Geography

How do pressure and surface winds affect the climatic conditions of a particular place? What other factors contribute to it?


SOLUTION
Winds move from a high-pressure area to a low-pressure area. During winter, there is a high-pressure area north of the Himalayas. Cold dry winds blow from this region to the low-pressure areas over the oceans to the south. In summer, a low-pressure area develops over interior Asia as well as over northwestern India. As a result, the low-pressure system attracts the southeast trade winds of the southern hemisphere. On crossing the equator, these trade winds-due to the Coriolis force-turn right towards the low-pressure areas over the Indian subcontinent. After crossing the equator, these winds start blowing in a south-westerly direction, and enter the Indian peninsula as the southwest monsoon. These are known as the South-west Monsoon winds. These winds blow over the warm oceans, gather moisture and bring widespread rainfall over the mainland of India. The upper air circulation in this region is dominated by a westerly flow. The rainfall received by India is largely due to the south-west monsoon winds. The duration of the monsoon is between 100 to 120 days. Hence, the bulk of rainfall received by the country is concentrated over a few months.
Other factors affecting the climatic conditions are:
(i) Coriolis force. The seasonal reversal of wind direction over the Indian subcontinent is the result of the Coriolis force. India lies in the region of north easterly winds. These winds originate from the subtropical high-pressure belt of the northern hemisphere. They blow south, get deflected to the right due to the Coriolis force, and move on towards the equatorial low-pressure area. Coriolis force also known as 'Ferrel's Law' is a force caused by the earth's rotation. It deflects the north-east trade winds towards right in the Northern Hemisphere and left in the Southern Hemisphere.
(ii) Jet Streams. They are a narrow belt of high altitude winds in the troposphere. Their speed varies from about 110 km/h in summer to about 184 km/h in winter. The movements of the westerly Jet Streams to the north of the Himalayas and the presence of the tropical easterly Jet Streams over the Indian Peninsula during the summer affects monsoon. Usually while the tropical eastern South Pacific Ocean experiences high pressure, the tropical Eastern Indian Ocean experiences low pressure. But in certain years, there is a reversal in the pressure conditions and the eastern Pacific has lower pressure in comparison to the eastern Indian Ocean. This periodic change in pressure conditions is known as the Southern Oscillation (SO).
(iii) Western cyclonic disturbances. The sub-tropical westerly Jet Stream blowing south of the Himalayas are responsible for the western cyclonic disturbances experienced in the north and north-western parts of the country during the winter months. Western disturbance is the term used in India, Pakistan, Afghanistan and Nepal to describe an extra tropical storm originating in the Mediterranean that brings sudden winter rain and snow to the northwestern parts of the Indian subcontinent. This is a non-monsoonal precipitation pattern driven by the Westerlies. The moisture in these storms usually originates over the Mediterranean Sea and the Atlantic Ocean.
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