Aviation Weather Hazards

Turbulence Overview

• Turbulence consists of irregular air motions resulting from eddies or vertical currents.
• Turbulence poses a potential threat to aviation safety.
• The severity of turbulence is relative to the aircraft, where light turbulence for a Boeing 747 may be severe for a Cessna 172.

Causes of Turbulence

• Turbulence has four primary causes: convective currents, mechanical disturbances, wind shear, and wake turbulence.

Convective (or Thermal) Currents

• Disturbed airflow from vertically moving air in convective currents defines convective (or thermal) turbulence

Mechanical Turbulence

• Mechanical turbulence refers to disturbed airflow from wind moving over obstructions like irregular terrain.

Wind Shear

• Wind shear involves a rapid change in wind speed or direction across a short distance.

Wake Turbulence

• Wake turbulence is the disturbed air behind an aircraft, arising from various factors, including wingtip vortices.
• Multiple causes of turbulence can occur simultaneously.

Stable and Unstable Air

• The atmosphere can be either stable or unstable.
• Stable air promotes smooth flying conditions as it resists vertical displacement and returns to its original level.
• Unstable air rises because it is buoyant, warmer, and lighter than the surrounding air.
• Warm air, being less dense, rises, while colder air descends.
• Warmer air holds more moisture and cools as it rises, approximately 10°C per kilometer.
• Rising air cools to its dew point, leading to condensation, fog, and cloud formation.
• Dew point is the temperature where air, cooled at constant pressure without water addition or removal, becomes saturated and condenses.
• Condensation occurs when gas transforms into liquid.

Indicators of Stable and Unstable Air

• Stable air signs include stratus clouds or fog, low visibility with pollutants, steady precipitation, consistent winds, and Instrument Flight Rules (IFR) visibility.
• Unstable air signs include cumuliform clouds, good visibility, gusty wind, showery precipitation, and thunderstorms.
• Stable air resists uplift, returning lifted air to its original position because it becomes colder and denser than surrounding air, resulting in smoother flights.
• Unstable air facilitates vertical motion; warm, lifted air surrounded by colder air ascends while colder air aloft descends, creating atmospheric instability and bumpy flight conditions.
• Identifying air mass type helps anticipate weather and flying conditions.
• Unstable air masses can cause cumulus clouds, turbulence, and heavy precipitation, making flying challenging.
• Stable air masses typically result in stratiform clouds, reduced visibility due to fog but smooth flying, potentially requiring instrument flight.

Categories of Turbulence

• Turbulence is categorized as either friction-induced or thermal-induced.
• Friction turbulence is due to restricted wind movement.
• Thermal turbulence is due to rapid temperature change in the atmosphere.

Friction Turbulence Types

• Mechanical, shear, and frontal turbulence all fall under friction turbulence.
• Mechanical turbulence results from surface friction (especially over irregular terrain), producing eddies.
• Eddy motion intensity relies on surface wind strength, surface nature, and air stability.
• Three factors affecting mechanical turbulence are air stability, ground roughness and wind strength.
• Stability influences turbulence's vertical reach; ground roughness affects friction; wind strength impacts friction and turbulence intensity.
• Loose ground material like sand or snow can visually indicate mechanical turbulence.
• If the air is unstable, loose materials may restrict visibility, while stable air might only cause drifting.
• Shear turbulence results from friction between opposing air currents.
• Strong wind shear, a drastic change in wind speed or direction over a short distance, causes shear turbulence.
• Frontal turbulence results from friction between opposing air masses near frontal surfaces.
• This is most noticeable when rising warm air is unstable and moist, with turbulence severity heightened by thunderstorm development.
• Turbulence is more tied to cold fronts, but warm fronts may also play a role to a lesser degree.

Thermal Turbulence Types

• The two types of thermal turbulence are daytime convection and cold air advection.
• Daytime convection turbulence results from vertical currents due to uneven heating of the Earth's surface,
• The strength of convection currents depends on air stability, differences in heating, and seasonal heating variations during the day and across seasons.
• Thermal turbulence is strongest in summer afternoons.
• Variations on the surface of the earth, resulting from uneven surface result in uneven heating of the air near the ground.
• Land heats up faster than water.
• Barren surfaces and fields heat faster than vegetated ground.
• Air heating near the ground causes varied convection currents over short distances.
• Cold air advection turbulence arises when cold air passes over warmer water or land, creating convection and turbulence.
• A strong temperature contrast can lead to strong turbulence

Intensities of Turbulence

• Turbulence is classified into light, moderate, and severe intensities.
• Light turbulence causes slight, erratic changes in altitude and attitude with minor strain against seatbelts and slight object displacement.
• Moderate turbulence leads to altitude and attitude changes without loss of control, airspeed variations, definite strain against seatbelts and noticeable object dislodging.
• Severe turbulence causes abrupt changes in altitude and airspeed variations, possible momentary loss of control, violent impact against seatbelts, and objects being thrown about.
• Turbulence intensity depends on the lifting agent and air stability; unstable air is typically more turbulent than stable air.

Downbursts

• A downburst involves a severe downward rush of air accompanied by damaging winds.
• Two types of downbursts are macrobursts. and microbursts.
• Macrobursts include a large downburst that has a horizontal diameter of 4km and can produce winds that reach 140kts; winds can last 5-20mins.
• Microbursts include a downburst of less than 4km in diameter and can cause winds that reach 140kts; winds last less than 5mins.
• Downdrafts have a vertical speed approaching 6000 feet per minute (1 NM/min or 60 knots).
• Classified as either wet (with) or dry (without).
• Wet microbursts occur when dry air enters saturated air, leading to rapid evaporation and cooling, causing air and precipitation to descend.
• Dry microbursts occur when precipitations, evaporates and air descends.
• Virga (precipitation not reaching the ground) associated with TCU/CB clouds often precedes microbursts.
• As virga evaporates, the air becomes colder, sinking and accelerating downwards.
• Microburst winds spread in all directions upon ground impact.

Wind Shear

• Wind shear is a key cause of turbulence and impacts aircraft control.
• It creates a sudden "tearing" effect from rapid wind speed/direction changes over a short distance.
• Wind shear's effect depends on wind direction/speed, affecting aircraft performance.
• Wind speed dictates how long it takes an aircraft to take off or depart.
• It influences motion relative to the ground.
• Wind shear occurs in temperature inversions.

Types of Wind Shear

• Directional is when two different winds meet.
• Speed is when two winds are close and blow in the same direction but at different speeds.
• It also exists along troughs, lows and and the fast-flowing air currents in the atmosphere known as jet streams
• It exists horizontally and virtically.
• It is located in flight, ground or both.

Where Wind Shear is Found

• In flight, wind shear is found below 3000 ft, at frontal air masses and around jet streams.
• Near the ground, wind shear is found at frontal shear, with thunderstorms, with temperature inversions, and around physical obstructions.

High-Level Wind Shear (HLWS)

• HLWS turbulence occurs at high altitudes, mainly with jet streams.
• Violent turbulence when entering/leaving jet streams is called clear air turbulence (CAT).
• CAT is sudden, severe turbulence occurring without visual warnings, caused by air masses moving at various speeds.
• CAT can cause structural stress on aircraft, physical injuries, and occurs without warning.

Low-Level Wind Shear (LLWS)

• LLWS is dangerous due to the aircraft's proximity to the ground.
• LLWS involves wind direction changes up to 180 degrees and speed changes up to 50 knots.
• LLWS commonly occurs with frontal systems, thunderstorms, and temperature inversions with strong upper-level winds, greater than 25 knots.
• LLWS is not a common event.
• LLWS was the reason for 60% of accidents that destroyed aircraft during the final approach and landing phase of flight.

Effects of LLWS on Aircraft

• Pilots can be assisted by notifying them of reported weather problems, such as low-level wind shear, visibility deterioration, and significant precipitation changes, minimizing potential accidents.
• During approach, for example, with a warm front, the wind ahead is at 040, while behind is at 220, causing the aircraft to drift left, requiring pilots to correct near the runway, especially if there are visibility issues.
• If an aircraft passes through the frontal surface, it could encounter a tailwind or calm conditions, it puts the plane at risk of overshooting the runway.
• Due to the aircraft's momentum, airspeed would increase, so the pilot must reduce the nose and airspeed, or the aircraft could be unsafe.
• Initial effects of wind shear during approach include airspeed decrease, nose pitching down, and altitude decrease.
• To resume a normal approach, thrust must be increased initially and is unsafe if a correction is not applied soon.
• Aircraft can proceed, hold or wait, or request another approach

Departing Aircraft

• LLWS can be hazardous during climb-out.
• For example, it can turn from headwind to tailwind.
• This results in loss of airspeed and altitude that can be fatal. If it is suspected, the pilot should determine the direction for climb-out to avoid unfavorable conditions and request the appropriate ATC clearance or delay the flight.

Chop Versus Turbulence

• Aircraft turbulence causes changes to altitude and attitude; aircraft chop results in bumps but no change in altitude or attitude.
• Chop and turbulence can be either light or moderate; only turbulence can be served.
• When the aircraft bumps slightly, it has chop-like characteristics; when the aircraft momentarily exhibits erratic altitude changes, it demonstrates characteristics that constitute turbulence.

Clear Air Turbulence

• CAT is especially dangerous for aircraft and it is important to understand in detail.
• CAT results when fast and slow-moving jet streams pass alongside each other and air is disturbed, resulting in the aircraft that is within will experience very bumpy conditions.
• CAT occurs at altitudes of 20,000 to 40,000 ft, or near mountain ranges, and is most serious near or just above the jet stream core.
• This is most hazardous because radar cannot detect it.
• CAT is closely associated with jet streams in a mostly cloudless sky, with the effect most noticeable on curved jets.

What to Remember About CAT

• It includes a rapid change of wind direction over a short distance.
• It occurs in patches with its area elongated by wind, usually at altitudes of as low as 15000ft to 40000ft.
• CAT includes change in speed over a distance.
• There are no clouds preset the altitudes at which clear air turbulence occurs.

Pilot Procedures for Reporting Turbulence

• Pilots must transmit details in the following order: position, time, altitude, type of aircraft, and turbulence type, whether it is light, moderate, or severe.

This content discusses various weather hazards critical to aviation safety. It highlights the dangers of clear air turbulence (CAT) and low-level wind shear (LLWS), including their characteristics and impact on aircraft. It also covers microbursts, temperature inversions, and the importance of weather reporting to mitigate risks during flight operations.