These downdrafts can be very violent. This effect is also noticeable in the case of hills and bluffs, but is not as pronounced. The main source of turbulence is the friction along the surface of Earth. This is called mechanical turbulence. Turbulence is also caused by irregular temperature distribution.
Mechanical turbulence is intensified in unstable air and is weakened in stable air.
These influences cause fluctuations in the wind with periods ranging from a few minutes to more than an hour. If these wind variations are strong, they are called wind squalls and are usually associated with convective clouds. They are an indication of approaching towering cumulus or cumulonimbus clouds. Effect of windflow over mountains. Gustiness and turbulence are more or less synonymous.