In the lower part of the atmosphere were general aviation pilots usually fly we experience low level, thunderstorm and mountain wave turbulence. Clear air turbulence is more relevant to those pilot flying the flight levels and crossing jet streams. Here we are going to concentrate on the effects and causes of low level turbulence so that it can be avoided or handled safely.
Low level turbulence can be split into different origins: mechanical, stable layer, dry convection and mesoscale circulations. We will examine them here one at the time.
When wind blows over the surface of the Earth it will experience friction and this will change the wind speed and direction. This layer is called the boundary layer as discussed in a previous article. Usually the boundary layer deepens when the turbulence intensifies and this depends largely on the wind speed.
By using the surface wind speed we can estimate of the thickness of the boundary layer. A rule is that when surface winds double the layer also doubles in height and as a result this layer can reach several thousand feet high. Keep in mind that turbulence intensities decrease with altitude to the top of the boundary layer. For very light aircraft (below 2000 kg) the turbulence will go up even higher.
This is a sudden change in wind speed and direction. This is caused when surface winds are strong and in close proximity to the ground and it is aggravated by obstructions on the ground. The result is large airspeed variations usually during takeoff and landing. One rule of thumb says that if wind speed exceed 20 kts you may experience airspeed fluctuations of 10 to 20 kts.
One can see that if the winds blows over an even flat surface (the sea or a lake for example) turbulence will be usually minimal or non existent. Any obstruction in the path of the wind will cause turbulence in the form of eddies. The intensity depends on the form and size of the obstruction and the speed of the wind.
On airports where hangers and buildings are relatively close to the runway you may expect some wind shear and LLT (low level turbulence) during takeoff and landing. To remodie this: add half the gust factor to your airspeed and this will give you a safe margin from the stall.
The effects felt downwind of any obstruction to the wind (trees, buildings) depends on the height and wind speed, you may expect influence on wind speed variations of at least 10 times the height of the obstruction in the down wind direction. Above the obstruction its not uncommon to see an increase in wind speed due to the venturi effect (as happens over a wing).
When the wind blows perpendicular over a building or other similar shaped structure, the wind forms a turbulent wake on the lee side of the obstruction. The boundary between laminar and turbulent flow can reach some 1.5 to 2.0 times the height. If the roof is slanted turbulent eddies or vortices can be generated causing problems.
These create a dangerous, corkscrew like turbulence which can be felt many miles downwind, even up to 20 miles downwind in the right conditions. This turbulence reacts on the aircraft like it is entering a pocket of unstable air moving in all directions around all axes of the aircraft. You may compare this turbulence with the wake vortices of a large aircraft.
While trying to keep control of the aircraft it feels like the ailerons and or rudder have no effect at all at certain moments. You can also expect a jolt of a couple of G's while leaving the turbulent air. Inform your passengers and have them in the seatbelts before they hurt themselves. Of course the effects depends on the type and weight of the aircraft you are flying.
Always look upwind to detect the cause of any low level turbulence and should you see these relics of ancient green technology: then you know where this turbulence is coming from. My advise: fly at least 2500 feet higher above the level of these things. Also, during landing you might experience a wingdrop due to this turbulence, be prepared for this! Opposite rudder, lower the nose and adding some power should take care of the issue.
This mechanical turbulence can also result in changes to cloud formation and cause or intensify low level stratus, mist or even rain further downwind of these man made obstructions.
See the next link for a clearer image: cloud formation due to obstructions and read the next article: Downwind turbulence by wind farms. A YouTube video about the serious issues with this kind of turbulence and location of these things: Popham Airfield.
The height (or depth) of the turbulence layer will increase in the presence of hilly or rugged mountainous areas. This depends on the slope, height and orientation compared to the wind direction, atmospheric stability and of course wind speed.
During preflight it helps to look at an aeronautical chart to judge the ruggedness of the terrain you intend to intend fly over. But, more importantly, verify not only the MSL height of the peaks and hills but also take into account the height compared to the surrounding terrain. This will give you a better idea of the influence of these obstructions to the wind and how turbulent it might become.
For large areas this will help the pilot planning the flight. Be aware that when winds exceed 25 to 30 kts over rough terrain (features over 1000 ft AGL) one can expect moderate or greater LLT and severe LLT with winds over 40 kts. The form of the terrain features may also influence the amount of turbulence.
Turbulence over hills depends on the characteristics and shape of these hills, if they tend to be smooth then the airflow will also be smooth. With steep slopes (25° to 40°) the effect of ascending air may be felt up to 1500 feet above the hill in stable air conditions.
If there are any sharp edges then the airflow tends to separate and form eddies and move further downwind. On the lee side you may expect to find a wake where the airflow reverses direction.
Sawtooth ridges tend to form whirl winds (eddies) when the wind flows perpendicular with the ridge. In dry and dusty conditions dust devils may also be a visible indicator for this kind of LLT.