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CHAPTER FIVE
AVIATION WEATHER
Figure 5-3 Airflow Over Irregular Terrain
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.
Mechanical Turbulence
Mechanical turbulence results from wind flowing over or around irregular terrain or man-made
obstructions. When the air near the surface of the Earth flows over obstructions, such as bluffs,
hills, mountains, or buildings, the normal horizontal wind flow is disturbed and transformed into
a complicated pattern of eddies and other irregular air movements (Figure 5-3). An eddy current
is a current of air (or water) moving contrary to the main current, forming swirls or whirlpools.
One example of mechanical turbulence may result from the buildings or other obstructions near
an airfield.
The strength and magnitude of mechanical turbulence depends on the speed of the wind, the
roughness of the terrain (or nature of the obstruction), and the stability of the air. Stability seems
to be the most important factor in determining the strength and vertical extent of the mechanical
turbulence. When a light wind blows over irregular terrain, the resulting mechanical turbulence
has only minor significance. When the wind blows faster and the obstructions are larger, the
turbulence intensity increases and extends to higher levels.
5-6
Weather Hazards of Turbulence, Icing, Ceilings, Visibility, and Ash Clouds


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