Why does the wind “blow”, and what factors are involved?
In the ‘free’ atmosphere on our rotating earth, the movement of air is forced by differences in atmospheric pressure between one location and another: this difference, over a specified distance, is known as the PRESSURE GRADIENT. It might be assumed that once there is a pressure gradient, that air would travel directly from high pressure to low: this doesn’t happen, because as soon as it begins to move, it undergoes an apparent deflection owing to the fact that we live on a rotating planet. In the Northern Hemisphere, the ‘deflection’ is towards the right of air motion; in the Southern Hemisphere it is towards the left. A balance is achieved whereby the force due to the Pressure Gradient (PRESSURE GRADIENT FORCE, or PGF) exactly equals the deflection due to planetary motion (the CORIOLIS DEFLECTION or ACCELERATION [CA]). The wind direction is that summarised in Buys Ballots Law (q.v.) The wind resulting from these ideal conditions is known as the GEOSTROPHIC WIND – a theoretical wind (
