Private Pilot Meteorology Notes PDF

349 PART 5 METEOROLOGY CHAPTER 25 FUNDAMENTALS OF METEOROLOGY 251 ATMOSPHERE 252 ATMOSPHERIC PRESSURE 253 TEMPERATURE AND HEAT EXCHANGE 254 ATMOSPHERIC MOISTURE  Chapter 25 FUNDAMENTALS OF METEOROLOGY The gas composition and structure of the Earth’s atmosphere...

349 PART 5 METEOROLOGY CHAPTER 25 FUNDAMENTALS OF METEOROLOGY 251 ATMOSPHERE 252 ATMOSPHERIC PRESSURE 253 TEMPERATURE AND HEAT EXCHANGE 254 ATMOSPHERIC MOISTURE  Chapter 25 FUNDAMENTALS OF METEOROLOGY The gas composition and structure of the Earth’s atmosphere has created a unique environment within the galaxy that supports life. The warming effect of the Sun and the presence of water work together to sustain life. The energy provided by the Sun via radiation leads to changes in air temperature, density and pressure that dictates much of the meteorological phenomena we experience. The transformation between the physical states of water within the atmosphere accounts for the types of precipitation. 251 ATMOSPHERE What is the atmosphere? The atmosphere refers to the mixture of gases that envelop the planet and is organised into five distinct layers (Figure 185). The innermost layer is the troposphere and the outermost layer is the exosphere. General aviation usually occurs within the lowest layer of the atmosphere, the troposphere (extends to approximately 35,000ft/10km altitude). However, larger commercial airliners fly outside the troposphere, within the stratosphere, while commercial space ventures attempt to venture into the outer mesosphere. Exosphere Thermosphere Mesosphere Sratosphere Troposhere 10km 30km 50km 400km Figure 185. The layers of the atmosphere. What is the composition of the atmosphere? The mixture of gases within the atmosphere is what we refer to as “air”. The relative composition of gases within the atmosphere is shown in Figure 186, with nitrogen and oxygen making up most of it. 353  METEOROLOGY 78% N₂ O 21% O₂ 1% Other : Argon 0.9% CO₂ 0.03% Misc 0.07% 78% 21% 1% Figure 186. Gas composition of the atmosphere. How does the atmosphere change with an increase in altitude? Moving away from the surface of the Earth and gaining altitude will reduce air density. This reduction continues until reaching the outermost regions of the exosphere, where air density becomes zero in the vacuum of space (i.e., there is no air). Air density refers to the weight of air per cubic metre (kg/m3). Despite air density reducing with altitude, the atmospheric composition remains the same – for instance, the relative percentages of gas particles are constant but the number of particles present decreases. Air temperature also decreases with an increase in altitude within the troposphere. The rate of decrease is approximately 2°C per 1000ft gain in altitude. What is the tropopause? The boundary layer between the troposphere and stratosphere is known as the tropopause. The tropopause is a layer where an increase in altitude no longer causes a decrease in temperature. Because of the abrupt change in temperature in the tropopause, it is common for weather phenomena to occur at this level. The tropopause level exists at different altitudes, depending upon latitude (distance from the equator). The tropopause is situated at a higher altitude over equatorial regions (approximately 50,000ft) than over the poles (approximately 30,000ft). This is due to warmer land temperatures in the tropical areas, resulting in a lower air density above it. As a result, temperature increases along with altitude in the stratosphere until a maximum temperature is reached at the stratopause (the boundary level between the stratosphere and mesosphere). How do gases within the atmosphere support life on Earth? This is obviously a very complicated question! In brief, the gases within the atmosphere contribute to life and the ecological systems on Earth in the following ways: 354  Chapter 25 FUNDAMENTALS OF METEOROLOGY Oxygen – Life is not sustainable outside the atmosphere as there are no gases and thus, no oxygen, which is a vital gas to sustain life. In addition, oxygen is needed for the combustion of fossil fuels (e.g. engine fuel) Carbon dioxide – This forms as a waste product of biological metabolism and fuel combustion. It contributes to global warming as a greenhouse gas (see below). Nitrogen – This is the most common gas within the atmosphere. Nitrogen participates in the life cycle of most ecosystems as a substrate for biochemical use. Water vapour – Water in its gaseous form is present in variable amounts within the troposphere. The amount of water vapour changes depending upon temperature and leads to the various types of precipitation: snow (solid), clouds, rain (liquid), vapour (gas). Water is another essential substrate to facilitate life. 252 ATMOSPHERIC PRESSURE What is atmospheric pressure? Atmospheric/air pressure is the force exerted by moving air particles. Pressure is the force applied (Newtons, N) per unit area (m2) for which the standard international (SI) unit is the Pascal (1Pa = 1N/m2). The most commonly used unit of pressure in meteorology is the hecto Pascal (hPa). 1 hPa is equivalent to 1 millibar (1hPa = 1 mBar). If air is denser (more gas particles in a given volume), air pressure will increase as more particles means they exert more force. What is the International Standard Atmosphere (ISA)? The International Civil Aviation Organisation (ICAO) published the ISA in 1993 which aimed to describe the atmospheric conditions on an “average day” to set the standard for engineering and performance comparisons in aviation. The ISA is defined as: Sea level pressure of 1013.2hPa Air temperature of 15°C Temperature lapse rate of 1.98°C per 1000 ft The ISA is based on the following assumptions: Air density of 1.225 kg/m3 Gravitational acceleration of 9.8 m/s2 Air is dry (i.e., contains no water vapour) It has a constant composition through all atmospheric layers The ISA described is valid for air operations from mean sea level to 36,000ft altitude. At 355  METEOROLOGY 36,000ft, the temperature is regarded as constant at -56.5°C and thus there is no temperature lapse rate (in the tropopause and stratosphere). Therefore, air pressure remains constant above 36,000ft altitude at 226hPa in ISA conditions. How is atmospheric pressure measured? Atmospheric pressure is measured using a barometer (derived from the Greek word Baros, meaning “weight”). There are various types of barometers based on different scientific principles (Figure 187). TYPE EXAMPLE PRINCIPLE Column of liquid (in this case Mercury) is forced up a scaled tube by the pressure exerted by the air/atmospheric pressure. The column stops moving up LIQUID MERCURY BAROMETER the tube when the force pushing the liquid up (air pressure) is equal to force pushing it down, the weight caused by gravity. Atmospheric pressure causes compression of a bellows or capsule that is directly opposed by a spring contained within it. This spring force is constant and thus the contraction/ ANAEROID ALTIMETER expansion of the device is proportional to the atmospheric pressure. This is connected to a calibrated pointer to enable the atmospheric pressure to be read on a dial. The stretching of a wire or movement of a diaphragm causes an alteration in the resistance/transmission of an electrical STRAIN/ DIAPHRAGM AUTOMATED WEATHER SYSTEM current passed through it. The change in resistance can be converted into a pressure reading. Figure 187. Table of the types of barometer, classified by principle of operation with examples. How does atmospheric pressure change with altitude? Atmospheric pressure (air pressure) drops with an rise in altitude. The average pressure lapse rate in the lower troposphere is 30hPa per 1000ft gain in altitude. The lapse rate is higher in higher-temperature areas and lower in lower-temperature areas. The reason behind this is twofold: Reduction in air density with altitude – The gravitational pull on air particles is at its highest near the surface of the Earth. This means that air particles are pulled together and congregate in greater concentrations at lower altitudes. Therefore, as altitude increases, there are fewer particles to exert a force (reduced air density), so air pressure decreases. 356  Chapter 25 FUNDAMENTALS OF METEOROLOGY Reduction in the weight of the column of air above – The force exerted on a surface by air depends upon the weight (force) of the air above it. As altitude increases, this column's height reduces, so air pressure also decreases. How is air pressure used for altimetry? An aircraft altimeter measures air pressure as a proxy to display altitude through the relationship between air pressure and altitude. Useful definitions QNH – Barometric atmospheric pressure (hPa) at a site of observation adjusted to sea level. The letters QNH represent an arbitrary historical code (not an acronym) assigned to this value during the days of Morse code communication. QFE – Barometric atmospheric pressure (hPa) at a site of observation. Similarly, this is an arbitrary historical code name. Altitude – Vertical distance in feet above mean sea level. Height – Vertical distance in feet above the ground. Pressure altitude – The altitude equivalent is air pressure corrected to ISA conditions (ft.). How is an altimeter calibrated for different atmospheric pressures? Figure 188. Photograph with altimeter subscale set to the current QNH of 1013hPa, whilst the aircraft on the ground. The altimeter now reads the airfield elevation of 180ft. 357  METEOROLOGY The pilot dials the subscale setting to calibrate the altimeter and account for changes in atmospheric pressure. Therefore, the subscale is set to 1013hPa in ISA atmosphere conditions. As a result, the altimeter will read zero feet when the aircraft is at sea level. However, it will be uncommon for sea level air pressure to precisely equal 1013hPa. If the subscale remained unchanged, a high-pressure weather system passing through would cause a reduction in the indicated altitude. Likewise, a low-pressure system would cause a corresponding increase. Therefore, the subscale sets the air pressure at mean sea level at a given location. This subscale setting is known as the QNH. If a place is not at sea level, then the QNH remains the air pressure for sea level at that location. This ensures that the altimeter reads altitude (i.e., height above sea level) at all times. With the QNH set, the altimeter will read the airfield elevation (Figure 188) when the aircraft is on the runway. After an aircraft takes off and climbs, air pressure will decrease and be displayed as an increase in altitude (height above sea level), with the dial calibrated as per the pressure lapse rate. Where can the QNH be obtained? The QNH is available through air traffic control and published in meteorological reports for aerodromes and specific areas. An alternative way to set the correct QNH is to set the altimeter itself to read the known elevation of the airfield (from the AIP aerodrome charts) when grounded at the airfield. When the aerodrome elevation reading is correct, the subscale pressure is QNH. What would happen if the QNH was set incorrectly on the altimeter? An incorrect subscale setting will allow the altimeter to show an arbitrary figure, as it would not be calibrated. Therefore, there will be no means to accurately gauge altitude during flight. Setting the QNH accurately is mandatory for reading aircraft altitude correctly. What is the QFE? The QFE is the atmospheric pressure at a site of observation (i.e., not at sea level). It is occasionally referenced but not officially utilised in New Zealand. If the QFE is set on the subscale (Figure 189) when an aircraft is grounded at an airfield, the altimeter will read zero (rather than the airfield elevation when QNH is set). Setting the QFE may be helpful in instances such as when conducting aerobatics over a fixed location where height above ground is critical. However, this datum is rendered useless when moving away from the area as altitude is used to label charts. 358  Chapter 25 FUNDAMENTALS OF METEOROLOGY QFE Height Above Datum QNH Altitude Above Sea Level Sea Level Figure 189. Diagram showing the differences in height and altitude readings when QFE and QNH are used as subscale settings, respectively. How does deviation from the ISA affect aircraft performance? Atmospheric conditions will rarely match the stated ISA atmosphere exactly. Air pressure and air temperature can be easily measured and they are used in aviation to deduce changes in air density. Physical gas laws dictate the relationship between gas pressure, density and temperature. In summary, density is proportional to pressure but inversely proportional to temperature. In other words, density increases with an increase in pressure but decreases with an increase in temperature and vice versa. Therefore, aircraft performance (primarily engine and aerodynamic performance) will vary, as shown in Figure 190. With reduced air pressure comes a reduction in air density with fewer oxygen particles present for combustion and fewer air particles to produce aerodynamic reactions, such as lift. This means that a decrease in air pressure reduces the efficiency and aerodynamic surfaces of an engine. The opposite is true for temperature, where a drop in air temperature results in an increase in the number of air particles available for combustion and aerodynamic reaction, creating greater efficiency. FACTOR INCREASE DECREASE Increase in efficiency of engine, Decrease in efficiency of engine, AIR PRESSURE propeller and wing. propeller and wing. Decrease in efficiency of engine, Increase in efficiency of engine, AIR TEMPERATURE propeller and wing. propeller and wing. Figure 190. Table of environmental factors which affect aircraft performance. 359  METEOROLOGY 253 TEMPERATURE AND HEAT EXCHANGE Useful definitions Conduction – The transfer of heat energy through a solid substance between the molecules it contains. For example, when heating a pan, the metal molecules of the pan conduct heat from one to another, and the whole pan base gets hot. Convection – The transfer of heat energy through a fluid substance (liquid or gas) by the movement of molecules through it, such as boiling water, where water particles transfer heat to other molecules. Radiation – The transfer of heat energy via waves that are transmitted through space, such as solar radiation. No molecules are required to transfer energy via radiation and therefore it can occur through a vacuum. Albedo – Deriving from the Latin term of “whiteness”, this refers to the ratio of radiation reflected from a substance. High albedo ratio substances (white substances) reflect more than low albedo ratio substances (dark substances). What types of radiation are emitted by the Sun? The Sun’s fusion reaction constantly emits solar radiation. This produces radiation of varied wavelengths , including radiation emission in the form of visible light, ultraviolet and infrared wavelength spectrums. Different wavelengths can also be emitted when solar flares occur, such as higher energy gamma and X-rays. What is the greenhouse effect? Solar radiation is emitted from the Sun and travels through space to Earth. Most of this radiation is transmitted through the atmosphere and hits the Earth’s surface. Before reaching Earth, it can be absorbed or reflected by the atmosphere or clouds. Ozone (O3) is contained within specific regions of the stratosphere (ozone layer) and is responsible for most solar radiation absorption. It is particularly crucial for absorbing the harmful spectrum of ultraviolet (UV) radiation before it reaches the Earth’s surface. The solar radiation that does go on to reach the Earth’s surface can either be reflected (determined by the albedo) or absorbed. The absorption of solar radiation causes the surface of the Earth to warm. This thermal energy can then be radiated back into the atmosphere as terrestrial radiation. Terrestrial radiation is emitted as infrared spectrum radiation and behaves differently from solar radiation as it possesses different characteristics (Figure 191). 360  Chapter 25 FUNDAMENTALS OF METEOROLOGY RADIATION FREQUENCY WAVELENGTH SOLAR HIGH SHORT TERRESTRIAL LOW LONG Figure 191. Table showing the characteristics of solar and terrestrial radiation. The longer the wavelength, the more infrared terrestrial radiation is absorbed by certain atmospheric gases. Water vapour, carbon dioxide and various other pollutants (greenhouse gases) are examples and means that the energy does not leave our atmosphere (Figure 192). This absorption causes the air temperature to increase by conduction and convection. This is the greenhouse effect which causes warming of the Earth and its surface, enabling sustenance of life. However, the effect of excessive, additional greenhouse gases added to the atmosphere exaggerates this effect which in turn leads to global warming. Sun Short Wavelength Radiation Long wavelength radiation reflected by atmosphere Atmosphere Figure 192. Diagram showing the change of light wavelength that causes warming of the Earth by the sun’s radiation. 361  METEOROLOGY What factors determine the effect of the Sun’s radiation on surface temperature? There are numerous factors that determine the effect that the Sun’s radiation has on the Earth’s surface, including: Time of day – Solar radiation increases as the height of the Sun increases. With the Sun higher in the sky, the heating effect on the Earth’s surface is more significant. Latitude – The latitude of your location on Earth dictates the intensity of the solar radiation. In the lower latitude regions (nearer the equator), solar radiation has a more concentrated effect and thus heating effect. Conversely, the poles receive far less intense solar radiation and therefore remain cold due to the solar radiation spreading over a larger area. Altitude – Higher altitude areas receive higher solar radiation because the distance the radiation travels through the atmosphere is reduced. Why does air temperature vary during the day? The Earth rotates around its axis every 24 hours, producing the day/night cycle. Therefore, there is a diurnal (over the day) change in air temperature. Air temperature varies depending on whether there is a net gain or net loss of thermal energy. Solar radiation is transmitted to the Earth’s surface during daylight hours and is reflected as terrestrial radiation to heat the air above it. At night, solar radiation no longer heats the land and thus it cools, causing a corresponding reduction in air temperature. The degree and rate of the rise/fall of air temperature depend upon the land surface and the environmental conditions, such as wind and clouds. Land with air pockets, such as sandy deserts, heat rapidly and to a large degree, leading to quickly rising air temperature during the day. However, it also loses heat quickly, hence the considerable diurnal variation in air temperature in desert regions. Cloudy conditions can reduce diurnal variation in air temperature as clouds can both reflect incoming solar radiation and absorb terrestrial radiation. This contrasts with air temperatures over water, which does not exhibit significant diurnal variation. This is because water is continuously moving and contains currents, so a vast body of water needs to be heated to increase the water temperature. Therefore, the air above the water is also slow to change temperature and remains relatively stable throughout the day. 254 ATMOSPHERIC MOISTURE Useful definitions Condensation – The conversion of a vapour or gas to a liquid. Evaporation – The conversion of a substance in liquid form to a gas. 362  Chapter 25 FUNDAMENTALS OF METEOROLOGY Deposition – The conversion of gaseous water vapour directly to solid ice without first becoming a liquid. Sublimation – The conversion of a solid directly to a gas without first becoming a liquid. Melting – The conversion of a solid to a liquid. Freezing – The conversion of a liquid to a solid. Latent heat – The energy required to convert a solid into a liquid (melting) or vapour (sublimation) or a liquid into a vapour (vaporisation) without change of substance temperature. Nuclei (condensation nuclei) – A particle, such as dust, ash or sea salt crystals, upon which water vapour condenses to form water drops or ice crystals. This is a requirement for creating visible moisture, such as fog, rain and clouds. Saturation – The point at which no more of a substance can be absorbed. Dew point – The temperature at which the air becomes saturated with water vapour. Relative humidity – The amount of water vapour in a given volume of air divided by the amount of air in that volume (%). Water vapour – Water that has evaporated and becomes contained within the air in its gaseous form. This is a necessary component of the formation of visible moisture. What are the phases of water transformation? The transition between the phases of water (solid, liquid and gas) occurs based on atmospheric conditions. With each phase change, latent heat/energy is transferred (Figure 193) to facilitate the process. NAME CHANGE IN PHASE LATENT HEAT CONDENSATION Gas to Liquid Released EVAPORATION Liquid to Gas Absorbed DEPOSITION Gas to Solid Released SUBLIMATION Solid to Gas Absorbed MELTING Solid to Liquid Absorbed FREEZING Liquid to Solid Released Figure 193. Table showing the changes in physical state of water. What is relative humidity? Relative humidity is the amount of water vapour present in the air, expressed as a percentage of the amount of water vapour that would be required to saturate the air. Saturation of air with water vapour occurs when there is an equal number of water molecules evaporating from liquid to gas to the number of water molecules condensing from a gas into a liquid. When the air reaches maximal saturation, it has a relative humidity of 100%. The formula to calculate relative humidity is: 363  METEOROLOGY Relative humidity (%) = weight of water vapour present x 100 maximum weight of water vapour that can be held What factors affect relative humidity? Given the calculation of relative humidity above, relative humidity can increase in one of two ways: Increasing the water vapour present within the air – While keeping the maximum saturation amount the same, the amount of water vapour within the air can be increased until the maximum saturation (100% relative humidity) is reached. Water vapour content increases = relative humidity increases Water vapour content decreases = relative humidity decreases Decreasing the amount of maximum water that can be held – While keeping the water content within the air the same, relative humidity will increase if the maximum amount to reach saturation is reduced. The water molecules within a liquid and gas have lower energy in colder temperatures. A reduction in energy means the particles have a reduced ability to escape from the liquid and form a gas. This means that the maximum possible air saturation tis reduced. Therefore, relative humidity is indirectly affected by the air temperature. Air temperature decreases = relative humidity increases Air temperature increases = relative humidity decreases 364 CHAPTER 26 PRESSURE SYSTEMS, AIR MASSES AND FRONTS 261 PRESSURE SYSTEMS 262 AIR MASSES 263 FRONTS  METEOROLOGY Due to the unequal heating effects of solar radiation, different air pressure systems are created. The movement, interaction and evolution of these systems lead to a continually changing environment that manifests in weather phenomena. 261 PRESSURE SYSTEMS What are the types of pressure systems? There are two types of pressure systems that exist and are known as anticyclones (“highs”) and depressions (“lows”). The circulation of winds around a high-pressure, anticyclone system occurs in an anticlockwise direction in the Southern Hemisphere and clockwise in the Northern Hemisphere. A ridge is an extension of an anticyclone. The opposite holds true for a low-pressure depression where the circulation of winds is in a clockwise direction in the Southern Hemisphere and anticlockwise in the Northern Hemisphere (Figure 194). An extension of a depression is known as a trough. An area that lies between an anticyclone and depression, where air pressure is neutral, is called a col. HEMISPHERE SOUTHERN NORTHERN Anticyclone (“High”) Anticlockwise Clockwise Depression (“Low”) Clockwise Anticlockwise Figure 194. Table showing wind directions around high and low pressure systems. What is convergence? Convergence is the tendency of air rotating around a pressure system to move toward the system's centre. Convergence is associated with a low-pressure system at the surface level and is therefore regarded as a convergent system. As air moves towards the centre at the surface, it forces air to rise from the centre, which eventually diverges again (Figure 195). The rising air within a depression cools and causes any water vapour contained within it to condense, forming clouds. Lows are associated with rising air, leading to overcast conditions that produce precipitation when enough moisture has condensed. What is divergence? The opposite process is referred to as divergence. In a divergent system, air tends to move away from the centre of a high-pressure system at surface level. The divergence occurring at the surface level means that convergence occurs at the higher levels (Figure 196). Convergence at a higher level allows air to fall from above to replace the diverging air.. As the 366  CHAPTER 26 PRESSURE SYSTEMS, AIR MASSES AND FRONTS air falls, it warms and thus prevents condensation from occurring. Highs are associated with sinking air and generally lead to clear weather. DIVERGENCE CONVERGENCE L CONVERGENCE H DIVERGENCE Ground Ground Figure 195. Diagram of a convergent air system Figure 196. Diagram of a divergent air system where where air converges at surface level, rises and air diverges at surface level, rises and converges at diverges at higher levels. higher levels. 262 AIR MASSES What is an air mass? An air mass is a body of air that extends horizontally over a large plane and has uniform properties throughout. The uniform, defining properties include moisture content, temperature and temperature lapse rate. What is an air mass source region? Source regions are large areas of defined, uniform areas on the Earth’s surface that dictate the characteristics of air masses. When an air mass is stationary over a large source region, the air mass will develop properties of that region and then becomes defined by this source region. Air masses can move and essentially retain their original properties but they can be altered by the surface they travel over. If an air mass moves slowly, its characteristic tends to change more according to the surface it moves over. 367  METEOROLOGY How are air source regions classified? Air source regions are classified as follows: Latitude of source region – This will largely dictate the temperature of the air mass. The closer to the equator the air mass originates from, the warmer the air mass will tend to be. With an increasing latitude of the source region, air masses can be described as “equatorial”, “tropical”, “polar” or “Arctic/Antarctic”. “equatorial” and “tropical” are classed as warm air masses, while “polar” and “Arctic/Antarctic” are classed as cold air masses. Characteristics of air mass – Humidity of the air mass is one of the defining characteristics. An air mass that originates over water will tend to contain more moisture with higher humidity and is referred to as “maritime”. Conversely, an air mass that originates over land will be drier with lower humidity is described as “continental”. 263 FRONTS What is a front? As air masses move across the Earth, their movement and interaction result in the weather that we experience. Their behaviour is predictable and therefore forms the basis of weather forecasting. Due to the inability of air masses to mix when they come into contact with one another, a distinct boundary between them develops. This boundary is known as a front and is defined by the air mass that is advancing. If a cold air mass is advancing, it is classified as a cold front and if a warm air mass is advancing, it is classified as a warm front. If neither air mass is advancing, it is known as a stationary front. How do fronts interact? When a cold air mass meets a warm air mass, the cold air mass (containing denser air) moves underneath, the less dense, warm air mass. This causes the warm air to be lifted and rise in the air. Again, this can either be by the relative movement of the cold air mass underneath the warm air mass (cold front) or vice versa (warm front). The transition/overlap of air masses can occur over a short distance (usually for cold fronts) or more considerable distances (usually for warm fronts). 368  CHAPTER 26 PRESSURE SYSTEMS, AIR MASSES AND FRONTS What conditions are typically associated with a cold front? Cumulus Cloud Warm Air Rising Cold Air Mass Warm Air Mass Cold Front Figure 197. Diagram showing the cross section of cold front as it moves underneath a warm front. Less dense warm air is forced to rise at a cold front. As cold fronts are steep and they exist over a relatively short distance (100 to 200km), they are associated with a sharp rise of the warm, moist air. The rising air is cooled, causing large cloud formations (cumulus and cumulonimbus, see below), and if there is sufficient moisture and condensation, there can be thunderstorms and hail. As the cold air mass moves over the surface of the Earth, there is a frictional drag force as it moves along the ground. A typical cold front is depicted in Figure 197. Other weather conditions that occur as an “idealised cold front” moves over an area are shown in Figure 198. 369  METEOROLOGY BEFORE AT AFTER AIR PRESSURE Steadily Decreasing Steady Increasing Pressure Colder than prior the TEMPERATURE Steady Sudden Decrease front passing, steady Veering and increasing in Slow decrease in WIND VELOCITY strength as front Abrupt Backing strength, steady approaches direction Cumulus, Clear conditions, Cumulus or Cumulonimbus CLOUD possibly scattered Altostratus and Nimbostratus cumulus/cumulonimbus (depending on severity) Very few light showers if Heavy showers and/ PRECIPITATION Light/patches of rain present or hail Very good outside of VISIBILITY Good Poor scattered rain showers Figure 198. Table showing the idealised conditions associated with a cold front. What conditions are associated with a typical warm front? Stratus Warm Air Rising Cold Air Mass Warm Air Mass Warm Front Figure 199. Diagram showing the cross section of warm front as it moves over a cold front. As a warm front advances, a warm air mass is forced over a cold air mass, causing the warm, moist air to rise (Figure 199). However, as a warm front is generally wider (extending up to 1000 kilometres), its rise is gentle. This leads to multiple layers of low-level cloud (stratus, see below) and if precipitation forms, it will be steady and continuous over the area where the air is rising. At the peak of the warm front, vertically developed clouds can exist that include stratocumulus and cumulonimbus (see below). 370  CHAPTER 26 PRESSURE SYSTEMS, AIR MASSES AND FRONTS Other weather conditions that occur as an “idealised warm front” moves over an area are shown in Figure 200. BEFORE AT AFTER AIR PRESSURE Steadily Decreasing Slow Decrease Steady, slight increase TEMPERATURE Steady, slight decrease Increase Steady Veering, increase in WIND VELOCITY Backing Steady strength Stratus, Nimbostratus. Cirrus, Cirrostratus and Also Cumulus/ CLOUD Low level cloud other high level cloud Cumulonimbus depending on severity Light, increasing in PRECIPITATION strength to heavy Reducing to drizzle Patches of rain/drizzle (steady) Fair outside of rain VISIBILITY Good outside of rain Poor (drizzle) Figure 200. Table showing the idealised conditions associated with a warm front. What is an occluded front? An occluded front occurs when a cold front advances and overtakes a warm front. The result of this is that warm front air will no longer meet the advancing cold front air at surface level. Rather, the warm air is occluded from the centre of the low-pressure system (situated at ground level), generally resulting in a reduction in temperature. The complex and multiple interactions of the fronts that occur during an occlusion are typically associated with strong winds, large cloud formations and heavy precipitation. An occluded front can be classified as either a cold or warm occlusion front, defined by the mechanics that produce the occluded front. Cold occlusion front Before an occlusion front is formed, a cold air mass advances towards a warm air mass, i.e., a cold front. A cold occlusion front is formed when the advancing cold air mass, while pushing underneath the less dense warm mass, encounters another cold air mass that is less cold than itself. The relative densities of the air masses mean that the coldest air mass will be pushed lower to the ground, while the less cold air mass rises and the warm air mass remains above both, becoming occluded from the ground (Figure 201). 371  METEOROLOGY Warm Air Mass Cold Air Mass Less Cold Air Mass Cold Front Warm Front Warm Air Mass Cold Air Mass Less Cold Air Mass Cold Occluded Front Figure 201. Diagram of a cold occlusion. 372  CHAPTER 26 PRESSURE SYSTEMS, AIR MASSES AND FRONTS Warm occlusion front A warm occlusion front occurs when the advancing cold air front that overtakes a warm front is less cold than the other cold front it encounters. In this instance, the advancing cold air will rise above the colder, more dense air it meets (Figure 202). The warm air remains trapped/ occluded between the two cold fronts. Less Cold Warm Air Mass Air Mass Cold Air Mass Warm Air Mass Less Cold Air Mass Cold Air Mass Warm Occluded Front Figure 202. Diagram of a warm occlusion. What are the potential dangers of flying in frontal conditions? Fronts are associated with variable weather conditions as described above. Flight conditions through fronts can be expected to be turbulent, associated with rain and reduced visibility. Wind shear effects can also be significant, adding to poor flying conditions. 373  METEOROLOGY How can these risks be mitigated? Weather forecasts should be monitored closely, actively observing current conditions for any cues that signal an approaching frontal system (e.g. a sudden change in pressure/ temperature). It is better to plan for the aircraft to remain grounded when the front is expected to pass through with this information. If grounded, it would be worthwhile to ensure that the aircraft is appropriately secured to prevent any damage from wind gusts as the front passes. 374 CHAPTER 27 WIND 271 AIR MOVEMENTS 272 JUDGING THE WIND 273 WIND SHEAR 274 SEA AND LAND BREEZES 275 MOUNTAIN WINDS  METEOROLOGY 271 AIR MOVEMENTS What causes air to move? A pressure gradient is produced when a high-pressure system exists next to a low-pressure system. This results in air moving from the area of high pressure to one of low pressure. The air movement is what we know as wind, which can be described in terms of both direction and speed (i.e., velocity). High- and low-pressure systems are depicted on meteorological pressure charts called synoptic charts (Figure 203). The magnitude, size and movements of air pressure systems can be identified. High pressures are illustrated with red “H”, and low pressures with a blue “L”. Isobars are graphical lines on the chart that signify areas of equal pressure (i.e., a pressure gradient of zero). Figure 203. Synoptic Mean Sea Level (“MSL”) chart showing common features seen in these charts. ©Meteorological Service of New Zealand. Used with permission. 376  CHAPTER 27 WIND What determines wind velocity? There are three main factors that determine wind velocity: Pressure gradient force Coriolis force Surface friction/terrain What are the effects of pressure gradient force on wind? The wind velocity generated depends on the pressure gradient – the larger the pressure difference between the high- and low-pressure systems, the higher the wind velocity. The pressure gradient is the primary determinant of wind speed. When the isobars on a synoptic chart are close together, it signifies a significant gradient, resulting in a strong wind. Conversely, light winds will be present when there is little or no pressure gradient. What are the effects of the Coriolis force on wind? A pure pressure-gradient force would cause air to travel perpendicular to the isobars seen on a synoptic chart, i.e., directly down the gradient from high to low pressure. However, air does not tend to move in a straight line from high- to low-pressure systems but instead rotates around them. This is due to the rotation of the Earth on its axis and is known as the Coriolis effect. The size and strength of the Coriolis force are dependent upon: Latitude – Lower latitudes (closer to the equator) experience a weaker Coriolis force. Speed of the object - As wind velocity increases, so does the strength of the Coriolis force. Size of the object – The Coriolis force affects all terrestrial objects and the strength of the force is proportional to the size of the object. The direction of wind rotation depends on the pressure system type and whether it is in the Northern or Southern Hemisphere. The Coriolis effect acts to turn the wind to the left (anticlockwise) in the Southern Hemisphere and turn to the right (clockwise) in the Northern Hemisphere (Figure 204). Air will move away from the centre of a high-pressure system toward an area of lower pressure. In doing so, the air is deflected to the left/anticlockwise direction in the Southern Hemisphere by the Coriolis force. This causes the anticyclone effect of a high-pressure system to be an anticlockwise, outward rotation of the wind around a high-pressure system. This means that wind tends to be light in the centre of a high-pressure system, but faster at the system's outer regions than the pressure gradient/isobars would suggest (Figure 205). 377  METEOROLOGY NORTH Earth’s Rotation Deflection Right WEST EAST Deflection Left SOUTH Figure 204. Diagram showing the deflection of wind due to Coriolis force in the Northern and Southern hemispheres. H Figure 205. Diagram showing winds moving around a high-pressure system. 378  CHAPTER 27 WIND This is in contrast with a low-pressure system where the air moves towards the centre (away from a high-pressure system). As the wind travels towards the centre, the Coriolis force will cause it to be deflected to the left (in the Southern Hemisphere). Therefore, this deflection relative to the low-pressure system creates a deflection to the right. Therefore, the overall effect of a low-pressure system is that winds travel in a clockwise direction towards the centre (Figure 206). Winds tend to be lighter in the outer regions of the system and become faster towards the centre of the low-pressure system. A hurricane is formed by a powerful low-pressure system, where the fastest winds are experienced in the “eye of the storm”. L Figure 206. Diagram showing winds moving around a low-pressure system. The resultant wind for large systems moves either anticlockwise around the high-pressure system or clockwise around the low-pressure systems. As there is a balance of the pressure gradient force versus the Coriolis effect force, the wind will travel in parallel to the isobars. This is known as a geostrophic wind (Figure 207). 379  METEOROLOGY Geostrophic Wind H L Pressure Coriolis Force Gradient Force Figure 207. Diagram showing the balance of the Coriolis force and the pressure gradient force to resulting in a geostrophic wind. What terms are used to describe a wind? Veering – A wind that turns in a clockwise direction. Backing – A wind that turns in an anticlockwise direction. Gust – A brief, intense rush of wind. Lull – The lowest strength of the wind during a particular time period. Squall – A sudden, violent gust of wind. Backing is going back in time – turning in an anticlockwise direction. What is the friction layer? The friction layer refers to the effect of the Earth’s surface on wind. Friction between air and the Earth’s surface causes wind to slow down and lose velocity. Smoother surfaces, such as water, create less friction and as a result, the friction layer is not as thick. This enables faster- moving wind. On the other hand, rough terrain creates more friction, so a thicker friction layer is formed. The magnitude of the effect of the friction layer is also determined by the air density. Denser air will be more viscous, slowing the air to a greater extent than less dense 380  CHAPTER 27 WIND air. The friction layer usually exists up to approximately 2000ft above ground level. What is the effect of the friction layer? Due to the reduced velocity of wind within the friction layer, there is a reduction in the strength of the Coriolis effect, as this effect is proportional to wind speed. Therefore, areas that have a significant friction layer results in wind travelling perpendicular to the isobars, i.e., directly from a region of high pressure to one of low pressure (without deviation by the Coriolis effect). In aviation, as you take off, you will initially climb through the friction layer. The wind can change direction and velocity when the friction layer boundary is reached. The aircraft will move from an area affected by the friction layer (with a slower wind that is less affected by the Coriolis effect) to an increased wind speed area with a more significant Coriolis effect. The net result (in the Southern Hemisphere) is that wind tends to increase in speed and back as you exit the friction layer. The reverse occurs on descending, where the wind tends to decrease in speed and veer as you enter the friction layer. How does the friction layer affect surface wind during the day and night? The change in the surface wind over the course of a day is part of a diurnal variation. As the Sun goes down and night follows, the surface wind over land (in the Southern Hemisphere) will veer. During the cooler night, the air becomes denser, leading to a more pronounced effect of the friction layer. Increased friction reduces the wind speed and reduces the left- turning/anticlockwise rotation caused by the Coriolis effect. Therefore, relative to the wind in the day, there is a right-turning/clockwise rotation where the wind is said to veer (Figure 208). Day Veer H Night L Figure 208. Diagram of wind veer from day to night. As the Sun rises the next day, the wind will begin to increase again due to a reversal of the process. During the warmer day, the air reduces in density and so the friction effect at the Earth’s surface also reduces. This leads to an increase in wind speed and, accordingly, a more 381  METEOROLOGY pronounced Coriolis effect. Therefore, there is an increase in the left-turning/anticlockwise deviation and, relative to the wind at night, the wind is said to back (Figure 209). Day Back H Night L Figure 209. Diagram of wind backing from night to day. To summarise, in the Southern Hemisphere, wind patterns are as follows: From day to night: The wind veers and speed decreases. From night to day: The wind backs and speed increases. This effect can also occur over the sea but it will be far less pronounced because the relatively smooth sea surface has a less pronounced friction layer and thus there is no significant change from day to night. 272 JUDGING THE WIND How can wind speed be determined? Wind speed can be determined by the angle a windsock makes with the pole - the angle increases as wind speed increases. For a standard-sized windsock, when the windsock angle is 0-degrees, the winds are 2 knots or less. A 15-degree angle indicates a wind speed of 5 knots. A 45-degree angle indicates a 10-knot wind, while a 60-degree angle indicates 15-knot winds. Fully extended (90-degrees), a windsock indicates at least a 25-knot wind. Windsocks can, however, sometimes be misleading. For example, they could be stuck in a position, damaged or be faulty. The position of the windsock can also make it susceptible to turbulence not affecting the runway. Also, different sized windsocks will have different wind angles than that standard-sized wind socks. 382  CHAPTER 27 WIND How can wind direction be assessed? A windsock can provide a good gauge of wind direction at an aerodrome. Approximate wind direction can also be determined using a variety of references during flight: Water – Looking at ripples on water, the concave side is generally on the upwind side (where the wind is coming from), but this can be difficult to determine. Wind lanes are long lines aligned with the wind, which form in strong winds, where the wind is funnelled over a body of water. Lastly, wind shadows are sheltered areas of water adjacent to areas of disturbed water. Wind shadows are caused by local terrain and the obstructed areas used to judge the wind direction. Smoke – Looking at the direction of smoke is a reliable method of determining wind direction. Aircraft ground speed and drift – A valuable method of how the aircraft behaves in terms of ground speed and drift. Global Positioning Systems (GPS) – Most modern systems provide real-time information of the wind experienced by the aircraft. What is Buys Ballot’s Law? Buys Ballot’s law is a rule of thumb to deduce where a low-pressure and high-pressure system are relative to your position w flight. In the Southern Hemisphere, the law states that when you stand with your back to the wind, the low-pressure system is to your right while the high-pressure system is to your left. In the Northern Hemisphere, it is reversed with the low-pressure system to your left and high to your right. This information has practical application. For example, if you are flying in New Zealand with the wind from your tail, turning to the right will turn your aircraft into a low-pressure system. This means you should be wary that the altimeter QNH setting may become incorrect and set too high. Furthermore, the likely change in meteorological conditions can be considered. 273 WIND SHEAR What is wind shear? Wind shear is a variation in wind velocity (speed or direction) over a short distance that results in zones of different wind velocities. These zones can be either on top of each other, resulting in vertical wind shear, or next to each other, resulting in horizontal wind shear (Figure 210). 383  METEOROLOGY Vertical Wind Shear Vertical Wind Shear Shear Zone Shear Zone Shear Zone Horizontal Wind Shear Shear Zone Horizontal Wind Shear Horizontal Wind Shear Horizontal Wind Shear Shear Zone Shear Zone Figure 210. Diagram to show horizontal and vertical wind shear. Vertical wind shear can cause you to transit from a high wind speed zone to a zone of low wind speed that could be in the opposite direction. This can be problematic, Shear Zone particularly on final descent when wind is required to maintain an indicated airspeed andZone Shear lift. Horizontal wind shear will have similar effects when transiting from one shear zone into another, where wind shear can be very significant and pose a danger to the aircraft. Pilots should be aware of areas where wind shear is common and warn passengers of likely turbulence. 274 SEA AND LAND BREEZES How does a sea breeze form? A sea breeze results from the Sun warming land at a faster rate than the sea. This creates a layer of warmer air over the ground than that over the sea. The warmer air rises and creates an area of low pressure immediately above the land. The area of low pressure draws in the cooler (relatively higher pressure) air from the sea, observed on land as an onshore sea breeze (Figure 211). This air will then form a return airflow at a higher level, moving back out to sea. 384  CHAPTER 27 WIND In regions situated closer to the equator, this effect is exaggerated due to the higher warming effect on the land leading to a rise of the air over land. Warm Air Rises Cooler Air Sinks Sea Breeze Warmer Land Cooler Sea Figure 211. The sea breeze process. What are the characteristics of a sea breeze? A sea breeze occurs during the day at a typical speed of 10 to 15 knots. Depending on the geographic location, a sea breeze may commence mid-morning (as the land begins to warm up), peaking mid- to late afternoon (when the land is the hottest) and dying off in the late afternoon/early evening. A sea breeze can extend inland of the coast approximately 10 to 20 nautical miles. What is sea breeze turbulence? The turbulence that results from both the horizontal and vertical movement of air is associated with the sea breeze that causes horizontal and vertical wind shear. This is due to the production of a sea breeze front where the cooler onshore air meets the warmer air over the land. Local coastal terrain is also particularly important in determining how air moves and the local effects caused by the sea breeze. Flight conditions are generally significantly smoother over sea than land, as there is a primarily horizontal shear effect until the land is reached, where the vertical component develops. 385  METEOROLOGY What is a land breeze? The land breeze process is the reverse of that of the sea breeze. In the evening, land loses heat more quickly than the sea. Therefore, there is relatively warmer, less dense air over the sea than cooler, more dense air over land. This causes the air to move offshore from land to sea (Figure 212). Land versus sea temperature differential varies according to the season, and thus these effects in turn have seasonal variation. Cooler Air Sinks Warm Air Rises Land Breeze Cooler Land Warmer Water Figure 212. The land breeze process. What are the characteristics of a land breeze? The land breeze is a light wind, usually only 3 to 4 knots. A land breeze requires the sea to be warmer than the land. In New Zealand, expect a land breeze to be most prevalent during the autumn months – when the land cools dramatically during a brisk autumn evening, while the sea still retains much of the warmth developed throughout the preceding summer months. 386  CHAPTER 27 WIND 275 MOUNTAIN WINDS What are valley and mountain breezes? Valley and mountain breezes are characteristic winds that occur in mountainous regions during the day and night, respectively. They share a similar process to sea and land breezes in coastal areas. During the day, the air over the upper regions of a mountain is heated more effectively than air at the same altitude over a valley. This creates lower-density air nearer the mountain, creating a horizontal pressure gradient. As this effect is more pronounced at higher mountain levels, air will flow towards and up the mountain. This is known as the valley breeze (from the valley to the mountain) and there will be return airflow from the mountain top. The reverse process occurs for a mountain breeze, which happens at night. Air higher up the mountain cools faster than air lower in the valley, resulting in a breeze that flows down the mountain. What are anabatic and katabatic winds? Anabatic and katabatic winds are exaggerated effects of valley and mountain breezes, respectively referring to the rising (anabatic) and falling (katabatic) of air. These types of winds are differentiated from breezes as they generally cause higher wind speed and can affect more extensive areas. The process that leads to the development of these winds is a combination of the gravitational effect on dense air and the mountain/valley breeze formation process. How is a katabatic wind produced? Katabatic winds (falling or flowing down a mountain) are caused by the enhanced cooling of air through direct contact with the surface of a mountain. This occurs most significantly near the top of the mountain, where terrain cools faster than lower down the mountain. This effect occurs most significantly during clear evenings and on snow-covered mountains, optimising conditions for radiation cooling. The air becomes denser and heavier through direct cooling of the air and thus heavier. Gravity drives this heavier air down the mountain and produces katabatic wind. What are the characteristics of katabatic winds? Katabatic winds are usually cold and dry. The wind speed depends on the steepness of the mountain and the temperature of the region. In extreme regions, such as Antarctic glaciers, with large, steep land formations and freezing temperatures, katabatic winds can reach gale force speeds. 387  METEOROLOGY How is an anabatic wind produced? Anabatic winds rise or flow up a mountain. This wind is produced following sunrise when the Sun's radiation has a more significant warming effect at a higher altitude than lower down the mountain. The effect of this is that it causes the air above the upper mountain terrain to be warmed more than air over the lower mountain terrain. As warmer air is less dense than cooler air, it flows up the mountain creating an uphill, anabatic wind. What are the characteristics of an anabatic wind? Anabatic winds are almost always lighter than katabatic wind, as the rising air must compete with gravity to gain altitude. 388 CHAPTER 28 CLOUDS AND PRECIPITATION 281 AIR STABILITY 282 CLOUD SCIENCE 283 CLOUD TYPES AND CHARACTERISTICS 284 PRECIPITATION 285 CLOUD REPORTING  METEOROLOGY 281 AIR STABILITY What are air stability and air buoyancy? Air stability determines the vertical movement of air. To conceptualise air stability, consider a bubble or parcel of air within the atmosphere. An air parcel is regarded as stable if it remains in one location or returns to a location after being disturbed. This is the opposite of unstable air, which does not tend to stay in one location and, if disturbed, continues to travel. What are the determinants of air stability? An air parcel's stability depends upon the relationship between the air pressure/density within a parcel relative to outside a parcel and how this relationship changes with altitude. Air pressure/density within a parcel or the surrounding environment is proportional to the temperature of the air. Warming of an air parcel will reduce its air pressure/density. Therefore, a warm parcel of air (lower air pressure) can be described as being positively buoyant and it will rise relative to the surrounding air. The change in temperature with an increase in altitude is referred to as a lapse rate. The surrounding environment changes according to the environmental lapse rates, while the air parcel temperatures are determined by adiabatic lapse rates (see below). What is the environmental lapse rate (ELR)? The environmental lapse rate (ELR) is the actual (which means it can be observed) decrease in air temperature with increasing altitudes. The environmental lapse rate determines the temperature of the air surrounding air parcels. In the ISA atmosphere, the environmental lapse rate is 1.98°C per 1000ft on an “average day”. However, the actual ELR for a specific region and time varies according to meteorological conditions. Figure 213 shows both a shallow and a steep ELR. A steep ELR means that the air temperature falls more quickly with altitude than ISA conditions (higher ELR) and vice versa for a shallow ELR (lower ELR). If an isothermal layer is present (Figure 214), the air temperature will remain the same despite a change in altitude (the ELR is zero). Likewise, if an inversion layer is present, the temperature will increase with altitude (a positive ELR). These features can be identified in ELR graphs. 390  CHAPTER 28 CLOUDS AND PRECIPITATION Shallow ELR Steep ELR (Warmer) ISA 10,000 (Cooler) 9000 8000 7000 6000 Altitude (Feet) 5000 4000 3000 2000 1000 Sea Level +15 ºC 0 ºC -15 ºC -30 ºC Reverse Temperature ( ºC ) Figure 213. Graph showing temperature versus altitude in ISA conditions, compared to a shallow and steep environmental lapse rate (ELR). Note, that the shallow ELR line appears steeper than steep ELR line because of the way it is plotted – a shallow ELR means that rate of fall of temperature is less than a steep ELR. 10,000 9000 8000 7000 6000 Altitude (Feet) 5000 4000 Isothermal Thermal 3000 Layer 2000 1000 Sea Level +30 ºC +15 ºC 0 ºC -15 ºC Temperature ( ºC ) Figure 214. Graph of temperature versus altitude showing an isothermal layer. 391  METEOROLOGY What is an adiabatic process? If a parcel of air expands, it will cool, whereas if it is compressed, it will warm. This is known as the dry adiabatic process. The word “adiabatic” refers to a physical process in thermodynamics that can occur in isolation from its surroundings (i.e., no external energy source is required). In the case of air parcels, this refers to secondary temperature changes that occur purely in response to changes in pressure. An air parcel that increases in altitude will always reduce in pressure and will therefore undergo adiabatic cooling. Conversely, an air parcel that is compressed will heat up. Since air pressure decreases with altitude, an air parcel that rises will expand and cool, while an air parcel that falls will compress and warm (Figure 215). 700 -5 ºC Decreasing 5 ºC Air Pressure (hPa) 15 ºC 1031 Sea 5,000 10,000 Level Altitude (Feet) Figure 215. Diagram showing gas expansion with altitude. What is an adiabatic lapse rate? The adiabatic lapse rate is the rate of change of temperature of an air parcel due to the adiabatic cooling process (secondary to an increase in altitude). There are two adiabatic lapse rates used: the dry adiabatic lapse rate (when no water vapour is present) and the saturated adiabatic lapse rate (when water vapour is present). Dry adiabatic lapse rates (DALR) The dry adiabatic lapse rate (DALR) refers to the rate at which a dry air parcel cools as it gains altitude. It describes temperature changes in response to changes in pressure and gas volumes when no water vapour is present. As described, when a parcel of air gains altitude, it expands, which causes it to cool. The DALR is on average 3°C per 1000ft. This is a constant 392  CHAPTER 28 CLOUDS AND PRECIPITATION and does not change with environmental conditions as it results from the purely physical adiabatic process. Saturated adiabatic lapse rate (SALR) The adiabatic effects change when water vapour is present within the air parcels. This is due to latent heat being released during the condensation of the water vapour. The latent heat released is absorbed by the air parcel, causing it to warm. Therefore, the adiabatic lapse rate for saturated air is less than for dry air, as the fall in temperature with altitude is partly offset by the absorption of latent heat. On average, the saturated adiabatic lapse rate is about 1.5°C per 1000ft. However, this is very much an average and the rate will be lower at high temperatures and a higher rate at low temperatures. How are the environmental lapse rate (ELR) and dry adiabatic lapse rate (DALR) related? The relationship between the ELR and DALR determines the stability of an air parcel and dictates cloud formation. When the ELR (which changes with conditions) is less than the DALR (which is relatively fixed), air will tend to be stable. This is because the air within a parcel will cool more with an increase in altitude than the surrounding air and thus will become relatively denser. Denser air will tend to fall and move towards its original position, making it stable (Figure 216). Air Parcel Cools More 10,000 -5 ºC -15 ºC Than Surrounding Air = More Dense Air Parcel Falls = STABLE AIR ELR 5 ºC 2 ºC/ 1,000ft Altitude (Feet) DLAR 3 ºC/ 1,000 ft 15 ºC Sea Level +15 ºC 0 ºC -15 ºC Temperature ( ºC ) Figure 216. Movement of an air parcel that defines stable air. 393  METEOROLOGY As with the DALR, when the ELR is lower than the SALR, air will be stable and when the SALR is greater than the ELR, air will be unstable. When air parcels saturated with water vapour are sufficiently cooled, the water will reach its condensation point. At this temperature/ altitude, cloud formation will begin. Clouds form at a lower level when there is a higher water vapour content of the air parcel and cooler surface temperatures. Likewise, higher surface temperatures and lower water vapour content lead to a higher cloud base. When the ELR is lower than the DALR, the opposite is true. As an air parcel rises, it will cool less than the surrounding air leading to less dense air within the air parcel compared to the surrounding air. This means that the parcel will continue to rise and is thus regarded as unstable (Figure 217). Air Parcel Rises = UNSTABLE AIR 10,000 Air Parcel Cools Less Than -15 ºC -25 ºC Surrounding Air = Less Dense. DLAR ELR 3 ºC/ 1,000 ft Altitude -4 ºC/ 1,000ft (Feet) 15 ºC Sea Level +15 ºC 0 ºC -15 ºC Temperature ( ºC ) Figure 217. Movement of an air parcel that defines unstable air. How are the ELR, SALR and DALR related? As the average value of the SALR (around 1.5°C) is close to the average value of the ELR (1.98°C for ISA conditions), only a small variation in the ELR is needed for it to become higher or lower than the SALR. This means the change from stable to unstable air movement occurs more readily in moist conditions. This is in contrast to the DALR (around 3°C), where a more considerable variation of the ELR is needed to exceed the DALR. So dry air has a higher tendency to be stable. A parcel of air can also initially be stable when dry but become unstable if it becomes saturated with water vapour. 394  CHAPTER 28 CLOUDS AND PRECIPITATION Air stability dictates flying conditions due to cloud formation, visibility, precipitation and turbulence. 282 CLOUD SCIENCE How does water enter the atmosphere? Water can enter the atmosphere as a vapour by either evaporation or transpiration. What is evaporation? When water is heated, it changes state from liquid to gas in the form of water vapour through a process called evaporation. This occurs when water particles have enough kinetic energy to move as gas particles. The rate at which evaporation occurs depends upon the factors shown in Figure 218. FACTOR EFFECT With an increase in temperature, the molecules of water move faster, allowing AIR TEMPERATURE more to escape into the air in the form of water vapor. In addition, warmer air is able to hold more water vapor. MOISTURE Air with a lower relative humidity, a lower moisture content, has a higher CONTENT OF THE AIR capacity to hold more moisture than air with a higher relative humidity. Evaporation takes place faster in air with a lower relative humidity. ATMOSPHERIC A lower atmospheric pressure makes it easier for water particles to escape PRESSURE into the air, so evaporation takes place faster. A wind over the water surface prevents the relative humidity of the air directly THE WIND above the surface of the water from rising. Therefore, wind increases the rate of evaporation. Figure 218. Factors which affect the rate of evaporation of water. What is transpiration? Transpiration refers to the particular process of water evaporating from plants. Plants take up water for cellular function eliminate the excess from their cells on the undersides of leaves, which then evaporates as water vapour into the atmosphere. How are clouds formed? The three elements that are required for cloud formation are: Moisture Cooling to the dew point (resulting in 100% saturation) Condensation nuclei 395  METEOROLOGY Clouds are formed when moist air is lifted, causing it to cool, eventually below its dew point. The dew point is the temperature that the air needs to be cooled to become saturated (relative humidity is 100%). Air with a high moisture content (humid air) has a higher dew point, i.e., the temperature decrease required to reach dew point is less. Therefore, humid air becomes more likely to saturate, condense and form clouds. Once moist air is cooled past its dew point, condensation nuclei provide a surface for water vapour to condense. These are tiny particles within the air, such as salt crystals, pollution and dust, that are invariably present in some form. What is the significance of a dew point? Dew point can be used as a proxy for relative humidity and help predict cloud formation. When the difference between the air temperature and dew point is small, relative humidity is high and condensation is likely. This generally leads to reduced visibility and a lower cloud base. When there is a larger discrepancy between air temperature and dew point, a high cloud base is expected as more air cooling is required. If both the dew point and air temperature are high and the dew point is reached, significant cloud formation will result from the large amount of water vapour required to saturate warm air. Increase In Water Vapour Content = Dew Point Increases Decrease In Water Vapour Content = Dew Point Decreases How can air be lifted to form clouds? When air is lifted, it causes air instability, cooling and cloud development. Methods of air lifting include: Convective lifting – Convection is the cycling of air in the atmosphere due to heat provided by the Sun. Solar radiation of the Earth’s surface heats the air close to the surface, causing it to rise as unstable air parcels. Higher surface temperature causes more lifting of air. Orographic lifting – When wind forces an air mass upward against sloping terrain (such as hills or mountains), also known as mechanical lifting. Frontal lifting – Occurs as a cold or occluded front which pushes warm air above it. This is generally characterised by a relatively fast frontal movement that produces more energetic lifting and substantial air instability. Slow widespread ascent – The slow widespread ascent of air over a wide geographic area that is generally associated with the convergence of air in a low-pressure area. How can cloud base be predicted on the windward side of a mountain using lapse rates? As mentioned, the DALR of air is approximately 3°C / 1000ft. Air will, therefore, decrease by 3°C per 1000ft gain in altitude, until it reaches the dew point when it becomes saturated with 396  CHAPTER 28 CLOUDS AND PRECIPITATION water, which condenses to form a cloud. Therefore, with the outside air temperature and dew point information from meteorological reports, the cloud base on the windward side can be predicted. Example An air mass is forced to rise due to orographic lifting approaching a large mountain range. What is the likely cloud base when the sea level temperature is 15°C and the dew point is 9°C? Difference between outside air temperature and dew point = 15°C - 9°C = 6°C Therefore, a fall in temperature of 6°C will result in condensation (cloud formation). Altitude increase required for 6°C fall in temperature = 6°C /DALR = 6°C /3°C per 1000ft = 2000ft How can cloud base be predicted on the leeward side of a mountain? Air reaches saturation point on its rise on the windward side of a mountain when the air temperature is equivalent to the dew point. Cloud formation begins to occur from this point. Air is forced to continue to rise until it reaches the top of the mountain. The cooling rate is now at the slower SALR of approximately 1.5°C/1000 feet rather than the DALR because the air is saturated. On the leeward side of the mountain, the saturated air is forced down and warms as per the SALR until the air temperature rises higher than the dew point. At this level, there will be no condensation, and the cloud will cease to exist (the leeward cloud base). The air further down the leeward side of the mountain will warm as per the DALR. Example From the previous example, assume the mountain range is 10,000ftin height. The cloud base on the windward side was calculated at 2,000ft (sea level temperature remains at 15°C, dew point remains at 9°C). The dew point on the leeward side is 3°C. The cloud base on the windward side was calculated at 2000ft (as above), where the air temperature equalled the dew point of 9°C. Therefore, a further gain of 8000ft is required to reach the mountain summit. The air temperature continues to fall on the windward side from the cloud base to mountain summit. The rate of fall now is dictated by the SALR. Therefore, the air temperature at the summit: = temperature at windward cloud base – temperature decrease due to SALR to summit = 9°C at 2000ft cloud base – 12°C (8 x 1.5°C/1000ft) = -3°C. The leeward side dew point is given as 3°C. Therefore, the air temperature difference between the mountain summit temperature and dew point on the leeward side: 397  METEOROLOGY = -3°C – (+ 3°C) = -6°C Therefore, the altitude decrease required to obtain a 6°C temperature rise is: = Temperature rise required of 6°C/SALR = 6°C / 1.5°C per 1000ft = 4 x 1000ft = 4000ft Therefore, the air on the leeward side will reach the dew point that is 4000ft below the summit of the mountain range, which will therefore be at the height of 6000ft. The cloud base on the leeward side of the mountain range will be at 6000ft. (Figure 219). In reality, the dew point is likely to reduce when the moisture has been removed from the air, and therefore the required cooling effect on the leeward side is even less, raising the leeward cloud base higher up the mountain. -3 °C at 10,000 Feet SALR -1.5 °C/1000ft SALR -1.5 °C /10,000 Feet 9 °C at 6,000 Feet 9 °C at 2,000 Feet DALR -3 °C /10,000 Feet Sea Level Sea Level Figure 219. Cloud base on the windward and leeward side of a mountain range. How can air temperature on a mountain range be predicted? The temperature for any mountain height can be predicted using the above method. Following this, cloud bases can be calculated with a given sea-level temperature and dew points on the windward and leeward sides of the mountain range. Next, the SALR or DALR may be used appropriately, depending on where on the mountain is situated for which the temperature is required to be calculated. 398  CHAPTER 28 CLOUDS AND PRECIPITATION 283 CLOUD TYPES AND CHARACTERISTICS What are the cloud naming conventions? The following terms are used in combination to describe the various cloud types: Nimbo – Rain is falling from the cloud Strato – Low-level clouds below 6,500ft Alto – Mid-level clouds between 6,500ft and 20,000ft Cirro – High-level clouds above 20,000ft Cumulus – Vertically developed clouds Stratus – Flat clouds that lack vertical development New Zealand Cloud Types Cirrus Ci Cirrostratus Cs Cirrocumulus Cc HIGH CLOUDS MIDDLE CLOUDS LOW CLOUDS Base usually above 6,000m Base usually between 2,000m Base usually below 2,000m (20,000ft) over New Zealand (6,500ft) and 6,000m (6,500ft) over New Zealand (20,000ft) over New Zealand, Cirrus (Ci) Stratus (St) but Ns may lower to near the hair-like or streaky ice cloud layer cloud Cirrostratus (Cs) Earth’s surface Cumulus (Cu) layer of ice cloud Altocumulus (Ac) heaped cloud White, fibrous-looking cloud made of ice crystals. This cloud Whitish veil-like high cloud made of ice crystals. It is usually Whitish high cloud made of ice crystals and composed of is often the first sign of an approaching front. Cirrus streaks translucent and has a smooth appearance. The sun, when small billow-like cloud elements. This cloud type is not often Cirrocumulus (Cc) billowy cloud at middle levels Cumulonimbus (Cb) are sometimes known as mares’ tails. viewed through Cs, is often seen to be surrounded by a observed. billowy ice cloud Altostratus (As) tall and rainy heaped cloud rainbow-like ring called a solar halo. This cloud often invades layer cloud at middle levels Stratocumulus (Sc) the sky well ahead of a frontal system and may thicken to As Nimbostratus (Ns) flattened heaped cloud as the front approaches. rainy layer cloud Photo: Unknown. Location: Unknown Photo: Peter Kreft. Location: Wellington Photo: John Crouch. Location: Hutt Valley Altocumulus Ac Altocumulus Lenticularis Ac Northwest Arch Ac/As/Cs Altostratus As Nimbostratus Ns A grey or whitish middle-level cloud that generally has some This middle-level wave cloud often forms when a layer of air This middle and high cloud often forms east of New A greyish or blueish middle-level cloud sheet. It usually Dark grey middle-level cloud usually associated with a shading and texture. Ac may follow Cs during the approach is lifted over hills or mountains in stable conditions. Ac lentic Zealand’s main mountain ranges as a result of an increasing develops from gradually thickening Cs, and it may thicken frontal system. The cloud base can be hard to see because of a front. can occur as single lens-shaped clouds or as many lens- northwest flow ahead of a frontal system. At first single Ac further and lower to Ns. Unlike Cs