Types of Turbulence

The atmosphere is turbulent everywhere, but often the intensity of the turbulence is so small that it has little effect on aircraft operations. Large intensities of turbulence are to be expected whenever horizontal wind shear or vertical wind  

Both types may occur either in clouds or in clear air. When conducting an aircrew brief, the Aerographers Mate uses the term CAT (clear air turbulence) to alert the aircrew whereto expect turbulence.

THERMAL TURBULENCE. Thermal tur-bulence is the effect (felt by aircraft) that is caused by wind shear (changes in wind speed and/or direction) directly produced by surface heating. As a surface is heated by solar radiation, the air above it is warmed through contact. The warm air becomes unstable and rises in uneven and irregular motions, causing eddies and gusts. If sufficient low-level moisture is present, convective cells may form giving rise to convective turbu-lence.

All turbulence caused by heating is classified as thermal turbulence. The term thermal heating of the surface.

Thermals. As Earths surface is heated by the Sun, different types of surfaces heat at different rates. Water heats slowly; forests heat slightly quicker; grasslands heat faster than forests; and desert sand heats the most rapidly. Each different surface warms the air above it through contact and re-radiation of long wave (thermal) energy. When the air near a homo-geneous surface becomes warmer than the air above it, the surface air begins to rise and mixing occurs. Cooler air from the upper layer sinks to replace the rising heated air. The rising parcels

of air are called thermals. If the thermal continues to develop and becomes large enough, it will produce turbulence. When different types of surface are located in the proximity of one another, thermals develop and intensify at different rates, which intensifies the scale and effect of the thermals. Typically, the slower warming areas will become the locus of a sinking column of air, and the faster warming areas will form the center of a sustained upward thermal. Thermals will normally develop a slight cyclonic spin. Strong prevailing winds will usually prevent thermals from developing. The formation of the stronger thermals usually coincides with max-imum solar heating, during the late afternoon.

Turbulence is felt when an aircraft flies through thermals into areas of stable or sinking air. A light "chop" may be felt when flying through numerous thermals with diameters smaller than the size of the aircraft, but light or moderate turbulence may be felt if the diameter exceeds the size of the aircraft. The vertical velocity of the thermal is an important factor in the severity of the turbulence. Moderate or severe turbulence may be expected when penetrating strong thermals over arid desert areas. These will usually be visible and avoidable because of the large amount of dust and debris picked up in the stronger updrafts.  

Some guidelines for briefing thermal tur-bulence to aircrews are as follows:  

Expect light thermal turbulence on days with light winds when the surface temperatures are expected to produce an unstable lapse rate in the lower few thousand feet. Keep in mind that large blacktop and concrete areas will heat faster.

Expect light turbulence in areas where dust devils are occurring or typically occur under similar conditions.

Expect moderate thermal turbulence in areas where extreme heating and low humidity are expected or where exceptionally large dust devils normally occur under similar situations.

Expect light thermal turbulence under areas of developing cumulus clouds. Thermals rarely exceed 10,000 feet AGL without forming clouds and being classified as convective turbulence.

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