High altitude turbulence, often called "clear-air turbulence," is typically found along the edge of a Jetstream in the upper levels of the atmosphere. Jetstreams are relatively narrow bands of strong wind along the border between hot and cold air masses. They are typically found at or near the tropopause at altitudes of 30,000 at the poles to 56,000 feet at the equator and can travel at nearly 200 mph.
The Jetstream winds blows from west to east but does shift to the north and the south along the way. The most severe turbulence is on the cold side of the Jetstream where the wind shear is the greatest due to the extreme difference in temperature and air density.
CAT can be encountered anywhere from a few thousand feet above to a few thousand feet below the tropopause. It is typically found in patches and a change of altitude of as little as 2,000 feet is often enough to exit the turbulence.
Pilots rely on reports from other pilots (PIREPS) to help them avoid the areas and altitudes where CAT has been encountered.
Thermal heating of the earth is another source of turbulence. A thermal is a vertical section of rising air in the lower altitudes of the Earth's atmosphere. The uneven heating of the Earth’s surface from solar radiation creates thermals. The Sun warms the ground, which in turn warms the air directly above it. The warmer air expands, becoming less dense than the surrounding air mass and begins to rise.
Clouds form as the moist air rises, cools, and condenses on dust particles in the air. When the moisture condenses, it releases energy known as latent heat, which allows the rising air to cool further, continuing the cloud's ascension. The air stops rising when it has cooled to the same temperature as the surrounding air and it then gets pushed to the side where it will begin to descend.
The colder air being displaced at the top of the thermal causes this downward flow surrounding the thermal.
If enough instability is present in the atmosphere, this process will continue long enough for cumulonimbus clouds (thunderstorms) to form, which support lightning and thunder. Generally, thunderstorms require three conditions to form: moisture, an unstable air mass, and a lifting force (thermals).
While the storm is first forming, most of the action inside is upwards, in the form of updrafts. Pockets of warm, moisture-laden air near the ground rise into cooler, drier air above, condense out their moisture into the water droplets that form the storm cloud, and continue to rise as that condensation adds more heat back into the air pocket.
As the storm continues to grow it is a rolling, churning mix of rising and sinking air. The friction of falling raindrops in the downdrafts grow stronger as some of the raindrops evaporate into the drier air below the cloud, causing cooling, which forces the air to sink faster. The downdrafts continue to grow stronger as the storm ages and more raindrops fall, until the downdrafts match the strength of the updrafts, and eventually dominate them as the storm begins to dissipate.
All thunderstorms, regardless of type, go through these three stages: the developing stage, the mature stage, and the dissipating stage.
Depending on the conditions present in the atmosphere, these three stages take an average of 45 minutes to complete.
If you were flying through the storm cloud as these updrafts and downdrafts were in force, they would turn the flight into a dangerous roller-coaster ride. However, even if the plane flies above the cloud or around it, it can still encounter turbulence from the eddies produced by the interaction of the faster-moving air inside the storms and slower-moving air around them.
Particularly strong downdrafts - called downbursts or microbursts - not only set up powerful winds blowing straight down towards the ground, but they also cause turbulent eddies as they hit the ground, deflect outwards and cause rolling air currents to form.
Turbulence outside the cloud is most prevalent in front of a storm and turbulence can often be present in clear air as many as 20 miles in front of a moving thunderstorm. The anvil at the apex of the storm often indicates the direction of movement. Behind a storm, turbulence is typically much less severe and in closer proximity to the clouds.
When flying around thunderstorms it is important to note that what you see with the naked eye is often several storms within what appears to be one storm cloud.
So, while it’s true the life cycle of a thunderstorm averages only 45 minutes, this applies to what is referred to as a cell. Viewing a thunderstorm on radar you can see the cells where the moisture is the most dense. Navigating through the storm while avoiding the areas of the highest potential of turbulence is beyond the scope of this article, but a very important skill set to acquire.
Mountainous terrain can cause turbulence as winds flow over and around a mountain or mountain range. Friction slows the winds closest to the surface which interacts with the faster airflow higher up and sets up a chaotic pattern of rolling, swirling flow between them. On the downwind (or leeward) side of mountains, the swirling eddies can be very large, with strong windshear between the inside of the eddies and the outside. When a plane flies through this chaotic flow, the turbulence it encounters can be very strong.
Depending upon the wind speed, a mountain wave can be several thousand feet above the mountain range, so it pays to be aware of the terrain below even it’s thousands of feet below.
The author has flown into a leeward mountain wave while three or four thousand feet above a range and the first indication was the aircraft’s airspeed began to decline as the autopilot fought to maintain altitude. Turning west in the direction of the mountain range we flew out of the leeward, down current and into the rising wave on the windward side. Interestingly, our altitude above the mountains afforded us a turbulent-free transition in to and out of the mountain wave.
When flying in to and out of airports surrounded by mountains it is very important to keep in mind the prevailing winds so you can visualize the currents. Pilots experienced in mountain flying also learn pretty quickly to set limits on wind speed for different airports.
Wake turbulence is another cause of clear-air turbulence.
When a heavy aircraft is flying slowly as it does on final approach and right after takeoff, its wings are working the hardest to produce the necessary lift; thus, creating the maximum differential between the low air pressure above the wing and the high pressure below the wing. This high pressure seeking low pressure creates what can be described as a horizontal tornado coming off the wing tips.
These “horizontal tornados,” or rotating vortices can linger in calm air for a significant amount of time after the passage of the aircraft and can cause a small aircraft to suddenly deflect or even flip on the ground or in the air.
Many aircraft are now made with winglets to reduce the vortices, but it’s still advisable to take precautions when you are following a heavy aircraft on approach or takeoff.
Unlike the picture above, wake turbulence is typically invisible so it is important to remember when following a heavy aircraft that wake turbulence and wing-tip vortices sink and drift with the wind. To avoid this phenomenon, therefore, you should alter your climb or descent gradient and path to stay above, and to the extent it’s possible, upwind from the flight path of the heavy aircraft.
While turbulence has many causes, it also has different levels of severity, which are rated as light, moderate, severe and extreme.
Light turbulence causes slight changes in altitude and/or attitude and airspeed fluctuations of 5 to 15 mph. Light turbulence is generally considered more of a comfort consideration than a safety issue.
Moderate turbulence causes changes in altitude and/or attitude but the aircraft remains in positive control at all times. Airspeed fluctuations of 15 to 25 mph.
Severe turbulence causes large, abrupt changes in altitude and attitude. Aircraft may be momentarily out of control.
Extreme turbulence causing aircraft to be violently tossed about and is practically impossible to control.
A PIREP, or pilot report should be made when turbulence is encountered that is moderate or greater. The report should include location, time (UTC), altitude, severity of the turbulence and if it’s in the clouds or in the clear, type of aircraft and when applicable, the duration of the turbulence.
Flying in turbulence cannot always be avoided; so, it’s advisable to follow a few recommendations when you find yourself flying in turbulence that is moderate or greater.
Depending upon the severity of the turbulence, you should consider the following recommendations:
1. Slow to your aircraft’s turbulence penetration speed*,
2. Attempt to hold wings level, but avoid abrupt control inputs to avoid adding to the load factor.
Maintaining attitude is more important than maintaining altitude if maintaining altitude might increase the load factor on the airplane.
You might also consider lowering the landing gear if below landing gear operational speed. This will lower the center of gravity of the aircraft and increase drag, which will help mitigate the effects of turbulence.
*An aircraft’s turbulence penetration speed is its maneuvering speed calculated for actual weight since published maneuvering speeds are based on the aircraft at gross weight.
If the Pilot’s Operating Handbook (POH) only lists one maneuvering speed and your aircraft is below gross weight, use the formula below to calculate an approximate maneuvering speed for your weight.
If your aircraft is less than gross weight, maneuvering speed is reduced. A good rule of thumb is to reduce maneuvering speed 1% for every 2% the aircraft is below gross weight. To simplify the math, if you are 20% below gross weight, reduce your maneuvering speed by 10%.
For more on the relationship between weight, angle of attack and maneuvering speed, click here.