Planetary or permanent winds blow from high pressure belts to low pressure belts in the same direction throughout the year. They blow over vast area of continents and oceans. They are easterly and westerlies and polar easterlies. (see fig.)
(i) The Easterlies
- The winds that blow from sub-tropical high pressure areas towards equatorial low pressure areas called trade or easterly winds.
- The word trade has been derived from the German word ‘trade’ which means track.
- To blow trade means ‘to blow steadily and constantly in the same direction’.
- Because of the Coriolis effect the northern trade winds move away from the subtropical high in north-east direction.
- In southern hemisphere the trade winds diverge out of the sub-tropical high towards the equatorial low from the southeast direction
- As the trade winds tend to blow mainly from the east, they are also known as the Tropical easterlies. (see fig.)
(ii) The Westerlies
- The winds that move poleward from the sub-tropical high pressure in the northern hemisphere are deflected to the right and thus blow from the south west.
- These in the southern hemisphere are deflected to the left and blow from the north-west. Thus, these winds are called westerlies (see fig.)
(iii) Polar Easterlies
- Polar easterlies blow from polar regions towards sub-polar low pressure regions.
- Their direction in the northern hemisphere is from north-east to southwest and from south-east to north-west in the southern hemisphere.