Cold air

2. Cold air Convection cell Convection cell • At the poles (areas which receive less solar radiation), the air is cold and descends towards the equator through the lower layers of the atmosphere. • At the equator (the area which receives the most solar radiation), the air warms up, rises and displaces the upper layers of the atmosphere towards the poles. Hot air moves from the equator to the poles, through the upper layers of the atmosphere, while there is a current of cold air through the lower layers, from the poles to the equator. This circulation model is that of the convection cell.

3. Convection cells Area of high pressure Area of low pressure Area of high pressure Area of low pressure Area of high pressure Convection cells • The rotation of the Earth means that the air which circulates from the poles to the equator is displaced to the right in the northern hemisphere and to the left in the southern hemisphere. • This phenomenon gives rise to various partial circulations on the Earth, which generate areas of high pressure (the air descends) and areas of low pressure (the air rises up). Due to the rotation of the Earth, the model of atmospheric circulation is not that of a single convection cell, but rather six: three in the northern hemisphere and three in the southern hemisphere.

4. Polar easterly winds Westerly winds Trade winds • The displacement of the air flow to the right in the northern hemisphere and to the left in the southern hemisphere gives rise to three wind belts: • The trade winds at the tropics. • The prevailing westerly winds. • The polar easterly winds.