower layers, the layer of turbulent mixing is very shallow. Cooling is confined to very low levels, resulting in the formation of very low stratus or fog. 2.1.7.1. Stratus Forecasting and Wind Speed. Wind speed is usually the dominant factor in determining fog or stratus formation (local topography is also an important consideration). While there’s no single critical wind speed threshold for fog/stratus formation, stratus will typically form due to nocturnal cooling with surface wind speeds exceeding 5-10 knots at a coastal location, 10-15 knots at an inland site, and over 25 knots in a valley location. 2.1.7.2. Empirical Rules. The level at which stratus forms over land is related to wind speed and the influence of local orographic features, but the dependence of cloud height on temperature and humidity prevents any simple relationship between cloud height and wind speed. The height of stratus in meters above level ground is 20 to 25 times the surface wind speed in knots (70 to 80 times for height in feet). If advected stratus clears during the morning, the dissipation temperature gives the best estimate of the temperature at which the cloud will re-form during the evening. 2.1.7.3. Dissipation of Stratus Using Mixing Ratio and Temperature. Manual analysis of the morning Skew-T is an excellent tool to determine the dissipation time of stratus. Use the following steps to determine the surface temperatures needed to begin dissipating and to completely dissipate stratus (see Figure 2.5). 2.1.7.3.1. Find the average mixing ratio between the surface and the base of the inversion. 2.1.7.3.2. Find the intersections of the average mixing ratio line and the temperature curve (the approximate height of the base is at point A, and the top of the stratus deck is at point B). 2.1.7.3.3. Follow the dry adiabat from point A to the surface, and label the surface intersection point as C. This point is the surface temperature required to start dissipation. 2.1.7.3.4. Follow the dry adiabat from point B to the surface, and label the surface intersection point as D. This point is the surface temperature required for complete dissipation. AFH15-101 5 NOVEMBER 2019 99 Figure 2.5. Stratus dissipation technique, using mixing ratio and temperature. 2.1.8. Forecasting Convective Cirrus Clouds. For purely convective cirrus (frontal or thunderstorm), the following rules of thumb apply. 2.1.8.1. When there is straight-line or anticyclonic flow at 200-300-mb downstream from a thunderstorm area, cirrus may appear the next day and advance ahead of the ridgeline. 2.1.8.2. When there is cyclonic flow at 200-300-mb downstream from a thunderstorm area, cirrus is not likely to appear (it may appear, however, if the cyclonic flow is weak.) 2.1.9. Forecasting Cirrus Clouds – Tropopause Method. Figure 2.6 provides a quick reference to forecast cirrus bases and tops based on the tropopause height; the figure resulted from a multi-year study of the relationship between the tropopause and cirrus bases/tops. To use this method, find the current tropopause height (in thousands of feet) and read across the figure to determine the average cirrus bases and tops (also in thousands of feet.) Figure 2.6. Tropopause method of forecasting cirrus bases and tops. 100 AFH15-101 5 NOVEMBER 2019 2.1.10. Forecasting Snow-Induced Clouds. When snow falls through an atmospheric layer with temperatures greater than 0°C, the snowflakes start to melt. If the dry- and wet- bulb temperatures at ground level are initially greater than 0°C, the snow ultimately reaches the ground without melting. This is due to an isothermal layer, with a temperature near 0°C, establishing itself near the ground. The air is also cooled below its wet-bulb temperature, supersaturation occurs, and stratus clouds form with bases at or very near ground level. 2.1.11. Forecasting Rain-Induced Clouds. Evaporation from falling rain may cause supersaturation and the formation of clouds. The base of the cloud layer will be at a height where the temperature lapse rate decreases significantly or becomes negative (a positive lapse rate exists when temperature decreases with height). 2.1.12. General Cloud Forecasting Rules of Thumb. The following rules are empirical in nature, and may need adjustment for location and current weather regime. 2.1.12.1. The cloud base of a layer warmer than 0°C is usually located where the dew point depression decreases to less than 2°C. 2.1.12.2. The cloud base of a layer between 0°C and –10°C is usually located at a level where the dew point depression decreases to less than 3°C. 2.1.12.3. The cloud base of a layer between –10°C and –20°C is usually located where the dew point depression decreases to less than 4°C. 2.1.12.4. The cloud base of a layer less than –25°C is usually located where the dew point depression decreases to less than 6°C, but can occur with depressions as high as 15°C. 2.1.12.5. For two adjacent layers in which the dew point depression decreases with height more sharply in the lower layer than in the upper layer, the cloud base should be identified with the base of the layer showing the sharpest decrease with height. 2.1.12.6. The top of the cloud layer is usually indicated by an increase in dew-point depression. Once a cloud base has been determined, the cloud is assumed to extend up to the level where a significant increase in dew- point depression starts. The gradual increase in dew-point depression that usually occurs with height is not considered significant. 2.1.12.7. 500-mb dew-point depressions of 4°C or less coincide with overcast mid-level cloudiness. 2.2. Turbulence. Turbulence poses a significant threat to Air Force personnel and operations. Encounters with severe and extreme turbulence can lead to structural damage to aircraft, and injury to passengers and crew. Accurate forecasts of turbulence are therefore essential to the success of Air Force operations. 2.2.1. Levels of Intensity. Turbulence is defined as the “Random and continuously changing air motions that are superimposed on the mean motion of the air” Turbulence intensity is based on the impact to aircraft flying through an area of concern: 2.2.1.1. Light Turbulence. The aircraft experiences slight, erratic changes in attitude and/or altitude, caused by a slight variation in airspeed of 5 to 14 knots with a vertical gust velocity of 5 to 19 feet per second. Light turbulence is typically found in the following areas: 2.2.1.1.1. Mountainous regions, even with light winds. AFH15-101 5 NOVEMBER 2019 101 2.2.1.1.2. In and near cumulus clouds. 2.2.1.1.3. Near the tropopause. 2.2.1.1.4. At low altitudes in rough terrain, when winds exceed 15 knots. 2.2.1.1.5. At low altitudes flying over varying terrain with different surface heating coefficients (such as a grassy field next to a concrete surface, or a shoreline where water meets land.) 2.2.1.2. Moderate Turbulence. The aircraft experiences moderate changes in attitude and/or altitude, but the pilot remains in positive control at all times. There are usually small variations in airspeed of 15 to 24 knots; vertical gust velocity is 20 to 35 feet per second. Moderate turbulence is found in the following areas: 2.2.1.2.1. In towering cumuliform clouds and thunderstorms. 2.2.1.2.2. Within 100 NM of the jet stream on the cold-air side. 2.2.1.2.3. At low altitudes in rough terrain when the surface winds exceed 25 knots. 2.2.1.2.4. In mountain waves (up to 300 miles leeward of a ridge), with winds perpendicular to the ridge exceeding 50 knots. 2.2.1.2.5. In mountain waves as far as 150 miles leeward of the ridge and within 2000 to 3000 feet of the tropopause when winds are perpendicular to the ridge is 25 to 50 knots. 2.2.1.3. Severe Turbulence. The aircraft experiences abrupt changes in attitude and/or altitude, and may be out of the pilot‘s control for short periods. There are usually large variations in airspeed greater than or equal to 25 knots and the vertical gust velocity is 36 to 49 feet per second. Severe turbulence occurs: 2.2.1.3.1. In and near mature thunderstorms. 2.2.1.3.2. Near jet stream altitude and about 50 to 100 miles on the cold-air side of the jet core. 2.2.1.3.3. Up to 50 miles leeward of a ridge, if a mountain wave exists and winds perpendicular to the ridge are 25 to 50 knots. 2.2.1.3.4. In mountain waves as far as 150 NM leeward of the ridge, and within 2000 to 3000 feet of the tropopause when winds perpendicular to the ridge exceed 50 knots. 2.2.1.4. Extreme Turbulence. The aircraft is violently tossed about and is nearly impossible to control. Structural damage may occur. Expect rapid fluctuations in airspeed of 25 knots or greater and a vertical gust velocity of 50 feet per second or greater. Extreme turbulence is rare, but is most likely to occur: 2.2.1.4.1. In mountain waves in or near a rotor cloud. 2.2.1.4.2. In severe thunderstorms. 102 AFH15-101 5 NOVEMBER 2019 2.2.2. Aircraft Turbulence Sensitivities. Different types of aircraft have different sensitivities to turbulence; Table 2.2 lists the categories for a variety of military and civilian aircraft in their default flight configurations. An aircraft‘s sensitivity to turbulence varies considerably with its weight (amount of fuel, cargo, munitions, etc.), air density, wing surface area, wing sweep angle, airspeed, and attitude. Turbulence information in Terminal Aerodrome Forecasts (TAFs) is specified for Category II aircraft; use caution when applying TAF turbulence data to a specific aircraft type, configuration, and mission profile. Table 2.3 is a turbulence conversion guide between different aircraft categories; modify the forecast for the aircraft type, as required. 2.2.2.1. Fixed Wing Aircraft Effects. Generally, the effects of turbulence on fixed wing aircraft are increased with: 2.2.2.1.1. Non-level flight. 2.2.2.1.2. Increased airspeed. 2.2.2.1.3. Decreased aircraft weight. 2.2.2.1.4. Increased wing surface area. 2.2.2.1.5. Decreased air density / increased altitude. 2.2.2.1.6. Decreased wing sweep angle (wings more perpendicular to the fuselage). 2.2.2.2. Rotary Wing Aircraft Effects. Generally, the effects of turbulence on rotary wing aircraft are increased with: 2.2.2.2.1. Increased airspeed. 2.2.2.2.2. Decreased aircraft weight. 2.2.2.2.3. Decreased lift velocity (the faster the liftoff, the less the turbulence). 2.2.2.2.4. Increased rotor blade arc (the longer the blade, the greater the turbulence). AFH15-101 5 NOVEMBER 2019 103 Table 2.2. Aircraft Turbulence Category Type. Aircraft Type (see Note 2) Military Identifier AH-1 Common Name Turbulence Category (see Note 1) Military Aircraft Turbulence Categories FAA Identifier HUCO Cobra/Huey Cobra B06 Kiowa (see Note 3) OH-58 (see Note 3) RQ-7B Shadow (see Note 6) UH-1 B212 Iroquois (Huey) C172 C150 TG15 H64 Mescalero Cessna 150 Duo Discus/Discus Glider Apache B2 Spirit B52 C5 DC93 GLF3 Stratofortress Super Galaxy Nightingale/Skytrain Gulfstream III GLF4 Gulfstream IV LJ35 C130 M28 DO328 GLF5 B737 Learjet 35 Hercules, Spectre, Commando II, etc. Skytruck Dornier 328 Wolfhound Gulfstream V BBJ, Clipper PA18 H47 Cubcrafters Top Cub Chinook V22 Osprey DO328 E8 S61 Dornier 328 JSTARS Sea King H53 Sea Stallion/Sea Dragon I (see Note 3) T-41D T-51A TG-15A/B AH-64 (see Note 3) B-2A (see Note 5) B-52H C-5M C-9A/C C-20B (see Note 5) C-20H (see Note 5) C-21A C-130 (see Note 7) C-145A C-146A C-37A/B C-40B/C (see Note 5) CC-18-180 CH-47 (see Note 3) CV-22 (see Note 4) DO-328 E-8 H-3 (see Note 3) H-53 (see Note 3) II 104 H-60 AFH15-101 5 NOVEMBER 2019 H60 BlackHawk/SeaHawk/PaveHawk K35R Stratotanker Open Skies Pilatus PC-12 Rivet Joint Talon Cirrus/Kaydett DG-1000 Club Glider King Air N/A Lakota (see Note 3) KC-135R/T OC-135B PC-12 RC-135 T-38A T-53A TG-16A U-21 U-28 UH-72 (see PC-12 T38 SR20 TG16 BE10 PC12 UH72 Note 3) VC-25 A-29 C-12 J C-12 C/D/F C-17A C27J C-32A B742 B190 BE-20 C17 C27 B752 Air Force One EMB 314 Super Tucano Airliner King Air/Super King Air Globemaster III Spartan Boeing 757, Air Force Two (see Note 5) EA-6B EC-130H EO-5C E-9A E-4B E-11A F-15C/D F-18 (A-D) F-18 (E/F/G) F-22 KC-10A KC-46A MC-12 MQ-1B/C MQ-9 QF-4 RC-26B RO-6A RQ-4 T-1A T-38C T-6A U-2S UV-18B A6 C130 E9 B742 E11 F15 F18 F18 F22 DC10 MC12 MQ1 MQ9 SW4 RQ4 BE40 T38 TEX2 U2 DHC6 Prowler Compass Call DHC-7-102/103 Bombardier Dash 8, Widget NAOC Bombardier Global Express/XRS Eagle Hornet Super Hornet (E/F)/Growler (G) Raptor Extender Pegasus Huron Predator/Gray Eagle Reaper Phantom (Drone) Metroliner DHC-8-311/315 Global Hawk Jayhawk Talon (for UPT) Texan 2 Dragon Lady Twin Otter III AFH15-101 5 NOVEMBER 2019 UV-20 A10C B-1B E-3B/C/G F-15E F-16C F-35A Civilian Identifier C-152 C-172 C-175 C-182 C-185 DA-20 PA-38 PAY-3 A-300 A-319 A-320 A-340-200 A-340-300 A-340-500 A-340-600 B-200 B-350 B-727 B-737-600 B-737-700 B-737-800 B-737-900 B-747 B-777 BE-20 C-208 C-310 C-402 C-414A C-421 CL-600 CRJ DC-8 G-520 105 PC6T A10 B1 E3TF/E F15 F16 F35 Pilatus Turbo Porter Thunderbolt II Lancer Sentry Strike Eagle Fighting Falcon Lightning II Civilian Aircraft Turbulence Categories FAA Identifier C152 C172 C175 C182 C185 DA20 PA38 PAY3 A306, A30B A319 A320 A342 A343 A345 A346 BE20 BE30 B721,B722,B72Q,R721,R722 B736 B737, C-40 B738 B739 B741, B742,B743,B74D,B744 B772, B773 BE20 C208 C310 C402 C414 C421 CL60 CRJ1, CRJ2, CRJ7, CRJ9 DC8 EGRT Cessna Aerobat Cessna Skyhawk Cessna Skylark Cessna Skylane Slywagon Diamond Katana Piper Tomahawk Piper Cheyenne Airbus A300 Airbus A319 Airbus A320 Airbus A340 Airbus A340 Airbus A340 Airbus A340 Beechcraft Super King Air Beechcraft Super King Air Boeing 727 Boeing 737-600 Boeing 737-700, BBJ Boeing 737-800 Boeing 737-900 Boeing 747 Boeing 777 Beechcraft Super King Air Cessna Caravan, U-27 Cessna 310, L-27 Cessna 402 Businessliner Cessna Chancellor Cessna Golden Eagle Canadair Challenger 600 Canadair Regional Jet Douglas DC 8, Super 62 Egret IV I II 106 Gulfstream IV & V L-13 L-23 LJ25/35/55/60 MD-80 PA-18 SR-20 B-737/200 B-757 B-767 DC -8 (Super 63) DC-10 AFH15-101 5 NOVEMBER 2019 GLF4, GLF5 Gulfstream IV, V L13 L23 LJ25/35/55/60 Blanik Glider Super Blanik Glider Learjet 25/35/55/60 MD81, MD82, MD83, MD87, MD88 PA18 SR20 B732 B752 B762, B763 DC86 McDonnell Douglas MD-80 Piper Super Cub Cirrus Boeing 737-200 Boeing 757-200 Boeing 767-200, 767-300 Douglas DC 8-60 Series DC10 McDonnell Douglas DC-10, III MD-10 DHC-6 DHC6 DeHavilland Twin Otter E-145 E145 Embraer Regional Jet 145 JS-41 JS41 BAe Jetstream 41 MD-11 MD11 McDonnell Douglas MD-11 Note 1: The Turbulence Categories in this table were derived using such aircraft considerations as wing span, wing area, aspect ratio, taper ratio, wing sweep, and others. The table therefore should be considered authoritative; however, an aircraft’s weight, airspeed, and/or altitude may change its turbulence category from its default value found in this table. Original source document is AFWAL-TR-81 3058. For updates and aircraft additions, contact AFLCMC/XZMG, DSN 785-2299/2310. Note 2: If an aircraft is not listed, the following conservative Turbulence Categories can be made: Jets and multi-engine prop/turbo-prop aircraft that fly at/above FL180 can be considered Category II. All other aircraft should be considered Category I (not related to AIRMETs/SIGMETs). Note 3: Turbulence Categories for helicopters is primarily determined from aircrew feedback. The methodology used for fixed-winged aircraft is not applied to helicopters due to their added complexity. Note 4: The CV-22 displays aspects of flight that include rotor-wing operations and therefore objective gust load calculations and turbulence categorization are not possible for rotor phase of flight (e.g. takeoff/landing). Note 5: Turbulence categories for aircraft with gust alleviations systems (passive or active) are likely less susceptible to turbulence than their computed category. AFH15-101 5 NOVEMBER 2019 107 Note 6: Turbulence categories for Small UAVs (Mean Aerodynamic Chord less than 2 ft), cannot be determined using the Gust Loads Formula and therefore should be considered Category I. Note 7: This turbulence category applies to all Modified/Basic Mission Designators and Model Series (except for the EC-130H/J models which are CAT III) Table 2.3. Turbulence Conversion Chart. 2.2.3. Causes of Turbulence. Turbulence is caused by abrupt, irregular movements of air that create sharp, quick updrafts/downdrafts. These updrafts and downdrafts combine, and create conditions that move aircraft unexpectedly. There are two basic atmospheric conditions that cause turbulence to occur: thermal conditions and mechanical mixing. 2.2.3.1. Thermal Turbulence. Surface heating can generate turbulent conditions; as solar radiation heats the surface, the air above it is warmed by contact. Warmer air is less dense than cooler air, so “bubbles” of warm air rise upward as updrafts. Uneven surface heating and cooling of elevated air parcels causes areas of downdrafts as well. These vertical motions may be restricted to the low levels, or may generate cumulus clouds that grow into thunderstorms. Thermally-induced turbulence has the following characteristics: 2.2.3.1.1. Normally confined to the lower troposphere (surface to 10,000 feet). 2.2.3.1.2. Maximum occurrence between late morning and late afternoon. 2.2.3.1.3. The main impact to flight operations is during terminal approach and departure and during low-level flights. 2.2.3.1.4. Moderate turbulence may occur in hot, arid regions, as the result of irregular convective currents from intense surface heating. 2.2.3.1.5. The strongest thermal turbulence is found in and around thunderstorms. Moderate or severe turbulence can be found anywhere within the storm, including the clear air along its outer edges. The highest probability of turbulence is found in the storm core, between 10,000 and 15,000 feet. 108 AFH15-101 5 NOVEMBER 2019 2.2.3.2. Mechanical Turbulence. Mechanical turbulence is caused by horizontal and vertical wind shear, and is the result of pressure gradient differences, terrain obstructions, or frontal zone shear. General characteristics of mechanical turbulence include: 2.2.3.2.1. Turbulence results from a combination of horizontal and vertical wind shears. 2.2.3.2.2. Turbulence layers are usually 2000 feet thick, 10 to 40 miles wide, and several times longer than wide. 2.2.3.2.3. Wind shear turbulence may result from strong horizontal pressure gradients alone. It occurs when the pressure gradient causes a horizontal shear in wind direction or speed. 2.2.3.2.4. Local terrain can magnify gradient winds to cause strong winds and turbulence near the surface. This creates eddy currents that can make low-level flight operations hazardous. 2.2.3.2.5. Most turbulence resulting from upper frontal zone shear occurs between 10,000 feet and 30,000 feet. 2.2.3.2.6. The jet stream causes most turbulence in the upper troposphere and lower stratosphere, usually occurring in patches and layers, with strongest turbulence on the low-pressure (cold air side) of the jet stream. 2.2.3.2.7. Strong turbulence is often associated with irregular and mountainous terrain. The greater the irregularity of the terrain and the sharper the slope of mountains, the greater the intensity and vertical extent of the turbulence. 2.2.3.2.8. Fronts may produce moderate or greater turbulence; the intensity depends on the strength and speed of the front (strong fronts may create updrafts of 1000 feet per minute in a narrow zone just ahead of the front). Over rough terrain, fronts typically produce moderate or greater low-level turbulence. Over flat terrain, fronts moving faster than 30 knots typically produce moderate or greater low-level turbulence. 2.2.4. Turbulence Environments. Several atmospheric and environmental factors are considered particularly favorable for turbulence production, as detailed below. 2.2.4.1. Strong vertical and/or horizontal wind shear. Both vertical and horizontal wind shear are key ingredients for the development of CAT. Horizontal and vertical shear create eddies that can produce turbulence in the presence or absence of static instability (i.e., convective processes). These eddies are capable of propagating great distances before dissipating into laminar (smooth) flow. Vertical wind shear of at least 6 knots/1,000 feet, or horizontal wind shear of at least 40 knots/150 miles, is generally needed to produce CAT. Shear of this magnitude can often be found near the jet stream, where wind speed changes significantly over relatively small vertical and horizontal distances. AFH15-101 5 NOVEMBER 2019 109 2.2.4.2. Decreasing potential temperature with height in the planetary boundary layer (PBL). At least some static instability is necessary for the development of turbulence, unless wind shear is very strong. In a dry PBL, the environment is statically unstable if the potential temperature decreases with height. This allows for the development of positively buoyant thermals that can contribute to turbulence production. Turbulence that develops in a statically unstable environment acts to reduce the instability and return the air to a laminar (non-turbulent) state. However, in some cases, such as in the PBL on a sunny day, the warm ground acts to continuously heat and destabilize the air, allowing turbulence to continue throughout the day. Note that if the environment is conducive to deep, moist convection, the lapse rate of the equivalent potential temperature should be used instead of potential temperature, because it accounts for the presence of water vapor. 2.2.4.3. Dynamic instability. 2.2.4.3.1. Dynamic instability can be evaluated by a Richardson Number of less than 1 and optimally less than 0.25 (see Table 2.4). If vertical wind shear is strong enough, turbulence can develop in a statically stable environment. When this occurs, the environment is considered to be dynamically unstable. Turbulence is produced dynamically when dense air initially resides beneath less-dense air, with a velocity shear between the layers. The flow in this initial state is laminar (see Figure 2.7 panel a). If the shear increases to a critical value, the flow becomes dynamically unstable and waves form on the surface (see Figure 2.7 panel b). These waves grow in amplitude until they break (see Figure 2.7 panels c-e). The breaking waves are known as KelvinHelmholtz waves. In the case of each wave, some lighter air rolls underneath denser air, leading to areas of static instability. The static instability and dynamic instability lead to turbulence production; eventually, the turbulence causes mixing and momentum transfer, reducing the shear and instability. The shear falls back below the critical value, the turbulence decays (see Figure 2.7 panel f), and the flow returns to a laminar state. This process is thought to occur during the onset of CAT, often near jet streams such as the nocturnal jet or the planetary-scale jet stream. In these cases, turbulence can persist for days. This entire process is illustrated in Figure 2.7. Figure 2.7. Kelvin-Helmholtz wave lifecycle. 110 AFH15-101 5 NOVEMBER 2019 2.2.4.3.2. Richardson number. Dynamic instability can be evaluated using the Richardson number (Ri) as indicated in Table 2.4 The Ri is the ratio of static stability to vertical wind shear. In general, turbulence will not develop when the Ri is greater than 1, because the static stability term is greater than the shear term, and static stability tends to drain off turbulent kinetic energy. When the wind shear increases to the point that it overpowers static stability, turbulence becomes more likely. As a general rule, turbulence is possible when the Ri is less than 1, and probable when the Ri is less than 0.25, which is the critical Richardson number. It is at this point that the shear is strong enough for the flow to become dynamically unstable and for Kelvin-Helmholtz waves to develop. Note that statically unstable air results in a negative Ri, and therefore implies dynamic instability. Table 2.4. Richardson Number calculation and interpretation. 2.2.4.4. Deep, moist convection (thunderstorms). Deep, moist convection always produces turbulence, because it is associated with both static instability and dynamic instability. Additionally, strong updrafts, downdrafts, and vertical and horizontal wind shear exist within and around thunderstorms. Thunderstorms also produce gravity waves, which are vertically propagating waves, driven by the temperature profile of the environment. These can cause severe turbulence as far as 20 miles from the nearest storm. Avoidance of incloud convective turbulence can be achieved through the use of radar and satellite imagery, or through visual identification. Near-cloud turbulence, however, presents a special challenge because it is not detectable with standard on-board and ground-based radar. In order to avoid near-cloud turbulence, FAA guidelines stipulate that aircraft should avoid by at least 20 miles (laterally) any thunderstorm identified as severe or giving an intense radar echo (especially under the anvil of a large cumulonimbus.) They also recommend that aircraft clear the top of a known or suspected severe thunderstorm by at least 1000-ft altitude for each 10 knots of wind speed at the cloud top (a guideline that likely exceeds the altitude capability of most aircraft.) AFH15-101 5 NOVEMBER 2019 111 2.2.4.5. Strong convergence at jet stream or tropopause level. Convergence is the compaction of a fluid caused by confluence of flow or deceleration of air parcels; convergence aloft at jet stream level or at the tropopause results in sinking motions, which in some cases can cause turbulence. Convergence aloft can also produce propagating gravity waves by disturbing a tropopause inversion above an area of frontogenesis, which may lead to turbulence. 2.2.4.6. Acceleration of air parcels at jet stream level to super-geostrophic speeds. Just as convergence aloft caused by confluence of streamlines or deceleration of air parcels can cause turbulence, diffluence and divergence aloft may also result in turbulent conditions. The geostrophic wind is usually an excellent approximation of the actual wind in the free atmosphere, especially at jet stream level. Turbulence can be created when geostrophic winds accelerate into or out of a jet streak and become super-geostrophic for a short time. 2.2.4.7. An “S-shaped” temperature profile above the tropopause. Stratospheric turbulence can be a major detriment to aircraft operating at ultra-high altitudes, such as the U-2 and Global Hawk. The most common stratospheric turbulence diagnostic is the presence or lack of an “S” shape in the Skew-T temperature profile above the tropopause, such as in Figure 2.8. The high risk area for turbulence is the middle of the “S”, where the temperature decrease (lapse rate) is adiabatic or super-adiabatic (≥ 9.8°C/km). This layer is characterized by strong static instability due to the steep lapse rates. Additionally, wind speeds above the tropopause are often in excess of 100 knots, so strong vertical and horizontal wind shear can be present as well. The U-2 community developed the following procedure for stratospheric turbulence forecasting utilizing the “S” layer: 2.2.4.7.1. Use the most recent Skew-T to locate the tropopause, which is characterized by a temperature increase of at least 2°C/km for several km. The tropopause height varies with latitude and season, but is often found between 300 mb and 100 mb. 2.2.4.7.2. Less than 3°C of warming per 1,000 feet above the tropopause indicates that turbulence is unlikely. 2.2.4.7.3. 3°C to 5°C of warming per 1,000 feet above the tropopause indicates that light to moderate turbulence is possible. 2.2.4.7.4. More than 5°C of warming per 1,000 feet above the tropopause indicates that severe turbulence is possible. 112 AFH15-101 5 NOVEMBER 2019 Figure 2.8. “S-shaped” tropospheric temperature profile, indicating potential turbulence. 2.2.5. Clear Air Turbulence (CAT). CAT is a form of mechanical turbulence, and includes all turbulence not associated with visible convective activity (such as high-level frontal and jet stream turbulence, or turbulence occurring in high-level, non-convective clouds). The typical meteorological conditions under which CAT forms are described below. 2.2.5.1. CAT surface and upper-level patterns. 2.2.5.1.1. Surface cyclogenesis. When cyclogenesis occurs, CAT is expected near the jet stream core, north-northeast of the surface low development (see left image in Figure 2.9). In some cases, the surface low redevelops north of the main jet, with a formation of a secondary jet (see right image in Figure 2.9); CAT typically occurs in a similar location along the secondary jet stream core. CAT intensity is dependent on several factors; the strength of cyclogenesis, the proximity to mountains, the intensity of the jet core, and the amplification and curvature of the downstream ridge. For cyclogenesis less than 1 mb/hour, expect moderate CAT. For cyclogenesis greater than or equal to 1 mb/hour, anticipate moderate to severe CAT. AFH15-101 5 NOVEMBER 2019 113 Figure 2.9. Surface cyclogenesis and jet-core CAT and secondary jet-core CAT. 2.2.5.1.2. Upper level lows. The potential for moderate CAT exists during the development of cut-off upper-level lows; the sequence shown in Figure 2.10 shows the areas of expected CAT during the various stages of cut-off low development. CAT usually first appears in the areas of confluent and diffluent flow; once the low is fully cut off, CAT diminishes to light in the vicinity of the low. Figure 2.10. CAT during the development of an upper-level low. Patterns b and c show the areas of moderate CAT as the low breaks off from the trough. 2.2.5.1.3. CAT criteria at 500 mb. The following patterns at 500 mb may be indicative of CAT: 2.2.5.1.3.1. Shortwave troughs near one another (double troughs). 2.2.5.1.3.2. A well-defined thermal trough. 2.2.5.1.3.3. A narrow band of strong winds with strong horizontal wind shears. 114 AFH15-101 5 NOVEMBER 2019 2.2.5.1.3.4. A closed isotherm cold pocket moving through an open flow pattern (i.e., height field with no closed contours). 2.2.5.1.3.5. 500-mb winds greater than 75 knots in areas with wind shifts greater than or equal to 20°, and tight thermal gradients. 2.2.5.1.3.6. Troughs associated with a surface frontal wave (often indicated by sharply curved isotherms around the northern edge of a warm tongue). 2.2.5.1.4. Shear lines in upper-level lows. In this scenario, the potential for CAT is greatest between the two curved portions of the jet near the narrow neck of the cut-off low (see Figure 2.11); this is the area near the shear line. Forecast moderate CAT when the jet stream is greater than or equal to 50 knots around the closed upper-level low; forecast severe CAT if the jet reaches 115 knots. Figure 2.11. CAT area along a shear line associated with an upper level low. 2.2.5.2. CAT Wind Patterns. 2.2.5.2.1. Diffluent winds. Most CAT occurs during formation of diffluent upper-level wind patterns; once the diffluent pattern becomes established, CAT may weaken in the diffluent zone. However, when a surface front is present (or forming), the potential for CAT increases in the areas of upper-level diffluent flow near the surface system (see Figure 2.12) AFH15-101 5 NOVEMBER 2019 115 Figure 2.12. CAT in a diffluent wind pattern. 2.2.5.2.2. Strong winds. CAT forms in areas of strong winds when isotherms and height contours are nearly parallel, and only minor variations exist in wind direction (about 20° per 4 degrees of latitude) with exceptionally tight thermal gradients. CAT can also occur along and above a narrow band of strong 500 mb winds when horizontal wind shears are strong on either side of the band, especially if the winds are highly ageostrophic. 2.2.5.2.3. Confluent jet streams. When two jet stream cores converge to within 250 NM, the potential for CAT increases. Since the poleward jet is usually associated with colder temperatures and is lower than the second jet, the poleward jet will often undercut the other, producing strong vertical wind shears. The potential CAT area ends where the jets diverge to a distance of greater than 5° latitude. 2.2.5.3. CAT Thermal Patterns. 2.2.5.3.1. Temperature gradients at and above 300 mb. Temperature gradients at the 300, 250, and 200 mb pressure levels provide key indicators for CAT potential; expect CAT when a temperature gradient greater than or equal to 5°C/120 NM is observed or forecast, and at least one of the following is observed: 2.2.5.3.1.1. Trough movement greater than or equal to 20 knots. 2.2.5.3.1.2. Wind shift greater than or equal to 75° in the region of cold advection. 2.2.5.3.1.3. Horizontal wind shear greater than or equal to 35 knots/110 NM (~200 km). 2.2.5.3.1.4. Wind component normal to the cold advection is greater than or equal to 55 knots. 2.2.5.3.2. Open-isotherm patterns. Noticeable “bulging” of a cold-air tongue in a relatively tight thermal gradient may occur at or near the base of the trough; in these cases, the isotherms curve more sharply than the height contours and moderate turbulence is possible between 25,000 and 35,000 feet. 116 AFH15-101 5 NOVEMBER 2019 2.2.5.3.3. Closed-isotherm patterns. CAT is often found in the development of a moving, closed cold-air isotherm at 500 mb when the height contours are not closed (see Figure 2.17.) In this situation, multiple reports of moderate or greater CAT were received; in these scenarios, CAT is likely between 24,000 and 37,000 feet. 2.2.5.4. CAT Trough and Ridge Patterns. 2.2.5.4.1. Shearing troughs. Rapidly moving troughs north of a jet may produce CAT in the confluent flow at the base of the trough (see Figure 2.13); the turbulent area is typically concentrated north of the jet stream core. Figure 2.13. CAT areas in shearing troughs. 2.2.5.4.2. Strong wind maximum to the rear of the upper trough. CAT potential is high when a strong north-south jet is located along the backside of an upper trough; it usually occurs in the area of decreasing winds between the base of the trough and the max wind upstream. Minimum wind speed changes required for CAT occurrence are greater than or equal to 40 knots within 10° of latitude. If the difference between the jet core and the minimum wind speed is greater than or equal to 60 knots, CAT is most likely to occur between the jet core and the base of the trough, centered on the warm-air side of the jet (see Figure 2.14.) AFH15-101 5 NOVEMBER 2019 117 Figure 2.14. CAT with wind maximum to the rear of the upper trough. 2.2.5.4.3. Deep pressure trough at 500 mb. With a deep pressure trough at 500 mb, CAT typically forms in a sharply anticyclonic, persistent isotherm pattern downwind of the trough. 2.2.5.4.4. Double trough configuration. Strong CAT is often associated with two troughs when they are close enough together that the trailing trough influences the airflow into the leading trough. This pattern is often associated with a flat or flattening ridge between the troughs, which advects warm air into the base of the lead trough. Although the double trough can be detected at a number of levels, the 500-mb product is the best to use. 2.2.5.4.5. Upper level ridges. Expect CAT on both sides of the jet near the area where the jet undergoes maximum latitudinal displacement in an amplifying ridge (see Figure 2.15.) Maximum CAT is located in the area of greatest anticyclonic curvature (usually within 250 NM of the ridge axis and elongated in the direction of the flow). Expect moderate or greater CAT when the vertical wind shear is greater than or equal to 10 knots/1000 feet, or when the winds are greater than 135 knots in the area of broad anticyclonic curvature. 118 AFH15-101 5 NOVEMBER 2019 Figure 2.15. CAT with upper level ridges. 2.2.5.5. Forecasting CAT using upper air data. 2.2.5.5.1. 700 mb and 850 mb height and temperature fields. At 700 mb and 850 mb, use the heights and temperatures to identify regions of thermal advection, wind components perpendicular to mountain ridges, mid or low level turbulence, and frontal boundaries. 2.2.5.5.2. 500 mb height, temperature, and vorticity fields. Focus on areas of thermal advection, short-wave troughs, and wind components perpendicular to mountain ridges. The 500 mb level can also be used to approximate jet stream positions and upper-air synoptic patterns; for example, place the subtropical jet near the –11°C isotherm, the polar front jet near the –17°C isotherm, and the northern branch jet near the –30°C isotherm. 2.2.5.5.3. 250 mb jet stream. Analyze the 250 mb winds closely to determine the current and future jet stream core position. 2.2.5.5.4. 200 mb height and temperature fields. Look for regions of strong isotherm packing in association with strong wind flow; the 200 mb isotherms align closely with the 500 mb vorticity pattern, and can indicate short waves and developing weather systems. 2.2.6. Mountain Wave (MW) Turbulence. The most severe type of terrain-induced turbulence is mountain wave turbulence. It most often occurs in clear air and in a stationary wave downwind of a prominent mountain range. It is caused by the mechanical disturbance of the wind by the mountain range. Mountain wave intensity depends on several factors: 2.2.6.1. Wind speed and direction. Winds flowing within 30 degrees of perpendicular to the ridgeline, with little change in direction with height, are most favorable for generating mountain wave turbulence. Mountaintop wind speeds of about 25 knots, increasing with height, are also favorable for generating mountain waves. Mountain waves can extend as far as 300 NM leeward of the mountain range when the wind component perpendicular to mountain range exceeds 50 knots. A wave can extend as far as 150 NM when the perpendicular component exceeds 25 knots. AFH15-101 5 NOVEMBER 2019 119 2.2.6.2. Height and slope of the mountain. High mountains with steep leeward and gentle windward slopes produce the most intense turbulence. 2.2.6.3. Upstream stability. Look for upstream temperature profiles that exhibit an inversion or a layer of strong stability near mountain top height, with weaker stability at higher levels; this can enhance mountain wave turbulence downstream. 2.2.6.4. Inversions. An inversion capping the tropopause induces a stronger downward wave and can cause wave amplification (and enhanced mountain wave turbulence). 2.2.6.5. There are several different types of clouds associated with MW turbulence; Figure 2.16 illustrates the structure of a strong mountain wave and the associated cloud patterns. Figure 2.16. Mountain wave cloud structure. 2.2.6.5.1. Cap clouds. Cap clouds “hug” the mountaintop and flow down the leeward side with the appearance of a waterfall. Cap clouds are potentially hazardous, since they can obscure the top of the mountain and are associated with strong downdrafts (5000-8000 feet per minute). 2.2.6.5.2. Roll clouds. Roll clouds, also called a rotor clouds, appear as a line of cumulus parallel to the ridgeline. They form on the leeside and have bases near the height of the mountain peak and tops near twice the height of the peak. Roll clouds are extremely turbulent, with strong updrafts (5000 feet per minute) on the windward side and intense downdrafts (5000 feet per minute) on its leeward edge. Roll clouds may form immediately on the lee of the mountain, or up to 10 miles downstream. 120 AFH15-101 5 NOVEMBER 2019 2.2.6.5.3. Lenticular clouds. Lenticular clouds are relatively thin, lens-shaped clouds with bases above roll clouds and tops extending to the tropopause. They have a tiered or stacked look due to atmospheric stability above the mountain ridge. All lenticular clouds are associated with turbulence. In polar regions, lenticular clouds can appear as high in the stratosphere as 80,000 feet; these are called “mother-of-pearl” (nacreous) clouds. 2.2.6.6. Favorable conditions for MW turbulence development: 2.2.6.6.1. Temperature of -60ºC or colder at the tropopause. 2.2.6.6.2. Jet stream over or just north of the ridge line. 2.2.6.6.3. A cold front approaching or stationary to the north of the mountain range. 2.2.6.6.4. Cold air advection across or along the mountain range. 2.2.6.7. MW turbulence occurrence indicators: 2.2.6.7.1. Rapidly falling pressure to the lee side of mountains with significant differences on the windward side. 2.2.6.7.2. Lee-side gusty surface winds at nearly right angles to the mountains. 2.2.6.7.3. Observations of ACSL, rotor clouds or cap clouds. 2.2.6.7.4. A lee-side cirrus trench (the Foehn gap). 2.2.6.7.5. A well-defined lee-side trough. 2.2.6.7.6. PIREPS indicating mountain wave turbulence. 2.2.6.7.7. Blowing dust picked up and carried aloft to 20,000 feet MSL or higher. 2.2.6.8. MW turbulence forecasting guidance. Used in conjunction, the parameters in Table 2.4 and nomogram in Figure 2.17 can provide guidance in forecasting mountain wave turbulence. Table 2.5. Low level mountain wave turbulence guidance chart. Low-level Mountain Wave Turbulence (Surface to 5000 feet above the ridge line) Wind component normal to the Turbulence Intensity mountain range at mountaintop is Light Moderate Severe greater than 24 knots, and: Surface pressure change across See Figure 2.17 See Figure 2.17 See Figure mountain is: 2.17 850 mb temperature difference Less than 6° C 6° C – 9° C Greater than across mountain is: 9° C 850 mb temperature gradient Less than 4° C – 6° C/60 NM Greater than across mountain is: 4°C/60 NM 6° C/60 NM Lee side surface gusts are: Less than 25 kt 25 – 50 kt Greater than 50 kt Winds below 500 mb greater than Increase turbulence by one degree of intensity (i.e., 50 knots: moderate to severe) AFH15-101 5 NOVEMBER 2019 121 Figure 2.17. Mountain Wave Turbulence nomogram. 2.2.7. Wake Turbulence. Every aircraft generates two counter-rotating vortices off each wingtip; wake turbulence results when an aircraft encounters vortices from another aircraft. Vortex generation begins when the nose wheel lifts off the ground and ends when the nose touches back down again during landings. A vortex forms at a wingtip as air circulates outward, upward, and around the wingtip. The diameter of the vortex core varies with the size and weight of the aircraft; the largest vortices can be up to 50 feet in diameter, with a much larger area of turbulence. 2.2.7.1. Wake turbulence dissipation. Wake vortices usually stay fairly close together (about 3/4 of the wing span) until dissipation; they sink at a rate of 400 to 500 feet per minute and stabilize about 900 feet below the flight path, where they begin to dissipate. Atmospheric turbulence increases the dissipation rate of wake turbulence, while ground effects and surface winds can alter the low-level vortex characteristics. As a vortex sinks into the boundary layer, it begins to move laterally at about 5 knots; a crosswind will decrease the lateral movement of a vortex moving toward the wind and increase the movement of a vortex moving with the wind. This could hold one of the vortices over the runway for an extended period or allow one to drift onto a parallel runway. Vortices persist longer during inversions. 2.2.7.2. Wake turbulence avoidance. The Federal Aviation Administration has published the following rules for avoiding wake turbulence (from their Aeronautical Information Manual): 2.2.7.2.1. If two aircraft fly in the same direction within 15 minutes of each other, the second should maintain an altitude equal to or higher than the first. If required to fly slightly below the first, the second aircraft should fly upwind of the first. 122 AFH15-101 5 NOVEMBER 2019 2.2.7.2.2. Vortex generation begins with liftoff and lasts until touchdown. Therefore, aircraft should avoid flying below the flight path of a recent arrival or departure. 2.2.7.2.3. Stable conditions combined with a crosswind of about 5 knots may keep the upwind vortex over the runway for periods of up to 15 minutes. 2.2.8. Gravity Waves and Stratospheric Turbulence. Stratospheric turbulence is fundamentally different from tropospheric turbulence – while tropospheric CAT is primarily caused by horizontal and vertical wind shears, stratospheric CAT is primarily caused by the breaking of gravity waves. A gravity wave is generated when an air parcel at equilibrium with its environment is rapidly vertically displaced (see Figure 2.18) – this can happen due to orographic forcing (mountains), or when an air parcel is trapped under an inversion (thunderstorm downburst near a cold front). As the air parcel is forced upwards, it expands, cools, and becomes heavier than its surrounding environment. The air parcel begins to sink, accelerating through the equilibrium point, where it compresses, warms, and becomes lighter than its surrounding environment, accelerating back upwards, where the process repeats. This up-and-down motion as the parcel travels downstream is a gravity wave. Gravity waves that propagate into the stratosphere increase in amplitude with height, due to decreasing air densities. Typical wavelengths range from 5 to 5000 km horizontally, and.1 to 5 km vertically; gravity waves on the order of 10-100 km wavelength typically generate turbulence felt by aircraft. Depending on atmospheric stability, they can last from about 5 minutes to over a day. Figure 2.18. Gravity waves on visible satellite imagery. 2.2.9. Forecasting Aids. The tools in this section are provided to aid in accurate turbulence forecasting; use them as applicable to assist in predicting turbulence conditions. 2.2.9.1. General turbulence locations. Refer to Table 2.5 for a summary of areas where turbulence is expected. AFH15-101 5 NOVEMBER 2019 123 Table 2.6. Expected turbulence locations. Always anticipate light-or-greater turbulence in the following areas: Thunderstorms Cold air advection Warm air advection Areas of strong thermal advection, such as: Strong upper-level fronts Rapid surface cyclogenesis Outflow areas of cold-region jet streams Tilted ridges Areas of larger vertical shear, particularly Sharp ridges below strong stable layers near: Tilted troughs Confluent jet streams Mountainous regions Areas of significant horizontal directional Diffluent upper-level flow and/or speed shear, such as: Developing cut-off lows Sharp anticyclonic curvature 2.2.9.2. Low-level turbulence (surface to 10,000 feet) forecasting flowchart. Use Figure 2.19 as a quick-reference guide for forecasting low-level turbulence for Category II aircraft; refer back to Table 2.3 to adjust to other aircraft types as necessary. Figure 2.19. Low-level turbulence forecasting flowchart – Category II aircraft. 2.2.9.3. Forecasting convective cloud turbulence. This method forecasts turbulence in convective clouds by analyzing two atmospheric layers (surface-9000 feet and above 9000 feet) on the Skew-T diagram (see Figure 2.20) The forecast method is designed for Category II aircraft; adjust with Table 2.3 as necessary for other types of aircraft. 124 AFH15-101 5 NOVEMBER 2019 Figure 2.20. Convective cloud turbulence forecasting, using the Skew-T. 2.2.9.3.1. Surface-9000 foot layer. Analyzing this layer estimates the buoyant potential in the lower atmosphere, and estimates turbulence in thunderstorms. First, use the convective temperature to forecast the maximum surface temperature, then project a dry adiabat from the convective condensation level (CCL) to the surface – this gives the convective temperature. Adjust this temperature using temperature curves for local effects. Next, subtract 11°C from the final forecast maximum temperature, and follow this isotherm to its intersection with the dry adiabat projected upward from the forecast maximum temperature. If the intersection is above 9000 feet, no turbulence is expected below 9000 feet MSL. If the intersection is below 9000 feet, follow the moist adiabat from the intersection of the isotherm and the dry adiabat upward to the 9000-foot level; the temperature difference between this moist adiabat and the free-air temperature curve determines the severity of the turbulence, as well as the limits of the layers of each degree of turbulence (refer to Table 2.6.) AFH15-101 5 NOVEMBER 2019 125 Table 2.7. Surface-9000 foot temperature difference vs. turbulence intensity. Layers where temperature Turbulence is forecast as: difference is: 0º to 6ºC Light 6º to 11ºC Moderate 11ºC or more Severe 2.2.9.3.2. Layer above 9000 feet. Follow the moist adiabat that passes through the CCL upward to the 400 mb level; the maximum temperature difference between this moist adiabat and the forecast free-air temperature curve is the central portion of the most turbulent area. The expected intensity of the turbulence based on upper-level temperature differential is shown in Table 2.7. Table 2.8. Layer above 9000 feet temperature difference vs. turbulence intensity. Layers where temperature Turbulence is forecast as: difference is: 0º to 2.5ºC Moderate 2.5º to 7ºC Severe 7ºC or more Extreme 2.2.9.4. Low-level turbulence nomogram. Figure 2.21 is a quick-reference nomogram, predicting turbulence using winds and temperature differences across a surface front. Figure 2.21. Low-level turbulence nomogram. 2.2.9.5. Satellite signatures and turbulence forecasting. 2.2.9.5.1. Deformation zone. Deformation zones are regions where the atmosphere is undergoing contraction in one direction and elongation or stretching in the perpendicular direction, relative to the motion of the air stream (Figure 2.22); a visible cloud border is often located near and parallel to the stretching axis. Moderate to severe turbulence is likely when: 126 AFH15-101 5 NOVEMBER 2019 2.2.9.5.1.1. Cyclogenesis is in progress, accompanied by a building or rapidly moving upper ridge to the east of the storm. 2.2.9.5.1.2. The cloud system is encountering confluent (opposing) flow caused by a blocking upper-level system (a closed low or anticyclone) downstream. 2.2.9.5.1.3. The low and associated comma cloud system are dissipating. 2.2.9.5.1.4. A flattening of the cloud border is occurring on the upstream side of the comma. Figure 2.22. Turbulence in a deformation zone. 2.2.9.5.2. Wave cloud signatures. 2.2.9.5.2.1. Transverse bands. Transverse bands are irregular, wave-like cirrus cloud patterns that form nearly perpendicular to the upper level flow (Figure 2.23); they are usually associated with the low-latitude subtropical jet stream and indicate large vertical and possibly horizontal wind shear. Wider and thicker transverse bands are more likely to contain severe turbulence, due to the added presence of thermal instability. Figure 2.23. Transverse bands. AFH15-101 5 NOVEMBER 2019 127 2.2.9.5.2.2. Billow clouds. Billow clouds (Figure 2.24) are cirrus or middle-level clouds which are regularly spaced, narrow, and oriented parallel to the upper flow. They are most often seen when a strong jet intersects either a frontal cloud system or a line of cumulonimbus clouds at a large crossing angle. The anvil debris of convective clouds in these situations extends well downstream from its source. Although individual waves dissipate quickly (less than 30 minutes), new waves can form nearby under favorable conditions. The longer the wavelength of the billows, the better the chance for significant turbulence. Figure 2.24. Billow clouds. 2.2.9.5.2.3. Water vapor imagery darkening. On water vapor imagery, elongated bands or large oval-shaped darkening regions are indicative of moderate or greater turbulence. This darkening is usually accompanied by cold air advection and convergence in the mid- and upper-levels of the troposphere, as stratospheric air descends into the upper troposphere. Moderate or stronger turbulence is likely when image darkening occurs (occurs over 80% of the time in one study), especially when the darkening persists for at least 3 hours. 2.2.9.5.2.4. Mountain waves. Mountain waves (Figure 2.25) are stationary waves situated downwind of a prominent mountain range, caused by the orographic disturbance of the wind. The waves exhibit a stationary, narrow clearing zone parallel to steep mountain ranges. They may also occur in Chinook wind synoptic situations, near or just east of the upper ridge and south of the jet stream. Figure 2.25. Mountain waves. 128 AFH15-101 5 NOVEMBER 2019 2.2.9.6. Vertical cross sections. Analyzing atmospheric vertical cross sections can provide tremendous insight into the atmospheric structures that contribute to turbulence development. Analyzing wind speeds (10 knot intervals) and temperature (at 5°C intervals) will reveal jet cores and strong vertical temperature gradients associated with atmospheric turbulence; frontal boundaries and areas of wind shear will also become evident. 2.2.9.7. Doppler weather radar. Weather radar provides multiple unique capabilities to detect and display turbulence indicators, such as frontal boundaries, low-level jets, gust fronts, and upper-level wind shear. 2.2.9.7.1. Spectrum Width. Though not conclusive, spectrum width values of 8-11 knots have been associated with moderate turbulence for Category II aircraft, and values 12 knots and higher may be indicative of severe turbulence. Use the spectrum width product to corroborate suspected turbulence areas found using other forecast products. 2.2.9.7.2. VAD Wind Profile. Use the radar’s wind profiler to examine the current and past vertical wind structure to identify turbulent features evolving over time (e.g., inversions, wind shifts, and development of jet streams). Look for areas of sharp turning in the winds with high wind speeds to identify strong local vertical wind shear. 2.2.9.7.3. Base velocity. Areas of sudden speed or directional shifts are associated with wind shear and atmospheric turbulence. Intense shear regions, such as the tops of thunderstorms associated with storm top divergence, can also be located using base velocity. 2.2.9.7.4. Vertically Integrated Liquid (VIL). High VIL values indicate a strong potential for severe convective weather, and associated wind shear and atmospheric turbulence. 2.2.9.8. AFW-WEBS Low-level and Upper-level turbulence guidance. AFW-WEBS provides global forecaster-in-the-loop turbulence outlooks for low-level (surface-18,000 feet) and upper-level (18,000-60,000 feet) forecasts (Figure 2.26); areas of expected turbulence are color-coded with bases and tops indicated in numerical format. AFH15-101 5 NOVEMBER 2019 129 Figure 2.26. AFW-WEBS authoritative upper level turbulence outlook. 2.2.9.9. The Stratospheric Layer Advanced Turbulence (SLAT) Index (see Table 2.8). The SLAT index is based on the “S-shaped” stratospheric temperature profile. SLAT values typically range from 0 to 15, with higher values indicative of moderate or greater stratospheric turbulence potential. High values are obtained from a large difference (but less than 10) between the temperatures at the top and bottom of the “S” layer, indicative of a steep lapse rate in the mixed layer; they also result from shallow inversion lapse rates and/or a thin mixing layer (small DZ) In the mountains, SLAT values can exceed 100, possibly indicative of vertically-propagating mountain waves. Table 2.9. Stratospheric Layer Advanced Turbulence Index. 2.2.9.10. Turbulence climatology. The 14th Weather Squadron provides global climatologies of monthly and annual turbulence frequency for several atmospheric layers (FL210-270, 300-350, and 350-400), these products are available on their website. 130 AFH15-101 5 NOVEMBER 2019 2.3. Icing. Structural icing interferes with aircraft control by increasing drag and weight while decreasing lift, while engine-system icing reduces the effective power of aircraft engines. The accuracy of icing forecasts begins with accurate predictions of precipitation, clouds, and temperature. Aircraft icing generally occurs between the freezing level and -40ºC (icing can occur at -42ºC in the upper parts of cumulonimbus clouds). The frequency of icing decreases rapidly with decreasing temperatures, becoming rare at temperatures below -30ºC. The normal atmospheric vertical temperature profile usually restricts icing to the lower 30,000 feet of the atmosphere. In the middle latitudes (such as in most of the United States, Northern Europe, and the Far East), icing is most frequent in winter; frontal activity is frequent, and the resulting cloud systems are extensive, creating favorable icing conditions. Polar regions are normally too cold in the wintertime to contain the concentration of moisture necessary for icing; locations at higher latitudes (such as Canada and Alaska) usually experience optimal icing conditions in the spring and fall. 2.3.1. Icing Formation Processes. Clouds are not water vapor, but instead consist of water droplets and/or ice crystals that form when the atmosphere becomes saturated. Once saturated, the atmosphere produces and maintains clouds through multiple processes, including the addition of water vapor, cooling and lifting by convective or mechanical/orographic processes, and convergence. 2.3.1.1. Terrain. Air lifted mechanically by terrain can spur development of a broad range of cloud types, from small cap clouds over mountain peaks to widespread cloud decks covering hundreds of kilometers. An example of a terrain effect is upslope easterly winds over the western high plains, which create widespread cloudiness as the air is forced westward over the gently rising terrain – this pattern generally occurs after passage of an arctic or polar front. These clouds often result in broad areas of icing conditions, which can last for days at a time. Icing hazards can also develop in orographic clouds, which tend to develop along mountaintops and ridges and can persist for days if the winds and moisture are consistent. Winds blowing perpendicular to ridgelines provide the most favorable conditions for orographic cloud development. 2.3.1.2. Fronts. Fronts act like “moving terrain”, forcing one air mass up and over another. Fronts can be areas of enhanced icing due to the presence of convection and ample moisture. Although the lifting over a moving cold air mass can have a broad extent, the most intense lifting tends to be limited to narrow bands of clouds near the surface frontal location. The icing threat posed by a cold front varies based on the strength and extent of the associated lift and ultimately, the aircraft‘s flight altitude and trajectory through the frontal cloud. A flight path perpendicular to the cloud band can reduce the icing threat, while a path parallel to the cloud band can be particularly hazardous due to the prolonged time within the cloud. AFH15-101 5 NOVEMBER 2019 131 2.3.1.3. Cyclones. Cyclonic circulations generate convergence of air at the center of lowpressure systems, resulting in large scale rising motion and cloud formation. Large scale dynamic processes, such as warm air advection and differential vorticity advection, also lead to broad regions of uplift and cloudiness. The area ahead of an active and stationary warm front, and behind the surface low center, is the primary region where icing occurs; this area provides optimal conditions for the formation of supercooled droplets and freezing precipitation. The extensive horizontal and vertical extent of a synoptic-scale cyclone can result in long exposures of aircraft to icing conditions, depending on the flight path and altitude. 2.3.2. Phase Transitions. Water is a unique substance; at typical tropospheric pressures and temperatures, it can exist in three phases – liquid, solid, and vapor. The transitions between these phases determine the likelihood and amount of liquid water available for icing. There are six main categories of phase change processes, described below. 2.3.2.1. Condensation. Condensation is the transition of water vapor to liquid water; this phase transition forms liquid water clouds. Clouds are formed when rising air cools to its dew point temperature; as an air parcel rises, it expands and cools adiabatically, and vapor condenses onto small airborne particles called cloud condensation nuclei (CCN). As the air continues to rise and cool, additional condensation takes place on these activated droplets and the droplets continue to grow. 2.3.2.2. Evaporation. Evaporation is the transition of liquid water to vapor; when clouds mix with the surrounding dry environment, droplets evaporate due to their exposure to subsaturated conditions. If enough dry air is mixed in, clouds will completely dissipate. This entrainment and mixing process is a common occurrence in both convective and stratiform clouds; stratus can change to stratocumulus and eventually dissipate as dry air is entrained. 2.3.2.3. Freezing. Freezing is the transition of liquid water to ice. Liquid water droplets do not necessarily freeze at 0° C; droplets may become supercooled, persisting at temperatures well below 0° C. In order for a supercooled droplet to freeze, it must come into contact with a small particle called an ice nucleus. The ability of these ice nuclei to catalyze droplet freezing is temperature dependent; at temperatures warmer than -12° C to -15° C, few active nuclei exist and clouds are likely to be composed primarily of liquid droplets rather than ice crystals. If a cloud lacks a sufficient concentration of ice nuclei, widespread areas of supercooled water can exist, and icing is likely. When temperatures approach -40° C, an ice nucleus is no longer needed and droplets freeze spontaneously. 2.3.2.4. Melting. Melting is the transition of ice to liquid water. Melting can remove accumulated ice from the airframe if the pilot is able to safely descend (or in more rare cases ascend) to temperatures warmer than 0° C. Knowledge of the freezing level altitude and depth of the above-freezing air is a critical part of icing forecasts; if the freezing layer extends down to the surface, there may be no escape from icing conditions other than flight above or around the cloud or horizontally toward warmer air. These actions are often impossible for smaller aircraft that have limited range, altitude, and ability to handle icing. 132 AFH15-101 5 NOVEMBER 2019 2.3.2.5. Deposition. Deposition is the transition of water vapor to ice. At a given temperature, the vapor pressure over a water surface is greater than that over an ice surface. If water droplets and ice crystals exist in the same environment (called mixed phase conditions), vapor molecules in the air will deposit on an ice crystal rather than condense onto a water droplet; the ice crystals grow at the droplets’ expense. Deposition creates subsaturation conditions, and the droplets evaporate to maintain water saturation, leaving additional water vapor available for ice crystal growth. In mixed-phase clouds, glaciation (the transition of the cloud from supercooled liquid to ice) takes place rapidly; it begins in the highest part of the cloud and moves downward, as ice crystals become larger and heavier and fall through the cloud. In stratiform clouds with tops colder than -15° C, significant icing conditions are not expected because at these colder temperatures, ice nuclei generally are active, forming ice and leading to glaciation of the cloud. The exception is in cumuliform (including stratocumulus) clouds with strong enough updrafts to supply both the liquid droplets and ice crystals with enough condensate for coexistence. 2.3.2.6. Sublimation. Sublimation is the transition of ice to water vapor; this process occurs in a sub-saturated, below-freezing environment where ice particles transition directly to water vapor without melting. 2.3.3. Icing factors. Icing severity and type depends on the properties of the aircraft as well as the atmospheric conditions; forecasters must focus on diagnosing the icing environment. The meteorological quantities most closely related to icing severity and type are detailed below. 2.3.3.1. Liquid water content. Cloud liquid water content (LWC) is extremely important for determining icing potential, but is difficult to quantify. LWC is the density of liquid water in a cloud, expressed either as grams of water per cubic meter (g/m3) or grams per kilogram (g/kg) of air. If the temperature is below freezing, the liquid water content is a measure of how much supercooled liquid water (SLW) is available to accrete on the aircraft. 2.3.3.2. Temperature. Temperature affects both the severity and type of icing. For icing to occur, the outside air and airframe temperatures must be below 0° C. Since supercooled droplets (SLD) need an ice nucleus to freeze, and ice nuclei are strongly temperature dependent, most icing takes place at temperatures between 0° and -20° C. The physical limit to icing is at -40° C, where liquid droplets freeze without the presence of ice nuclei. 2.3.3.3. Droplet size. Droplet size has a significant influence on icing conditions when they are larger than 40 microns; these larger drops persisting in subfreezing temperatures are called supercooled large drops (SLD) and can present a significant icing hazard. SLD includes freezing drizzle (diameters 40 to 200 microns) and freezing rain (diameters greater than 200 microns). Droplet size influences the collection efficiency of drops on the airframe; small droplets have little mass and momentum, so they tend to be swept around the airframe as the airplane passes. As droplet size increases, they will begin to accumulate near the leading edge of the wing, where the air diverges to go around the airfoil. 2.3.3.4. Altitude. Theoretically, there is no altitude limit for icing; icing conditions may be present from the surface to the stratosphere. In practical terms, however, optimal icing conditions are only found in a narrow band of the atmosphere. Studies have shown that 50% of all icing cases occur between 5000 and 13,000 feet, with a peak occurrence near 10,000 feet. AFH15-101 5 NOVEMBER 2019 133 2.3.4. Icing types and amounts. 2.3.4.1. Rime Icing. Rime ice is a milky, opaque, and granular deposit with a rough surface (Figure 2.27); it forms by the instantaneous freezing of small, supercooled water droplets as they strike the aircraft. This instantaneous freezing traps a large amount of air, giving the ice its opaqueness and making it very brittle. It is most frequently encountered in stratiform clouds, but can also occur in cumulus clouds. Rime ice is lightweight, brittle, and fairly easily removed. 2.3.4.2. Clear Icing. Clear ice is a glossy ice, identical to the glaze forming on trees and other objects as freezing rain strikes the Earth; it’s the most serious type of icing because it adheres so firmly to the aircraft. Conditions most favorable for clear ice formation are high water content, large droplet size, and temperatures slightly below freezing. Clear icing is most frequently encountered in cumuliform clouds and during freezing precipitation, and can be smooth or rough (Figure 2.28) It’s smooth when deposited from large, supercooled cloud droplets or raindrops that spread, adhere to the surface of the aircraft and slowly freeze. If mixed with snow, ice pellets or small hail, it is rough, irregular, and whitish; the deposit then becomes very blunt-nosed with rough bulges building out against the airflow. Clear ice is hard, heavy, and tenacious; its removal is difficult. 2.3.4.3. Mixed Icing. Due to small-scale variations in liquid water content, temperature, and droplet sizes, an airplane can encounter both rime and clear icing along its flight path – this results in mixed icing. Ice particles become embedded in clear ice, building a very rough accumulation, sometimes in a mushroom shape on leading wing edges. Figure 2.27. Rime Icing. 134 AFH15-101 5 NOVEMBER 2019 Figure 2.28. Clear Icing – smooth and rough varieties. 2.3.4.4. Icing Type and Temperature. The type of icing is dependent on multiple variables (liquid water content, airframe characteristics, etc.), but temperature can be a good indicator of expected icing type. The general relationship between temperature and icing type is outlined in Table 2.9 Table 2.10. Icing type based on temperature. Temperature of Expected icing type atmospheric layer 0º to -10ºC Clear -10º to -15ºC Mixed -15ºC to -40ºC Rime 2.3.4.5. Icing Amounts. Icing potential is dependent upon aircraft type and design, flight altitude, and airspeed as well as the atmospheric conditions. General classifications for icing amounts are defined in the Federal Aviation Administration’s Flight Information Handbook, and are listed in Table 2.10 AFH15-101 5 NOVEMBER 2019 135 Table 2.11. Icing amount definitions. Icing amount Definition Ice becomes perceptible. The rate of accumulation is slightly greater than the rate of sublimation. It is not hazardous unless encountered for an Trace extended period of time (over one hour, even though de-icing/anti-icing equipment is not used. Light The rate of accumulation may create a problem if flight is prolonged in this environment (over one hour). Occasional use of de-icing/anti-icing equipment removes/prevents accumulation. It does not present a problem if the de-icing/anti-icing equipment is used. Moderate The rate of accumulation is such that even short encounters become potentially hazardous, and use of de-icing/anti-icing equipment or diversion is necessary. Severe The rate of accumulation is such that de-icing/anti-icing equipment fails to reduce or control the hazard. Immediate diversion is necessary. 2.3.5. Icing in Precipitation. Clear icing caused by droplets larger than cloud-size (greater than 40 microns) poses an especially hazardous icing problem; this type is often referred to as supercooled large droplet (SLD) ice. Large droplets tend to form a very lumpy texture, which significantly disrupts airflow and aerodynamics. These drops can flow large distances along the airfoil, impacting the aircraft farther aft than smaller cloud-sized droplets and accreting on surfaces beyond the reach of de-icing equipment. 2.3.5.1. Physical mechanisms for icing development. SLD are formed in two ways: through melting of ice and subsequent supercooling of the drops (warm layer process), or through droplet growth processes within a supercooled environment (collisioncoalescence). In the first case, the presence of the ice phase is needed; in the second case, it is not. In either case, the presence of freezing precipitation at the surface is a good initial indicator of SLD aloft. 2.3.5.1.1. Warm layer process. During a precipitation event, warm intrusions aloft often result in favorable conditions for SLD formation, and the possibility of SLD clear ice accretion increases The precipitation types most often associated with a warm layer process during the cold season are either freezing rain (ZR) or freezing drizzle (ZL). Freezing rain and freezing drizzle can result from snowflakes falling through and melting in a layer of warm air aloft (usually at least 2° to 3° C), then continuing to fall into a layer of subfreezing air below. The warm layer must be deep enough to melt frozen precipitation; if the low-level cold layer is too cold or too deep, the supercooled drops (ZL or ZR) can refreeze to ice pellets. 136 AFH15-101 5 NOVEMBER 2019 2.3.5.1.2. Collision-coalescence. ZR and ZL can also form by collision and coalescence of droplets. This process doesn’t require a preliminary ice phase (as the warm layer process does), and no warm layer is required. Clouds typically have a wide distribution of drop sizes, resulting in a distribution of fall speeds. If the distribution is large enough, some drops will collide with one another and coalesce into larger drops. When the largest drops reach approximately 20 microns in size, the collisioncoalescence process begins; it can rapidly transform cloud-sized droplets into larger drizzle drops (between 200 and 500 microns) or even raindrops (greater than 500 microns). 2.3.5.2. Meteorological considerations for icing. 2.3.5.2.1. Stratiform clouds. Stable air masses often produce stratiform clouds with extensive areas of relatively continuous potential icing conditions; icing intensities in stratiform clouds generally range from light to moderate, with maximum intensity in the cloud‘s upper portions. Both rime and mixed icing are observed in stratiform clouds. High-level stratiform clouds (such as cirrostratus contain mostly ice crystals and produce little icing. Stratiform cloud icing typically occurs in mid- and low- level clouds, in a layer between 3000 and 4000 feet thick, and rarely occurs more than 5000 feet above the freezing level. Multiple layers of stratus clouds may be so close together that flying between layers is impossible. In these cases, maximum depth of continuous icing conditions rarely exceeds 6000 feet. 2.3.5.2.2. Cumuliform clouds. Unstable air masses produce cumuliform clouds with a limited horizontal extent of potential icing conditions. Icing generally occurs in the updraft regions in mature cumulonimbus, but is confined to a shallow layer near the freezing level in a dissipating thunderstorm (see Figure 2.29) Icing intensities range from light in small cumulus to moderate or severe in towering cumulus and cumulonimbus. The most severe icing occurs in cumulus clouds just prior to entering the cumulonimbus stage. Although icing occurs at all levels above the freezing level in building cumulus, it is most intense in the upper half of the cloud. The zone of icing in cumuliform clouds is smaller horizontally but greater vertically than in stratiform clouds. Icing (usually clear or mixed) is more variable in cumuliform clouds, because many of the factors conducive to icing depend largely on the particular stage of the cloud‘s development. AFH15-101 5 NOVEMBER 2019 137 Figure 2.29. Cumuliform cloud icing locations. 2.3.5.2.3. Cirriform clouds. Icing rarely occurs in cirrus clouds, even though some non-convective cirriform clouds do contain a small proportion of water droplets. Moderate icing can occur in the dense cirrus and anvil tops of cumulonimbus, however, where updrafts may contain considerable amounts of supercooled water. 2.3.5.2.4. Frontal systems. For significant icing to occur above a frontal surface, lifted air must cool to temperatures below freezing, and be at or near saturation. If the warm air is unstable, icing may be sporadic; if it is stable, icing may be continuous over an extended area. While precipitation forms in the relatively warm air above the frontal surface at temperatures above freezing, icing generally occurs in regions where cloud temperatures are colder than 0º. Generally, this layer is less than 3,000 feet thick. Icing below a frontal surface most often occurs in freezing rain or drizzle; as precipitation falls into the cold air below the front, it may become supercooled and freeze on impact with aircraft. Freezing drizzle and rain occur with both warm fronts and shallow cold fronts. 2.3.5.2.4.1. Warm frontal icing characteristics (Figure 2.30) Warm frontal icing is usually widespread, and can extend well ahead of the front. Light rime icing occurs in altostratus up to 300 miles ahead of the warm frontal surface position; mixed/clear icing can occur 100-200 miles ahead of the surface position. 138 AFH15-101 5 NOVEMBER 2019 Figure 2.30. Icing with a warm front. 2.3.5.2.4.2. Cold frontal icing characteristics (Figure 2.31) Icing associated with cold fronts is typically not as widespread as it is with warm fronts, since cold fronts generally move faster and have less cloudiness. Clear icing is more prevalent than rime icing in the cumuliform clouds associated with the cold front; light icing usually occurs in the extensive layers of supercooled stratocumulus clouds behind the front, while moderate icing occurs in the supercooled cumuliform clouds up to 100 miles behind the cold front surface position (most likely directly above the frontal zone). Icing conditions in the stratiform clouds of a widespread, slowmoving cold frontal cloud shield are similar to warm frontal icing. Figure 2.31. Icing with a cold front. AFH15-101 5 NOVEMBER 2019 139 2.3.5.2.4.3. Stationary/occluded frontal characteristics. Icing associated with occluded and stationary fronts is similar to that of warm or cold frontal icing. Moderate icing also frequently occurs also with deep, cold, low-pressure areas where frontal systems are indistinct; icingcan be severe in freezing precipitation. 2.3.5.3. Other Icing Conditions. 2.3.5.3.1. Terrain. Icing is more likely (and more severe) in clouds over mountainous regions than over other terrain; mountain ranges cause upward vertical motion on their windward side. Strong upslope flow can lift large water droplets as much as 5,000 feet into sub-freezing layers above a peak, resulting in supercooled water droplets. In addition, when a frontal system moves across a mountain range, the normal frontal lift combines with the mountain‘s upslope effect to create extremely hazardous icing zones. 2.3.5.3.2. Induction icing. In addition to the hazards created by structural icing, an aircraft can also be affected by icing of its power generation structures (i.e., the engine). Ice can develop on air intakes under the same conditions favorable for structural icing; when it occurs, icing is most common in the air induction system, but may also be found in the fuel system. The main effect of induction icing is power loss due to blocking of the air before it enters the engine. On rotary-wing aircraft, a loss of manifold pressure combined with air intake screen icing may force the immediate landing of the aircraft. 2.3.5.3.2.1. Air intake ducts. In flights through clouds containing supercooled water droplets, air intake duct icing is similar to wing icing. The ducts can be susceptible to icing in other conditions, however, even when skies are clear and temperatures are above freezing. During taxiing, takeoff, and climb, reduced pressure forms in the intake system (Figure 2.32), which can lower temperatures to the condensation and/or sublimation point. If condensation/sublimation occurs, ice may form on the intake, which decreases the radius of the duct opening and limits the air intake. Ice formed on these surfaces can later break free, causing potential foreign object damage (FOD) to internal engine components. Figure 2.32. Intake icing. 140 AFH15-101 5 NOVEMBER 2019 2.3.5.3.2.2. Carburetor icing. Carburetor icing can be treacherous, and may lead to complete engine failure (Figure 2.33); it can form under seemingly benign conditions even when structural icing doesn’t occur. Ice in the carburetor may partially or totally block the flow of the air/fuel mixture into the engine, leading to reduced engine performance or even total engine failure in the most severe icing cases. Carburetor icing forms when moist air, drawn into the carburetor, is cooled to a dew point temperature less than 0°C (frost point). Carburetor icing can occur in a wide range of atmospheric conditions, but it’s most likely when relative humidity is high (above 80%) and temperatures are between 21°C (70°F) and -7°C (20°F) (Figure 2.34) Figure 2.33. Carburetor icing. AFH15-101 5 NOVEMBER 2019 141 Figure 2.34. Carburetor icing potential. 2.3.6. Icing Products and Procedures. The products and procedures listed in this section can assist in determining the potential for icing. 2.3.6.1. AIRMETS, SIGMETS, PIREPS, and AIREPS. Use these products to verify icing forecasts, to locate icing areas that impact the forecast area, and to identify synoptic icing conditions. PIREPS and AIREPS are critical data sources, since they originate from aircrews and act as in-situ observations; solicit aircrews aggressively for reports, so other aircrews may benefit from their reporting. 2.3.6.2. Upper air temperature and dew point. Consult upper-air soundings along the flight route for temperatures and dew point spreads at flight level, and refer to Table 2.11 to determine icing potential. In addition, pay close attention to the upper-level flow to identify upstream icing, which may advect into the route of flight by the time the aircraft reaches the area. 142 AFH15-101 5 NOVEMBER 2019 Table 2.12. Icing potential based on temperature and dew point depression. Unfavorable atmospheric conditions for icing Temperature Dew point depression Forecast 0ºC to -7ºC Greater than 2ºC none -8ºC to -15ºC Greater than 3ºC none -16ºC to -22ºC Greater than 4ºC none lower than -22ºC any spread none Favorable atmospheric conditions for icing Dew point Temperature Advection Forecast Probability depressio n Neutral/weak CAA Trace 75% 2°C or 0°C to -7°C less Strong CAA Light 80% -8°C to -15°C 3°C or less 0°C to -7°C 2°C or less -8°C to -15°C 3°C or less Neutral/weak CAA Trace 75% Strong CAA Light 80% None – associated areas with vigorous cumulus buildups due to surface heating Light 90% 2.3.6.3. Upper air composite data. Upward vertical motion in the vicinity of a jet stream maximum, combined with adequate moisture and CAA, produce a high probability of icing; when these features are located in close proximity, icing is likely. Use additional information in this section to determine icing type and intensity. 2.3.6.3.1. Vorticity. Use a 500 mb chart to show areas of positive and negative vorticity advection (PVA/NVA). Overlay the vertical velocity product (OVV) to show vertical motion. 2.3.6.3.2. Wind speed (jet stream). Use 300 and 200 mb charts to highlight locations of jet streams, with emphasis on wind speed maxima and minima. 2.3.6.3.3. Moisture. Analyze the 850, 700, and 500 mb charts for moisture. Sufficient moisture, combined with cold-air advection, indicates icing potential in those regions. 2.3.6.3.4. Thermal advection patterns. Evaluate the 1000-500 mb, 1000-700 mb, or 1000-850 mb thickness products for thermal advection patterns; CAA increases the possibility of icing. 2.3.6.4. Icing from freezing precipitation. Analyze surface isotherms in one color, and overlay 850 mb isotherms in another color. In a third color, overlay 850 mb moisture data (dew point depressions of 2°, 3°, and 4° C). Look for areas on the composite chart with high moisture, surface temperatures below freezing, and 850 mb temperatures above freezing; precipitation in these areas is likely to be freezing rain or freezing drizzle, with icing likely. AFH15-101 5 NOVEMBER 2019 143 2.3.6.5. Vertical cross sections. Vertical cross sections can show the amount of moisture in the atmosphere and the associated temperatures