Turbulence stands out as one of the most unpredictable weather phenomena significantly impacting pilots and aviation. Essentially, turbulence is irregular air motion resulting from eddies and vertical air currents. Its effects can range from minor, annoying bumps during a flight to severe disruptions capable of momentarily wresting control from an aircraft or even causing structural damage. It’s a common atmospheric condition associated with weather fronts, wind shear, thunderstorms, and various other meteorological factors.
Understanding the Effects of Turbulence on Aircraft
Turbulence manifests in ways that directly affect an aircraft’s motion. When turbulent whirls are roughly the same size as the aircraft itself, they induce chaotic and unpredictable rolls, pitches, and yaws. This can make for an uncomfortable flight and, in severe cases, a dangerous one.
Turbulence Intensity Levels: A Classification
To effectively communicate the severity of turbulence, it is typically classified into categories: light, moderate, severe, and extreme. The degree of turbulence is determined by both the atmospheric conditions initiating it and the stability of the air mass.
Light turbulence is characterized by momentary, slight changes in altitude and/or aircraft attitude, or a gentle bumpiness. Passengers may feel a slight strain against their seat belts during light turbulence.
Moderate turbulence is similar to light turbulence but more pronounced in intensity. While there is no loss of control of the aircraft, occupants will experience a definite strain against their seat belts, and unsecured objects within the cabin may be dislodged.
Severe turbulence involves large and abrupt changes in altitude and/or attitude, often accompanied by significant variations in indicated airspeed. During severe turbulence, the aircraft may be momentarily out of control. Occupants will be forcefully pressed against their seat belts.
Extreme turbulence represents the most hazardous level. In extreme turbulence, the aircraft is violently tossed around, making control practically impossible. This level of turbulence carries a risk of structural damage to the aircraft.
Chop is another term used to describe a specific type of turbulence. Chop is characterized by rapid and somewhat rhythmic bumpiness, often felt as a continuous series of minor jolts.
| Turbulence Intensity Classification |
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| Intensity |
| Light |
| Moderate |
| Severe |
| Extreme |
The Four Primary Causes of Turbulence
Turbulence in the atmosphere arises from four primary sources, each with distinct characteristics and mechanisms. Understanding these causes is crucial for pilots and meteorologists alike.
1. Mechanical Turbulence: The Impact of Terrain and Obstacles
Mechanical turbulence is generated by the friction between the moving air and the Earth’s surface. This effect is particularly pronounced over irregular terrain and areas with man-made obstacles. The friction disrupts the smooth flow of air, creating eddies and irregular air motions, especially in the lower atmospheric levels.
The intensity of mechanical turbulence is influenced by several factors: the strength of the surface wind, the nature of the terrain, and the stability of the air. Generally, a surface wind speed of 20 knots or higher is required to generate significant mechanical turbulence. Rougher terrain and more unstable air will exacerbate the turbulence. Among these factors, air stability is paramount. Unstable air, especially when heated from below, promotes vigorous and extensive vertical air motion, leading to more pronounced turbulence. In unstable conditions, eddies tend to grow in size, whereas in stable air, they dissipate more slowly without significant growth.
Even large structures like hangars and buildings can induce eddies in strong winds, and these turbulent effects can extend a considerable distance downwind. Strong winds are often gusty, characterized by rapid speed fluctuations. Sudden, short-term increases in wind speed, known as squalls, can also contribute to severe turbulence.
Mountain waves represent a particularly significant form of mechanical turbulence. These turbulent eddies develop downwind of mountain ridges and are stationary relative to the mountains that cause them. Mountain waves are known to produce some of the most intense turbulence associated with mechanical forces. It’s important to note that the stability of the lower troposphere above and to the lee of the mountain is a critical factor; the most intense turbulence is typically associated with stable air in these regions.
Favorable conditions for the formation of mountain waves include:
- Winds of 25 knots or greater blowing perpendicular to the mountain ridge.
- Minimal change in wind direction with increasing altitude.
- Wind speeds increasing with altitude.
- A stable atmosphere. This often involves cold air advection across or along the mountain range, a layer of low stability near the ground, a very stable layer at mountain top level above the surface layer, and a less stable layer above the stable layer. This complex layering causes air parcels forced upward over the mountain crest to sink back towards their initial altitude, creating an oscillatory pattern.
- Mountain waves can extend from the surface to slightly above the tropopause.
- They may extend 100 miles or more downstream from mountain crests.
- The main updraft and downdraft components of a mountain wave can vertically displace an aircraft by up to 5,000 feet per minute.
- Downdrafts can extend to the surface on the leeward side of the mountain.
- The most intense turbulence is often found at low levels, leeward of the mountains, particularly in or near rotor clouds if present.
- Mountain waves can be indicated by specific cloud types, including Cirrocumulus Standing Lenticular (CCSL), Altocumulus Standing Lenticular (ACSL), and Rotor clouds, with rotor clouds often associated with the most severe turbulence.
2. Thermal (Convective) Turbulence: Rising Air and Uneven Heating
Thermal, or convective, turbulence is another significant cause of atmospheric disturbances. It is particularly prevalent on warm summer days when the sun heats the Earth’s surface unevenly. Different surface types absorb and radiate heat at varying rates. Barren ground, rocky areas, and sandy surfaces heat up more quickly than grass-covered fields, and much more rapidly than water bodies. This differential heating sets isolated convective currents in motion, with warm air rising and cooler air descending. As an aircraft flies through these rising and descending currents, it encounters bumpy conditions.
Turbulence from thermal convection extends from the surface up to the top of the convection layer. Above this layer, smoother flight conditions are typically found. If cumulus, towering cumulus, or cumulonimbus clouds are present, the turbulent layer extends all the way to the cloud tops. The intensity of thermal turbulence increases as the strength of convective updrafts intensifies. Pilots often prefer to fly during the early morning or evening hours when thermal activity is less severe, especially in weather conditions conducive to thermal turbulence.
Convective currents may not always be visible as cumuliform clouds, leading to “dry thermals.” Dry convection is favored by warm surface temperatures, uneven surface heating, and steep lapse rates near the surface.
Convective currents can become strong enough to generate air mass thunderstorms, which are associated with severe turbulence. Turbulence is also common in the lower levels of a cold air mass moving over a warmer surface. Heating from below in this scenario creates unstable conditions, resulting in gusty winds and bumpy flying conditions.
Thermal turbulence significantly impacts aircraft during landing approaches. As an aircraft descends towards a landing area, it encounters convective currents of varying intensity originating from the ground along the approach path. These thermals can displace the aircraft from its intended glide path, potentially causing it to overshoot or undershoot the runway.
3. Frontal Turbulence: Air Mass Interaction Zones
Frontal turbulence arises in frontal zones where contrasting air masses meet. The lifting of warmer air by the sloping frontal surface, combined with friction between the two opposing air masses, creates turbulence. This turbulence is most pronounced when the warm air is moist and unstable. If thunderstorms develop along the front, the turbulence can become extremely severe.
Turbulence is more commonly associated with cold fronts, which are typically characterized by more abrupt lifting and stronger vertical motions. However, turbulence can also occur, albeit often to a lesser degree, in warm fronts as well.
4. Wind Shear Turbulence: Changes in Wind Speed and Direction
Wind shear is defined as a change in wind direction and/or wind speed over a specific horizontal or vertical distance. Wind shear is a significant source of turbulence in the atmosphere. Atmospheric conditions prone to wind shear include areas with temperature inversions, along troughs and lows in pressure, and around jet streams. When the change in wind speed and direction is substantial, it can generate severe turbulence. Clear air turbulence (CAT), which occurs at high altitudes (typically above 15,000 feet AGL), is often associated with jet streams and wind shear.
Temperature inversions, where temperature increases with altitude, are zones with potential for vertical wind shear. The strong stability associated with inversions prevents mixing between the stable lower layer and the warmer layer above. The greatest wind shear, and consequently the most intense turbulence, is usually found at the top of the inversion layer. Turbulence related to temperature inversions frequently occurs due to radiational cooling, which is the nighttime cooling of the Earth’s surface, leading to the formation of surface-based inversions.
Turbulence associated with lows and troughs is primarily due to horizontal directional and speed shear. Turbulence is commonly found along troughs at any altitude, within lows at any altitude, and poleward of lows in the mid and upper altitudes of the atmosphere.
A jet stream is a core of strong horizontal winds that follows a wave-like pattern as part of the general atmospheric circulation. Jet streams are located in areas with large horizontal temperature differences between warm and cold air masses. Turbulence related to jet streams is typically found along zones of strong isotach gradients (lines of equal wind speed). Most frequently, turbulence is located on the poleward side of cyclonic jet streams and on the equatorward side of anticyclonic jet streams. Turbulence is further enhanced when a jet stream becomes “arched” or amplified around troughs and ridges in the upper-level flow.
Clear Air Turbulence (CAT): Invisible High-Altitude Hazard
Clear Air Turbulence (CAT) is a specific type of turbulence that is not associated with cumuliform clouds, including thunderstorms. By definition, CAT occurs at or above 15,000 feet. Despite its name, CAT is not exclusively confined to cloud-free air; in fact, approximately 75% of CAT encounters occur in what is visually perceived as clear air.
| Click above image for larger image |
General characteristics of clear air turbulence include:
- It occurs in patches rather than continuous zones.
- The turbulent area is typically elongated in the direction of the wind.
- It is usually found above 15,000 feet.
- It is associated with marked changes in wind speeds, either vertically (vertical shear) or horizontally (horizontal shear).
- CAT layers are often around 2,000 feet deep.
- They can be approximately 20 miles wide.
- And extend for about 50 miles in length.
- CAT is typically transitory in nature, meaning it is not persistent in one location.
- It is most frequent during the winter months.
- And least frequent during the summer.
At high altitudes, CAT areas are usually patchy, with patches varying significantly in dimensions. They can be as thick as 10,000 feet, as wide as 500 miles, and as long as 1,000 miles.
The occurrence of CAT can extend to very high altitudes and is often associated with specific wind flow patterns that produce shear:
- A sharp upper-level trough, especially one moving at speeds greater than 10 knots. (See Figure A. below)
- A closed low aloft, particularly if the flow is merging or splitting. (See Figure B. below)
- To the northeast of a cutoff low aloft, as depicted in Figure C. below.
Turbulence Associated with Thunderstorms: Extreme Aviation Risk
Turbulence associated with thunderstorms is exceptionally hazardous and poses a significant risk to aircraft. It has the potential to overstress aircraft structures or cause a complete loss of control. The powerful vertical currents within thunderstorms can displace an aircraft vertically by as much as 2,000 to 6,000 feet. The most intense turbulence is typically found in the vicinity of adjacent rising and descending air currents.
Gust loads within thunderstorms can be severe enough to cause an aircraft to stall even when flying at rough air (maneuvering) speed or to cripple it at design cruising speed. Maximum turbulence generally occurs near the mid-level of a thunderstorm, between 12,000 and 20,000 feet, and is most severe in clouds with the greatest vertical development.
Severe turbulence is not confined to the interior of the thunderstorm cloud itself. It can extend up to 20 miles away from severe thunderstorms and is typically greater downwind than upwind. Severe turbulence and strong outflowing winds can also be present beneath a thunderstorm. Microbursts, localized columns of sinking air within a thunderstorm, are particularly dangerous due to the extreme wind shear associated with them.
Managing Turbulence from Convection Currents and Obstructions
Convection currents are a common cause of bumpiness experienced by pilots flying at low altitudes in warmer weather. During low-altitude flights over varied surfaces, pilots will encounter updrafts over pavement or barren land and downdrafts over vegetated areas and water. This type of turbulence can often be mitigated by flying at higher altitudes where the convective effects are less pronounced. When larger convection currents develop into cumulus clouds, pilots typically find smoother air above the cloud level.
Avoiding turbulence caused by convection currents by flying above the cloud level.
Convection currents also introduce challenges during landings, affecting the rate of descent. For example, a pilot on a normal glide approach might land short of or overshoot the intended landing spot depending on the presence and intensity of convection currents.
| Varying surfaces affect the normal glidepath. Some surfaces create rising currents which tend to cause the pilot to overshoot the field. | Descending currents prevail above some surfaces and tend to cause the pilot to land short of the field. |
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While local convection effects can be managed, turbulence caused by wind flowing around or over obstructions presents a more significant, often invisible hazard. The only effective way for pilots to avoid this type of turbulence is through pre-flight awareness and knowledge of areas where unusual wind conditions are likely.
Effect of Obstructions on Wind
When wind encounters an obstruction, it breaks into eddies – gusts characterized by sudden changes in speed and direction. These eddies can be carried a considerable distance downwind from the obstruction. Pilots flying through such turbulence should anticipate bumpy and unsteady flight conditions. The intensity of this turbulence depends on the size of the obstacle and the wind velocity and can be a serious hazard, particularly during takeoffs and landings. For instance, during landings, it can cause an aircraft to “drop in” suddenly; during takeoffs, it might prevent the aircraft from gaining sufficient altitude to clear obstacles in its flight path. Takeoffs and landings in gusty conditions should be performed at higher speeds to maintain adequate control.
This effect is amplified when larger obstructions like bluffs or mountains are involved. On the windward side of a mountain, the wind flowing upslope is relatively smooth, and the upward current can assist in carrying the aircraft over the peak. However, on the leeward side, the wind follows the terrain contour downwards, often with significant turbulence, potentially forcing an aircraft towards the mountainside.
Airplanes approaching hills or mountains from windward are helped by rising currents. Those approaching from leeward encounter descending currents.
Stronger winds exacerbate the downward pressure and associated turbulence on the leeward side of obstructions. Therefore, when approaching a hill or mountain from the leeward side, pilots should gain ample altitude well in advance. It is generally recommended to clear mountain ridges and peaks by at least 2,000 feet. If there is any doubt about adequate clearance, the pilot should immediately turn away and gain more altitude. In canyons or narrow valleys between hills or mountains, the wind often deviates from its normal course, flowing through the passage with increased velocity and turbulence. Pilots flying over such terrain must be vigilant for wind shifts and exercise extra caution, especially when attempting a landing.
Impacts of Turbulence and Friction on Winds: Lee Waves and Surface Effects
Lee waves cause updrafts and downdrafts
General winds are modified by terrain and other factors to produce surface winds. The figure above illustrates lee slope eddy effects on a larger scale. Tall mountain ranges can modify strong winds aloft, creating waves and large eddies on their leeward sides. Winds dip downwards due to pressure differences on the lee side, initiating wave actions in strong winds. Lens-shaped clouds, known as altocumulus lenticularus, may form at the crests of these waves. These clouds are likely to develop on the leeward side of mountain ranges oriented perpendicular to winds of 40 knots or more. While these clouds typically occur at high altitudes, the associated strong winds may not always be felt at the surface. However, occasionally, these strong winds aloft can descend to the surface, or eddy winds can reverse the direction of prevailing winds. Depending on location, surface winds can be significantly altered by these processes.
Friction and air turbulence generated at the Earth’s surface act to slow down low-level winds. Turbulence arises from two main sources: mechanical and thermal. Surface roughness, often due to vegetation cover, causes friction, resulting in mechanical turbulence. Surface heating during the day generates thermal turbulence as heat-driven convection currents rise from the surface and mix with the air flowing above.
Channeling and mechanical turbulence also play a role in modifying winds. When a general wind flows through a pass or saddle in a mountain range, wind velocities may increase as the air is constricted. As the air exits the pass on the leeward side, it spreads out, often with associated eddy actions.
Winds on the leeward sides of ridges can shift in direction and speed, making them unpredictable and erratic. These eddy winds are typically described as gusty and inconsistent.
| Vegetation may also cause updrafts and downdrafts | Thermal turbulence caused by surface heating |
Another significant factor influencing low-level winds is thermal turbulence caused by surface heating. Different land surfaces absorb, reflect, and radiate varying amounts of heat. Warm air rises and mixes with the air moving across the terrain. This mixing action affects surface winds, often making them gusty and erratic.
Turbulence from various sources modifies surfaces winds
This overview provides a concise explanation of general winds, their development, and how they are influenced by terrain and turbulence.
Conclusion
Turbulence is a multifaceted atmospheric phenomenon crucial for aviators and weather enthusiasts to understand. From mechanical and thermal turbulence near the surface to frontal and wind shear turbulence at higher altitudes, and the particularly hazardous clear air turbulence and thunderstorm-related turbulence, each type presents unique challenges. Recognizing the causes, intensity levels, and impacts of turbulence is paramount for ensuring flight safety and operational efficiency in aviation.
