• the near-spherical shape of earth and the north/south movements of the sun relative to the earth as the seasons progress
  
  
• marked differences in heating of land and sea surfaces
  
  
• the rotation of the earth which causes winds to be deflected to the right in the northern hemisphere and to the left in the southern hemisphere. This is known as the Coriolis effect and results in the formation of giant eddies (cyclones and anticyclones).

Figure 1. This diagram shows three major convective cells between the equator and the pole. Easterly winds predominate near the equator and also in the lower atmosphere at the poles. Elsewhere westerlies are dominant and they reach a peak in jet streams (shown in red) where wind speeds of 200 to 300 km/h are not uncommon. The major pressure and wind zones are shown within the circle representing the earth.

Figure 2. This diagram is the same as the one above except that the major pressure and wind zones have been replaced by a typical isobaric weather map.

These factors also give rise to seasonal monsoons, jet streams in the upper atmosphere, and other complications, as well as the giant eddies (cyclones and anticyclones) in middle latitudes.

The diagrams also show major pressure and wind zones. As we move from the equator towards the poles they are:

The trade winds from both hemispheres converge towards the doldrums and a zone of low pressure, the equatorial trough, that girdles the earth. Air settles gently in the subtropical high pressure belt and spirals outwards, some of it moving equatorward and joining the trades, some of it moving poleward and joining the westerlies. All the zones move northwards and southwards and change in nature with the seasons. These diagrams give a very simplified picture of day to day wind and weather patterns. For example, the ascent of air in the doldrums is far from uniform. It occurs mainly in cells and groups of cells which give rise to areas of showers and thunderstorms and sometimes tropical storms which are interspersed with large areas of much more settled weather.