Fill in the Blank : When the strong wind passing over the building, the force acting on building is in ........
Class 11 Physics
Fluid Mechanics
Answers
Wind usually refers to the movement of air parallel to the earth's surface. In this Digest we are concerned only with winds in the lowest few hundred feet of the atmosphere. The driving forces for such movements are pressure differences caused by unequal heating of the air. For a steady wind, however, the direction of flow does not follow the steepest pressure gradient from a "high" to, a low" as one might expect. In fact, the direction of flow is more nearly parallel to the isobars (lines connecting points of equal pressure) rather than perpendicular to them. This is because every object moving across the earth's surface is deflected to the right in the northern hemisphere (to the left in the southern) as a result of the rotation of the earth. This deviating effect, called the Coriolis force, is small and is usually disregarded except in the atmosphere and the ocean. The pressure gradient causing wind, however, is also small. Normally, wind requires several hours to develop, and although flow begins perpendicular to the isobars, it is gradually deflected to the right as time passes, so that when a steady state is finally attained the wind blows more nearly parallel to the isobars. The pressure gradient is then balanced by the Coriolis force and the frictional drag force, plus or minus centrifugal force if the path happens to be curved.