Pressure Belts - 02
SUB-POLAR LOW PRESSURE BELT
- Located between 45°N and S latitudes and the Arctic and the Antarctic circles (66.5° N and S latitudes).
- Owning to low temperatures in these latitudes the sub polar low pressure belts are not very well pronounced year long.
- On long-term mean climatic maps, the sub polar low-pressure belts of the northern hemisphere are grouped into two centers of atmospheric activity: the Iceland low and the Aleutian depression (Aleutian low).
- Such belts in the southern hemisphere surround the periphery of Antarctica and are not as well differentiated.
- These are dynamically produced due to
- Coriolis Force produced by rotation of the earth on its axis, and.
- Ascent of air as a result of convergence of westerlies and polar easterlies (we will more about these in next topic – wind systems).
- Sub polar low-pressure belts are mainly encountered above
- During winter, because of a high contrast between land and sea, this belt is broken into two distinct low centers – one in the vicinity of the Aleutian Islands and the other between Iceland and Greenland.
- During summer, a lesser contrast results in a more developed and regular belt.
- The area of contrast between cold and warm air masses produces polar jet streams which encircles the earth at 60 degrees latitudes and is focused in these low pressure areas.
Due to a great contrast between the temperatures of the winds from sub-tropical and polar source regions, extra tropical cyclonic storms or lows’ (temperate cyclones or frontal cyclones) are produced in this region.
POLAR HIGH PRESSURE BELT
- The polar highs are small in area and extend around the poles.
- They lie around poles between 80 – 90° N and S latitudes.
- The air from sub-polar low pressure belts after saturation becomes dry. This dry air becomes cold while moving towards poles through upper troposphere.
- The cold air (heavy) on reaching poles subsides creating a high pressure belt at the surface of earth.
- The lowest temperatures are found over the poles.
- SEASONAL WIND
- The climate of India is strongly influenced by monsoon winds.
- The sailors who came to India in historic times were one of the first to have noticed the phenomenon of the monsoon. They benefited from the reversal of the wind system as they came by sailing ships at the mercy of winds. The Arabs, who had also come to India as traders named this seasonal reversal of the wind system ‘monsoon’
- The monsoons are experienced in the tropical area roughly between 20° N and 20° S.
- To understand the mechanism of the monsoons, the following facts are important.
- (a) The differential heating and cooling of land and water creates low pressure on the landmass of India while the seas around experience comparatively high pressure.
- (b) The shift of the position of Inter Tropical Convergence Zone (ITCZ) in summer, over the Ganga plain (this is the equatorial trough normally positioned about 5°N of the equator – also known as the monsoontrough during the monsoon season).
- (c) The presence of the high-pressure area, east of Madagascar, approximately at 20°S over the Indian Ocean. The intensity and position of this high-pressure area affects the Indian Monsoon.