Wondering what is turbulence and what causes turbulence? This post has everything you need to know!
1. What is turbulence?
The word ‘turbulence’ can strike fear into any inexperienced flier and a flight can quickly turn into a nightmare for the passenger when beverages start moving around the cabin. Rest assured, however, the aircraft is designed to fly through even the worst turbulence and the pilots are trained to handle the situation.
Turbulence refers to a sudden and irregular movement of air within the atmosphere that can occur at sea level or the cruising altitude of commercial aircraft at around 35000ft.
The intensity of turbulence can range from small bumps that passengers may not even feel to large drops in altitude, sometimes by hundreds of feet, which can injure passengers who have not fastened their seatbelts.
2. What causes turbulence?
Atmospheric turbulence can be caused by thunderstorms, weather fronts, mountain ranges, jet streams, and even other aircraft.
These conditions combined create turbulence that we experience when flying. However, turbulence can be subdivided into different categories.
These are thermal turbulence, mechanical turbulence, mountain wave turbulence, frontal turbulence, thunderstorm turbulence, Clear Air Turbulence (CAT), and wake turbulence.
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3. Thermal Turbulence
Thermal turbulence is one of the most common forms of air turbulence.
This form of turbulence is encountered most frequently during the daytime in late spring, summer, and early autumn. An airplane is most likely to encounter thermal turbulence during the hottest months of the year because this form of turbulence is caused by warm air rising from the ground which has been heated by the midday sun.
These columns of rising air are often called ‘thermals’ by pilots. When flying out of an airport on a hot summer day, you may think it is the powerful thermals rising from the hot concrete below that are causing the choppy air.
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However, it is the interaction between the thermals and high-altitude winds which causes the turbulence we experience. The convective thermals block colder, high-altitude wind from travelling in a straight line so it must deviate around the thermals like water flowing over a rock in a river.
As a result of this, swirling wind patterns start to form downwind of the thermals, these are called ‘eddies’ by meteorologists. These eddies are what aircraft passengers feel as turbulence. The intensity of thermals on a hot day and the speed of the wind dictate how powerful the turbulence will be when flying.
Therefore, it can be said that because of the impacts of climate change, we can expect more intense turbulence while flying due to hotter temperatures which will produce more powerful thermals and winds.
4. Mechanical Turbulence
Mechanical turbulence is another form of turbulence, that is most common near ground level. Mechanical turbulence is caused by man-made objects such as skyscrapers and irregular terrain features like forests or hills.
When the wind is blowing in a certain direction, friction is generated between the air and these objects which causes eddies to form and therefore turbulence.
Like thermal turbulence described above, the intensity of mechanical turbulence depends on several factors such as wind speed, the roughness of the terrain, and the surface temperature.
This form of turbulence is often found in built up areas which can cause a hazard to light aircraft because large buildings such as hangars and control towers can cause strong eddies to form and be carried downwind over the runway which can make landings more difficult to inexperienced pilots.
5. Mountain Wave Turbulence
Mountain wave turbulence is a form of mechanical turbulence and it is formed where a mountain obstructs the horizontal movement of air in the atmosphere. As the wind tries to move up and over the mountain, eddies are formed on the leeward side as air tries to descend the mountain.
As a result of this phenomenon, turbulence will be more intense for aircraft on the leeward side of a mountain than on the windward side.
Even though commercial airliners fly above most mountains in the world, mountain wave turbulence can still be a hazard as some pilots, flying in high altitude aircraft, have reported mountain wave turbulence at altitudes as high as 60,000ft which is over 30,000ft higher than Mt Everest and with vertical speeds of over 8000ft a minute (Australian Transport Safety Bureau, 2021).
For mountain turbulence to form, the atmosphere must have the following conditions:
- Wind flow around ridge height that is at least 30 degrees from perpendicular to the ridge line and with a speed of at least 28 mph, increasing with height.
- Stable air above the crest of the mountain ridge with less stable air above and a stable layer of air below the ridge (SKYbrary Aviation Safety, 2021).
6. Frontal Turbulence
Frontal turbulence is a form of air turbulence that can be found where a warm front (edge of a warm air mass) meets a cold front (edge of a cold air mass).
Frontal turbulence can be caused by several reasons such as rising warm air which can make aircraft rise or drop in altitude suddenly, an unstable frontal surface or an abrupt change in wind direction caused by the different air densities between the masses of air.
It is generally accepted by most meteorologists that the most severe frontal turbulence is caused by fast-moving cold fronts. (Publishing, 2021)
7. Thunderstorm Turbulence
Thunderstorm generated turbulence is one of the most common and arguably the most dangerous form of air turbulence. Thunderstorm generated turbulence can be found inside monstrous thunderstorm clouds called Cumulonimbus which can reach heights upwards of 50,000ft or more.
The violent air turbulence inside these clouds is caused by immense updraughts and downdraughts which are so powerful they can cause physical damage to the frame of an aircraft foolish enough to fly into one of these clouds.
There are even cases of aircraft dropping by as much as 2000-6000ft in altitude while flying through these clouds which really illustrates how hazardous storm cells are to aviation and why they must be avoided at all costs. Some inexperienced pilots may also think it is safe to fly below the storm cloud, but this can be a fatal mistake.
Strong downbursts of wind emanating from inside the storm cloud can cause even large commercial aircraft to suddenly drop in altitude. A microburst is a smaller, localised downburst of air (around 2.5 miles in diameter) that can suddenly surge downwards from a thunderstorm at speeds of up to 100mph and damage aircraft and even buildings on the ground.
A microburst’s relatively small size is a major hazard to aircraft on final approach because the wind direction can suddenly change from a strong headwind to a strong tailwind in a few seconds which can cause an aircraft to stall if it is slow enough.
This is what happened to Delta Air Lines Flight 191 which crashed on the 2nd August 1985 during a thunderstorm at Dallas/ Fort Worth International Airport with the loss of 137 passengers and crew.
8. Clear-Air Turbulence
Clear-air turbulence or CAT is one of the most frightening form of air turbulence for nervous passengers because it can occur without warning even if there are no visible weather systems near the aircraft.
This form of turbulence is often encountered when an aircraft is cruising at altitudes at or above 20000ft, so it is not associated with clouds in the lower atmosphere such as cumulus and cumulonimbus.
In fact, over 75% of clear-air turbulence occurs in clear air (weather.gov 2021). Clear-air turbulence occurs at the meeting point between two large masses of air at different temperatures and traveling at different speeds.
The jet streams are commonplace to encounter clear-air turbulence. Jet streams are narrow, fast-moving currents of air located at high altitudes, normally close to the Tropopause (the boundary layer between the troposphere and the stratosphere) and are caused by the temperate gradient between large masses of air.
These winds circle the earth unobstructed at speeds between 80-140mph, but in extreme cases, they can sometimes reach speeds of 275mph (National Geographic 2021). Clear-air turbulence is generated where a cold fast-moving jet stream meets a warm, slow-moving mass of air. Eddies are formed at the edges of the jet stream.
These eddies cause clear-air turbulence because the wind is rapidly changing speed by hundreds of miles per hour in a relatively short distance. The sudden changes in the wind speed and/or direction is also called wind shear by meteorologists.
Clear-air turbulence can last for up to 50 miles in any direction and can be 2000ft deep making it harder for pilots to avoid. Clear-air turbulence is also most frequent during the winter and less frequent in the summer.
9. Wake Turbulence
Wake turbulence can be one of the most dangerous forms of turbulence not because it damages the frame of the aircraft but because the turbulence can severely impact the handling of the aircraft.
Unlike the previous forms of air turbulence, wake turbulence is generated by the passage of an aircraft in flight. Every fixed wing aircraft generates wake turbulence because it is a function of the aircraft producing lift, which generates two counter-rotating vortices left in the wake of the aircraft. These vortices are also called wingtip or wake vortices.
These vortices can pose a hazard to aircraft during the take-off and landing phases of flight as the wingtip vortices generated by an aircraft can induce a violent roll and yaw motion on the aircraft that flies through the wake of the previous aircraft.
The intensity of wake turbulence depends on factors such as the overall mass and wingspan of the aircraft that generated the wingtip vortices and the aircraft that is flying through the wake turbulence.
Another factor is the time between an aircraft generating the wingtip vortices and a trailing aircraft flying through the wingtip vortices.
Since wingtip vortices dissipate over time and get weaker with distance, pilots and ATC are always recommended to wait for wing tip vortices to dissipate after a plane takes off before clearing another aircraft for departure.
Large aircraft such as an Airbus A380 will generate larger, stronger, and more persistent wingtip vortices than a lighter aircraft such as a Boeing 737 so a 737 will have to wait until the A380’s wingtip vortices have dissipated before taking off.
If the positions were reversed, then the A380 wouldn’t need to wait as long because the 737’s wing tip vortices will have very little impact on the much heavier A380’s handling (Skybrary.aero 2021).
10: How common are turbulence related injuries?
Turbulence can cause serious injuries to passengers and crew if they are not prepared for it.
According to the Federal Aviation Administration (FAA), turbulence is the most common cause for airline injuries and is responsible for around 75% of weather-related accidents and injuries.
Clear-air turbulence is a common culprit for these injuries because it can occur suddenly, violently and with no warning. However, even though turbulence can injure passengers it doesn’t mean you need be worried about the next time you fly.
Injuries related to turbulence are very rare and the crew are trained to handle the situation if it comes about.
In one year, the FAA estimates that 58 people get severely injured by turbulence, on the other hand 6 million people fly airliners all over the world in only 24 hours so the likelihood of receiving an injury due to turbulence is very low (Slack Davis Sanger 2018).
11: How do pilots avoid turbulence?
Airline pilots will use several methods to improve the overall experience and safety of a flight. To mitigate the dangers of thunderstorm turbulence the obvious solution is to avoid thunderstorm clouds completely.
If the pilots notice they are heading towards a thunderstorm they will usually plot a course around or over the thunderstorm without the passengers even knowing. New technology has also greatly reduced the total number of injuries related to turbulence.
An example of this is Delta Airlines which released an app in recent years that automatically finds smooth air while flying and now 12000 pilots are using it.
Since thermal turbulence is most common during the hottest hours of the day, passengers can reduce their chances of experiencing turbulence by booking flights in the evening or at night when it is cooler.
Passengers can also reduce their chances of experiencing clear-air turbulence by flying in the summer and autumn months since the jet streams are colder and more powerful in winter (Delta News Hub 2018).
References (what causes turbulence)
- Atsb.gov.au. 2021. Mountain wave turbulence
- Skybrary.aero. 2021. Mountain Waves – SKYbrary Aviation Safety.
- Publishing, I., 2021. Frontal Turbulence. [online] Navyflightmanuals.tpub.com.
- Weather.gov. 2021. Turbulence
- Society, N., 2021. jet stream. [online] National Geographic Society.
- Skybrary.aero. 2021
- Slack Davis Sanger. 2021. Turbulence-Related Injuries Your Rights | Slack Davis Sanger
- Delta News Hub. 2021. 3 ways Delta’s groundbreaking turbulence app is changing the way we fly.
Do you have any questions or experiences with turbulence? Please leave a comment in the section below – we would love to hear from you!
Kudzi Chikohora is a B737 pilot with around 2,000 hours flying in Europe. He holds a masters degree in Aerospace Engineering and is a chartered engineer and a member of the Royal Aeronautical Society.
Kudzi completed his pilot training via the self funded modular pilot training route and created kcthepilot.com to share pilot training and aviation content.