Answer: The Earth is surrounded by a thin layer of air called the atmosphere. The air in the atmosphere moves in response to differences in temperature at the equator (warm) and the poles (cold). This movement of air is called global atmospheric circulation. The movement of air across the planet occurs in a specific pattern that is dependent on:

• Latitudinal variation of atmospheric heating.
• The emergence of pressure belts.
• The migration of belts following the apparent path of the sun.
• The distribution of continents and oceans.
• The rotation of the Earth.

The whole system is driven by the equator, which is the hottest part of the Earth. Air rises at the equator, leading to the creation of low pressure and rainfall. When the air reaches the edge of the troposphere (tropopause), it cannot go any further and so it travels to the north and south. The air becomes colder and denser, and falls, creating high pressure and dry conditions at around 30° north and south of the equator. Large cells of air are created in this way. The following are the 3 major cells:

• The Hadley Cell: This is driven by warm rising air. The Coriolis effect causes the air that has risen at the equator and moving to the poles to deflect and become increasingly westerly high up in the atmosphere that the circulation breaks down and the air sinks back towards the ground in the subtropics (30° the ‘horse latitudes’). Once the air has descended back to the ground, it returns to the equator and is deflected to the east – they are known as the easterly trade winds.
• Polar Cell: The same thing happens in the polar regions, but it is driven by sinking cold air. The sinking, cold air at the poles moves back towards the equator, but because it is slow-moving, it becomes increasingly easterly as the underlying earth moves faster, and the circulation eventually breaks down at around 70° latitude. The air then rises again and returns to the poles, resulting in another atmospheric cell.

Ferrel Cell: This lies in between the Polar and Hadley cells, and is a little more complicated. Put simply, it is the net effect of air motions from all the storms or ‘depressions’ that occur in the mid-latitudes. Air sinks in the sub-tropics and rises around 60-70°. In this region, westerly surface winds occur. These cells drive airflow, atmospheric pressure, and rainfall. The rising and sinking of air cause high and low pressure at the surface, respectively. The large-scale winds of the atmosphere initiate large and slow-moving currents of the ocean. Oceans in turn provide input of energy and water vapour into the air. These interactions take place rather slowly over a large part of the ocean.