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Mechanical and Thermal Turbulence

Surface winds often vary considerably in both speed and direction over short intervals of time. They tend to blow in a series of gusts and lulls with the direction fluctuating rapidly. This irregular air motion is known as turbulence, which may be either mechanical or thermal in nature. At the surface, turbulence is commonly identified in terms of eddies, whirls, and gusts; aloft it is associated with "bumpy" flying.

The depth of the air layer through which the frictional force is effective also varies with the roughness of the surface; it is shallower over smooth surfaces and deeper over rough topography. The depth may also vary with the stability of the lower atmosphere. A low inversion will confine the frictional effect to a shallow surface layer, but a deep layer can be affected if the air is relatively unstable. These effects vary widely both with time and between localities. Usually the friction layer is considered to be about 2,000 feet deep. The top of the friction layer is the gradient wind level above which the windflow tends to parallel the isobars or pressure-surface contours.

Mechanical turbulence

Surface friction produces mechanical turbulence in the airflow. The effect of friction is least over smooth water and greatest over mountainous topography. The flow of stable air near the surface is similar to the flow of water in a creekbed. At low speeds the currents of air tend to follow the general contours of the landscape. But when the speed increases--as when a creek rises--the current "tumbles" over and around hills and ridges, structures, trees, and other obstacles, and sets up eddies in all directions (for example, see Effects of Vegetation on Surface Winds). Mechanical turbulence increases with both wind speed and the roughness of the surface.


Thermal turbulence

Thermal turbulence is associated with instability and convective activity. It is similar to mechanical turbulence in its effects on surface winds, but extends higher in the atmosphere. Since it is the result of surface heating, thermal turbulence increases with the intensity of surface heating and the degree of instability indicated by the temperature lapse rate. It therefore shows diurnal changes, and is most pronounced in the early afternoon when surface heating is at a maximum and the air is unstable in the lower layers. It is at a minimum during the night and early morning when the air is more stable. Mechanical and thermal turbulence frequently occur together, each magnifying the effects of the other.

Thermal turbulence induced by the combination of convection and horizontal wind is the principal mechanism by which energy is exchanged between the surface and the winds aloft. Unstable air warmed at the surface rises to mix and flow along with the winds above. This turbulent flow also brings air with higher wind speeds--greater momentum--from aloft down to the surface, usually in spurts and gusts. This momentum exchange increases the average wind speed near the surface and decreases it aloft. It is the reason why surface winds at most places are stronger in the afternoon than at night.

Eddies

Eddy formation is a common characteristic of both mechanical and thermal turbulent flow. Every solid object in the wind path creates eddies on its lee side. The sizes, shapes, and motions of these eddies are determined by the size and shape of the obstacle, the speed and direction of the wind, and the stability of the lower atmosphere. Although eddies may form in the atmosphere with their axes of rotation in virtually any plane, it is usual to distinguish between those which have predominantly vertical or horizontal axes. A whirlwind or dust devil is a vertical eddy, as are eddies produced around the corners of buildings or at the mouths of canyons with steep sides. Large, roughly cylindrical eddies that roll along the surface like tumbleweeds are horizontal eddies.

Eddies associated with individual fixed obstructions tend to remain in a more-or-less stationary position in the lee of the obstruction. If they break off and move downstream, new ones form near the obstruction. The distance downwind that an obstacle, such as a windbreak, affects the windstream is variable. For most obstructions, the general rule of thumb is that this distance is 8 to 10 times the height of the obstacle.

Rotation speeds in eddies are often much greater than the average wind speeds measured with mechanical anemometers. These higher speeds are often of short duration at any point, except where stationary eddies are found, but are still significant in fire behavior. Whirlwinds, for example, develop speeds capable of lifting sizable objects. Eddies moving with the general windflow account for the principal short-term changes in wind speed and direction known as gustiness.


Subsections found in Mechanical and Thermal Turbulence

Encyclopedia ID: p418



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