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There are four types of induced or dynamic tertiary circulations. They are eddies, turbulence, large scale vertical waves, and Foehn winds.


An eddy is a circulation that develops when the wind flows over or adjacent to rough terrain, buildings, mountains or other obstructions. They generally form on the lee (downwind or sheltered) side of these obstructions. The size of the eddy is directly proportional to the size of the obstruc-tion and speed of the wind. Eddies may have horizontal or vertical circulations that can be either cyclonic or anticyclonic.

Horizontal eddies form in sheltered areas downwind of rough coast lines or mountain chains. An example of a horizontal eddy is the weak cyclonic circulation that develops in the channel off the coast of Santa Barbara, Califor-nia. The winds frequently blow parallel to the northern California coast line during the winter fog and stratus season. The Santa Barbara channel often remains fog-free because the waters are pro-tected from winds which transport the fog inland. However, when the winds are sufficiently strong, friction along the rough coastal range produces a weak cyclonic eddy over the channel. This cyclonic flow, though weak, is sufficient to advect fog into the region. 

Vertical eddies are generally found on the lee side of mountains, but with low wind speeds, stationary eddies or rotating pockets of air are produced and remain on both the windward and leeward sides of obstructions. (See fig. 3-3-7.) When wind speeds exceed about 20 knots, the flow may be broken up into irregular eddies that are carried along with a wind some distance downstream from the obstruction. These eddies

Figure 3-3-7.—Eddy currents formed when wind flows over uneven ground or obstructions.

may cause extreme and irregular variations in the wind and may disturb aircraft landing areas sufficiently to be a hazard. 

A similar and much disturbed wind condition occurs when the wind blows over large obstruc-tions such as mountain ridges. In such cases the wind blowing up the slope on the windward side is usually relatively smooth. On the leeward side the wind spills rapidly down the slope, setting up strong downdrafts and causing the air to be very turbulent. This condition is illustrated in figure 3-3-8. These downdrafts can be very violent. Air-craft caught in these eddies could be forced to col-lide with the mountain peaks. This effect is also noticeable in the case of hills and bluffs, but is not as pronounced.


Turbulence is the irregular motion of the at-mosphere caused by the air flowing over an uneven surface or by two currents of air flowing past each other in different directions or at dif-ferent speeds. 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. The warmer air rises and the colder air descends, caus-ing an irregular vertical motion of air; this is called thermal turbulence.

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.

Figure 3-3-8.—Effect of windflow over mountains.

Gustiness and turbulence are more or less synonymous. Gustiness is an irregularity in the wind speed which creates eddy currents disrupt-ing the smooth air flow. Thus, the term gust is usually used in conjunction with sudden intermit-tent increases in the wind speed near the surface levels. Turbulence, on the other hand, is used with reference to levels above the surface. Gustiness can be measured; turbulence, however, unless en-countered by aircraft equipped with a gust probe or an accelerometer, is usually estimated.

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