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Microscale and Thermal TurbulenceThe microscale is the smallest scale in which we classify weather phenomena. Microscale swirls of air have widths of a few meters and last less than a few minutes. Like mesoscale flow, microscale flow is forced by heating and obstructions to flow. These cause tiny motions in the air to form. These motions are known as eddies, and they twist and turn the air, mixing it with the air molecules that surround the eddies. Eddies caused by heating are grouped and called thermal turbulence. Those caused by obstructions are called mechanical turbulence. Though both thermal turbulence and mechanical turbulence interact with each other, we will first look at each influence separately.
Thermal TurbulenceDifferential heating of a surface will cause some air molecules to become excited, while others have less energy. The interactions between excited molecules and slower ones cause tiny eddies to form. These little whirlwinds add to turbulence in the friction layer and to instability. The development of friction near the surface involves both molecular viscosity and eddy viscosity. Molecular viscosity is the slowing of motion in a fluid due to the random movements of molecules and their interaction with each other at different speeds. This effect is primarily very near the surface of the earth (within a few millimeters). Eddy viscosity is the friction, or drag, created by turbulent eddies and their interaction. Molecular viscosity has only a slight effect on air flow, but does aid in the formation of eddies, which induce larger magnitudes of drag force higher above the earth's surface. Eddy viscosity is the primary influence on friction in the lower atmosphere. Friction increases as the eddy viscosity increases. Eddy viscosity is affected by the roughness of the surface and the obstructions present, which leads us into mechanical turbulence.Confused? Have a question? If so, check out the Frequently Asked Questions (FAQ) page or send mail to the OS411 tutor (os411tutor@shodor.org) with your question! Report technical/content problems here |
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