Winds Aloft
Usually, we separate winds into surface winds and winds aloft. There is no sharp separation between them, but rather a blending of one into the other. We think of surface winds as those winds measured with instruments mounted on surface-borne masts or towers. Winds aloft are those measured with airborne equipment from the surface layer up to the limit of our interest. In ascending from the surface through the lower atmosphere, there is a transition in both speed and direction from the surface to the top of the friction layer, which is also called the mixing layer. The depth of this friction or mixing layer is dependent upon the roughness of the terrain and the intensity of heating or cooling at the surface. The winds aloft above the mixing layer are steader in speed and direction, but they do change as pressure centers move and change in intensity.
Pressure systems higher in the troposphere may differ markedly from those near the surface. At progressively higher altitudes, closed pressure systems are fewer. Furthermore, it is common for the troposphere to be stratified or layered. With height, there may be gradual changes in the distribution of Highs and Lows. These changes produce different wind speeds and directions in the separate layers. With strong stratification the wind direction may change abruptly from one layer to the next. The difference in direction may be anywhere from a few degrees to complete reversal. In the absence of marked stratification above the friction layer, wind direction at adjacent levels tends to be uniform, even though the speed may change with altitude. A common cause of stratification in the lower troposphere is the overriding or underrunning of one air mass by another. Thus, the layers often differ in temperature, moisture, or motion, or in any combination of these.
Marked changes in either wind speed or direction between atmospheric layers often occur with an inversion which damps or prevents vertical motion, whether it is convection over a fire or natural circulation in the formation of cumulus clouds. Even though a wind speed profile--a plot of wind speed against height--of the upper air might indicate only nominal air speeds, the relative speeds of two air currents flowing in nearly opposite directions may produce strong wind shear effects. Wind shear in this case is the change of speed or direction with height. Clouds at different levels moving in different directions, tops being blown off growing cumulus clouds, and rising smoke columns that break off sharply and change direction are common indicators of wind shear and disrupted vertical circulation patterns.
Wildland fires of low intensity may be affected only by the airflow near the surface. But when the rate of combustion increases, the upper airflow becomes important as an influence on fire behavior. Airflow aloft may help or hinder the development of deep convection columns. It may carry burning embers which ignite spot fires some distance from the main fire. The winds aloft may be greatly different in speed and direction from the surface winds.
Encyclopedia ID: p421
