Windshear, the friction layer and low level turbulence
Turbulence is simply disturbed, rough air. What the aircraft actually feels is a sudden change in the wind, which changes the aircraft's energy, and that jolt is the turbulence. So the root cause of every kind of turbulence in this chapter is windshear.
Windshear
Windshear is a sudden change in the speed or direction of the wind, vertical currents included.
Turbulence is met in three places: the friction layer, in cloud, and in clear air. This page covers the friction layer, the next covers the two low level sources, and the last two pages cover mountain waves and clear air turbulence.
The friction layer
The friction layer is the lowest part of the atmosphere, from the surface up to about 2000 to 3000 ft, where the ground drags on the wind. Its depth depends on:
- Terrain roughness. Rougher ground throws the disturbance higher.
- Wind speed. A stronger wind is deflected more.
- Stability. Stable air resists vertical movement and keeps the layer shallow.
By day, thermals reduce the low level stability and deepen the friction layer. By night, surface cooling increases the stability and shrinks it, and on a clear night it can form a low level inversion. Below that inversion the wind is light and backed, above it the wind is much stronger, so there is sharp windshear at the inversion. Climbing or descending through it can give moderate to severe turbulence.
Thermal and mechanical turbulence, and mountain wave conditions
Within the friction layer there are two sources of turbulence, one from heating and one from obstacles. When the obstacle is a whole mountain range, the second source can set up a wave that reaches for hundreds of miles.
Thermal turbulence
Insolation drives convection currents. Their strength depends on the surface: rock and concrete heat fast and throw up strong currents, grass and woodland heat slowly and give weak ones.
Mechanical turbulence
Caused by physical obstructions to the airflow: hills, coasts, trees, buildings. It is stronger with a higher wind and rougher ground. In stable air the eddies stay close to the obstacle. In unstable air they are carried up and mixed away.
Mountain waves, the three conditions
Mountain waves, also called standing or lee waves, form on the downwind side of a range when all three of the following hold:
| Ingredient | Requirement |
|---|---|
| Wind direction | Within 30° of perpendicular to the range, with little change in direction as height increases |
| Wind speed | At least 15 kt at the summits, and increasing with height. Joshi puts it at 7 m/s for small mountains, 15 m/s for large ones |
| Stability | A stable layer near summit level, an isothermal layer or inversion, with less stable air above and below |
A definite air flow across the ridge, within 30 degrees perpendicular to the ridge. A wind of atleast 7 m/s for smaller mountains and 15 m/s for larger mountains. Stable layer below the ridge, stable air above the crest with less stable air above.IC Joshi, Aviation Meteorology, chapter 17
The wind direction is perpendicular to the mountain range, plus or minus 30 degrees, without significant change in direction as altitude increases. The wind speed at the summits is at least 15 kt with speed increasing as altitude increases. A marked layer of stability around the altitude of the summits.Oxford ATPL Vol 9, 8
Mountain wave hazards, rotors and the wave clouds
Once the wave is set up it can run 100 to 200 miles downwind and reach above the tropopause. Flight in the smooth part of a wave can be gentle, but the wave carries hazards that have overstressed airframes.
Where the danger is
- The rotor zone sits beneath the wave crests. The strongest rotor is about one wavelength downwind of the ridge, level with or slightly above the ridge crest, and is marked by a roll cloud. The turbulence there is comparable to a severe thunderstorm, with 2 to 4 g common.
- Vertical currents of 10 to 25 m/s occur over large mountains. An aircraft caught in the descending current loses height continuously until it has cleared the whole length of the ridge, so generous terrain clearance is essential.
- The altimeter is unreliable in waves, from lag in the capsules and from temperature error.
The wave clouds
- Lenticular clouds, lens shaped, sit on the wave crests, a few thousand feet above the summits and downwind. Ragged edges mean turbulence.
- Rotor, or roll, clouds sit under the crests, level with or slightly above the ridge. The first one downwind marks the strongest rotor.
- Cap cloud covers the ridge itself and can be swept down the lee slope as a Föhn wall.
All of these form on the upwind side and dissolve on the downwind side, so they appear stationary while the air rushes through them. If the air is dry, no cloud forms at all, even with a strong wave running.
Rotor streaming
If the wind is strong only at low levels and weakens or reverses higher up, the standing rotors are replaced by rotor streaming, violent rotors that break off the lee slope and travel downwind.
Clear air turbulence, jet streams and reporting
Clear air turbulence is high level bumpiness in cloud free air, with no visual warning at all. It is the reason the seat belt sign matters at cruise.
What CAT is
CAT is caused by strong windshear between bodies of air moving at very different speeds. It is most common in the upper troposphere, roughly 23 000 to 39 000 ft, near the tropopause, and it is closely tied to jet streams and to upper level frontal zones where temperature contrasts are sharp.
CAT is made much worse where a jet stream crosses mountainous terrain and mountain waves are present. Turbulence at an upper level trough, where the wind direction changes sharply, is worse than at a ridge, which is more gently curved. Over India, CAT is most frequent from October to May over the north, from the subtropical jet, and is most severe from December to February.
If you hit it high up
Slow to the rough air penetration speed and also descend. At high altitude the margins to high and low speed buffet are small, and descending widens them.
The reporting scale
| Intensity | Inside the aircraft | IAS change, g |
|---|---|---|
| Light | Slight strain on the belts, walking easy | 5 to 15 kt, under 0.5 g |
| Moderate | Definite strain on the belts, unsecured objects dislodged, walking and service difficult | 15 to 25 kt, 0.5 to 1.0 g |
| Severe | Violent against the belts, objects tossed about, aircraft briefly out of control | Over 25 kt, over 1.0 g |
Pilots report time, location, level, intensity and aircraft type. The UK does not use the term Extreme for turbulence.