Figure 18 – Planetary winds

♤ The prevailing westerlies are relatively more variable than the trade winds both in direction and intensity. There are more frequent invasions of polar air masses along with the travelling cyclones and anti-cyclones. These moving cells of low and high pressures largely affect the movement of westerlies. The westerlies are stronger in the cold. In the southern hemisphere, westerlies are so powerful and persistent due to absence of land between 400- 600 S that these are called ‘roaring forties’, ‘furious fifties’ and ‘screaming sixties’ along 400 S, 500 S and 600 S latitudes.

♤ Winds move away from polar high pressure to sub-polar low pressure along the surface of the earth in Polar cell. Their direction becomes easterlies due to coriolis force. These are called polar easterlies.

♤ Winds coming from the sub-tropical and the polar high belts converge to produce cyclonic storms or low pressure conditions. This zone of convergence is also known as polar front (see fronts and cyclones).