The general distribution of winds throughout the lower atmosphere is known as planetary winds.
These are of three types:
i) Trade winds
- Trade winds blow in a belt lying between 5°N-30°N in the northern hemisphere and 5°S-30°S in the southern hemisphere.
- As we all know that air travels through high pressure to low pressure. There is low pressure on the equator , while high pressure at the sub tropics. So, air moves from tropics toward equator. Due to corolis force and rotation of earth winds move toward left hand side in southern hemisphere and right hand side in northern hemisphere.
- Zones of sub-tropical highs in latitudes about 30°-35°N and 30°-35°S are areas of descending air and are characterize by calms light variable winds, comparatively dry air and quiet, stable weather conditions. These zones of latitudes are called Horse latitude.
- The Westerly winds blow across latitudes 35°-60° of both hemispheres.
- The air streams flowing pole wards from the Sub-tropical high pressure areas deflects eastward in the Northern he sphere to form South-westerlies.
- Similar winds in the Southern hemisphere are known as North-westerlies.
- Unlike the trade winds, the westerlies are very variable in force and direction especially in the Northern hemisphere.
- In the Southern hemisphere, on the other hand, the Westerlies blow with great strength and regularity throughout the year over the almost uninterrupted expanse of ocean and have given the name Roaring forties to the region specially between latitudes 40°S and 50°S.
- Sometimes the name is applied to the winds themselves as they give a roaring sound on account of high speed.
iii) Polar easterlies
The Polar easterlies blow from the Polar high pressure area to the Temperature low pressure area. On their equator ward journey they are deflected westward to become North easterlies in the Northern hemisphere and South easterlies in the Southern hemisphere.