How Does the Global Air Circulate?

Answer:

An excess of heat build-up over the equator causes convection. As this hot air rises, it forms a band of low pressure cells over the equator. Collectively, these form the Inter Tropical Convergence Zone (ITCZ) which is effectively the "heat equator" of the earth.

Surface winds along the ITCZ are generally light, giving rise to the region being called the "doldrums".

Air rising over the equator cools and settles back to the earth's surface over the mid-latitudes, forming the semi-permanent anti-cyclones (an area known as the "horse latitudes"). These systems in turn give rise to the trade winds north and south of the equator.

Cold polar and warm equatorial air masses meet along a broad front between the mid-latitudes and the poles, causing air to rise and so form the mid-latitude cyclones. These frontal systems form a broad band of low pressure cells along these latitudes.

Extremely cold, dry air sinks over the poles forming the permanent polar high pressure areas.

All of the above systems shift north and south seasonally, following the ITCZ heat equator.