ABSTRACT

The atmosphere is in constant motion on scales ranging from short-lived, local wind gusts to storm systems spanning several thousand kilometres and lasting for about a week, and to the more or less constant globalscale wind belts circling the earth. Before considering the global aspects, however, it is important to look at the immediate controls on air motion. The downwardacting gravitational field of the earth sets up the observed decrease of pressure away from the earth’s surface that is represented in the vertical distribution of atmospheric mass (see Figure 2.13). This mutual balance between the force of gravity and the vertical pressure gradient is referred to as hydrostatic equilibrium (p. 23). This state of balance, together with the general stability of the atmosphere and its shallow depth, greatly limits vertical air motion. Average horizontal wind speeds are of the order of one hundred times

greater than average vertical movements, although individual exceptions occur – particularly in convective storms.