Figure 5-2. Strength of Thermal Currents Vary with Composition of Surface

AVIATION WEATHER

CHAPTER FIVE

Clear Air Turbulence

CAT normally occurs outside of clouds and usually occurs at altitudes above 15,000 feet MSL,

due to strong wind shears in the jet stream. CAT is not limited to jet streams, in fact CAT can be

found in each of the four categories of turbulence, but the most severe CAT is associated with jet

streams. You may also notice the Wind Shear category of turbulence is only CAT.

Thermal Turbulence

The thermal or convective turbulence forms as a result of heating from below. Localized vertical

convective currents resulting from surface heating or cold air moving over warmer ground cause

thermal turbulence. Strong solar heating of the Earth's surface can result in localized vertical air

movements, both ascending and descending. For every rising current, there is a compensating

downward current usually slower in speed since it covers a broader area. Such vertical air

movements can also result from cooler air being heated through contact with a warm surface.

Figure 5-2 Strength of Thermal Currents Vary with Composition of Surface

The strength of thermal currents depends in part on the extent to which the Earth's surface has

been heated, which in turn, depends upon the nature of the surface (Figure 5-2). Notice in the

illustration, dry- barren surfaces such as sandy or rocky wasteland and plowed fields absorb heat

more readily than surfaces covered with grass or other vegetation, which tend to contain more

moisture. Thus, barren surfaces generally cause stronger convective currents. In comparison,

water surfaces are heated more slowly. The difference in surface heating between land and water

masses is responsible for the turbulence experienced by aircrews when crossing shorelines on hot

summer days.

Weather Hazards of Turbulence, Icing, Ceilings, Visibility, and Ash Clouds

5-5