7. Planetary winds

The Planetary winds are also known as permanent winds or prevailing winds as they continuously blow in a particular direction throughout the year. Pressure belts control them. The planetary wind is of three types: the trade wind, the Westerlies and the Polar Easterlies.

 

In both the hemispheres, Trade winds blow from the subtropical high-pressure belt (East) to the Equatorial low-pressure belt (West). They blow with great regularity, force and in a constant direction throughout the year. Trade winds are named so because these winds were very helpful for ships of traders to travel west who depended on the winds while sailing in the seas.

Wind from the tropical high-pressure belts to subpolar low-pressure belts as variable westerlies in both the hemispheres. Under the effect of Coriolis force, they become the south-westerlies (blow from South West to North East ) in the northern hemisphere and north-westerlies (blow from North West to South East) in the southern hemisphere.

 

In the southern hemisphere, i.e. from the $40°S\ to\ 60°S$, Westerlies blow with much greater force throughout the year. These are called Roaring Forties at $40°$, Furious Fifties at $50°$ and Screaming Sixties at $60°$ latitudes.

Polar easterlies are weak, cold and dry polar winds that blow from the polar high-pressure belt to the subpolar (temperate) low-pressure belt. These winds blow from the North-East direction in the Northern Hemisphere and the South-East direction in the Southern Hemisphere.