Air circulating north and south of the subtropical high pressure zones
generally blows in a westerly direction in both hemispheres, giving rise
to the prevailing westerly winds. These westerly moving air masses again
become heated and start to rise, creating belts of subpolar lows.
The circulation pattern discussed above is mainly true on the surface
movement. The atmosphere, like the oceans, have differences throughout
the air column which cause changing weather. For example, when warmer
air lies above polar air it, causes wind to blow parallel to where they
meet. This produces a "jet" of strong air on a continuous
basis referred to as the "Jet Stream."
Atmospheric circulation is further complicated by the distribution of
land and water masses on the surface of the Earth and the topography of
the land. The oceans are the source of moisture and the elevation of the
land surface helps control where moist air will rise. Climatic zones
depend not only on latitude, but also on the distribution and elevation
of land masses. In general, however, most of the world's desert areas
occur along the mid-latitudes where dry air descends along the
mid-latitude high pressure zones.