FACTORS AFFECTING INDIA’S CLIMATE
- The Tropic of Cancer passes through the middle of the country from the Rann of Kuchchh in the west to Mizoram in the east
- Almost half of the country, lying south of the Tropic of Cancer, belongs to the tropical area
- All the remaining area, north of the Tropic, lies in the sub-tropics.
- Therefore, India’s climate has characteristics of tropical as well as subtropical climates.
- India has mountains to the north, which have an average height of about 6,000 metres.
- India also has a vast coastal area where the maximum elevation is about 30 metres.
- The Himalayas prevent the cold winds from Central Asia from entering the subcontinent.
- It is because of these mountains that this subcontinent experiences comparatively milder winters as compared to central Asia.
Pressure and Winds
- The climate and associated weather conditions in India are governed by the following atmospheric conditions:
- Pressure and surface winds;
- Upper air circulation;
- Western cyclonic disturbances and tropical cyclones.
- 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.
- Generally, these winds carry very little moisture as they originate and blow over land.
- Therefore, they bring little or no rain. Hence, India should have been an arid land, but, it is not so
- An apparent force caused by the earth’s rotation. The Coriolis force is responsible for deflecting winds towards the right in the northern hemisphere and towards the left in the southern hemisphere. This is also known as ‘Ferrel’s Law’.
- 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 north-western India.
- This causes a complete reversal of the direction of winds during summer.
- Air moves from the high-pressure area over the southern Indian Ocean, in a south-easterly direction, crosses the equator,
- Turns right towards the low-pressure areas over the Indian subcontinent.
- These are known as the Southwest 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. An important component of this flow is the jet stream
- These jet streams are located approximately over 27°-30° north latitude, therefore, they are known as subtropical westerly jet streams
- Over India, these jet streams blow south of the Himalayas, all through the year except in summer.
- The western cyclonic disturbances experienced in the north and north-western parts of the country are brought in by this westerly flow.
- In summer, the subtropical westerly jet stream moves north of the Himalayas with the apparent movement of the sun.
- An easterly jet stream, called the tropical easterly Jetstream blows over peninsular India, approximately over 14°N during the summer months.
- These are a narrow belt of high altitude (above 12,000 m) westerly winds in the troposphere. Their speed varies from about 110 km/h in summer to about 184 km/h in winter. A number of separate jet streams have been identified. The most constant are the mid-latitude and the sub-tropical jet stream.
Western Cyclonic Disturbances
- The western cyclonic disturbances are weather phenomena of the winter months brought in by the westerly flow from the Mediterranean region.
- They usually influence the weather of the north and north-western regions of India
- Tropical cyclones occur during the monsoon as well as in October -November, and are part of the easterly flow.
- These disturbances affect the coastal regions of the country.
THE INDIAN 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.
- 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.
- 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 monsoon trough during the monsoon season).
- 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.
- The Tibetan plateau gets intensely heated during summer, which results in strong vertical air currents and the formation of high pressure over the plateau at about 9 km above sea level.
- The movement of the westerly jet stream to the north of the Himalayas and the presence of the tropical easterly jet stream over the Indian peninsula during summer.
Inter Tropical Convergence Zone
- The Inter Tropical Convergence Zone (ITCZ,) is a broad trough of low pressure in equatorial latitudes.
- This is where the northeast and the southeast trade winds converge.
- This convergence zone lies more or less parallel to the equator but moves north or south with the apparent movement of the sun.
- Changes in the pressure conditions over the southern oceans also affect the monsoons.
Southern Oscillation or SO
- Normally when 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.
- If the pressure differences were negative, it would mean below average and late monsoons.