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. 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. 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.