JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
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 that the Wind Shear category of turbulence is only CAT.
Thermal Turbulence
Thermal (or convective) turbulence is caused by localized vertical convective currents resulting
from surface heating or cold air moving over warmer ground. 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 that is 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. The turbulence that forms as a result of heating from below
is called thermal, or convective, turbulence.
The strength of convective 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 4-1). Notice in the
illustration that 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.
Figure 4-1 - Strength of Convective Currents Vary With Composition of Surface
When air is very dry, convective currents may be present even though convective-type clouds
(cumulus) are absent. The upper limits of the convective currents are often marked by haze lines
or by the tops of cumulus clouds that form when the air is moist. Varying surfaces often affect
the amount of turbulence experienced in the landing pattern and on final approach.
Figure 4-1 — Strength of Convective Currents Vary With Composition of Surface
Version 3.2/Dec 08 4-3