Climate of India UPSC

Climate of  India

Weather is the momentary state of the atmosphere which changes quickly while climate refers to the average of the weather conditions over a longer period of time. 
Monsoon connotes the climate associated with seasonal reversal in the direction of wind. India has hot monsoonal climate which is the prevalent climate in south and southeast Asia. 

Factors Determining the Climate of India:

Factors related to location and relief

1. Latitude

• Northern part of the India lies in sub-tropical and temperate zone and the part lying south of the Tropic of Cancer falls in the tropical zone. 

• The tropical zone being nearer to the equator, experiences high temperatures throughout the year with small daily and annual range. 

• Area north of the Tropic of Cancer being away from the equator experiences extreme climate with high daily and annual range of temperature. 

2. Himalayan Mountains

• Himalayas in the north along with its extensions act as an effective climatic divide. 

• The towering mountain chain provides an invincible shield to protect the subcontinent from the cold northern winds.

• Himalayas also trap the monsoon winds, forcing them to shed their moisture within the subcontinent

3. Distribution of Land and Water: 

• India is flanked by the Indian Ocean on three sides in the south and girdled by a high and continuous mountain-wall in the north. 

• As compared to the landmass, water heats up or cools down slowly. This differential heating of land and sea creates different air pressure zones in different seasons in and around the Indian subcontinent.

• Difference in air pressure causes reversal in the direction of monsoon winds. 

4. Distance from the Sea:

• With a long coastline, large coastal areas have an equable climate. 

• Areas in the interior of India are far away from the moderating influence of the sea. Such areas have extremes of climate. 

5. Altitude

• Temperature decreases with height. Due to thin air, places in the mountains are cooler than places on the plains. 

6. Relief

• The physiography or relief of India also affects the temperature, air pressure, direction and speed of wind and theamount and distribution of rainfall. 

• The windward sides of Western Ghats and Assam receive high rainfall during June-September whereas the southern plateau remains dry due to its leeward situation along the Western Ghats. 

Factors Related to Air Pressure and Wind:

1. Distribution of air pressure and winds on the surface of the earth

2. Upper air circulation caused by factors controlling global weather and the inflow of different air masses and jet streams

3. Inflow of western cyclones generally known as disturbances during the winter season and tropical depressions during the south-west monsoon period into India, creating weather conditions favourable to rainfall. 

Mechanism of Weather in the Winter Season:

Surface Pressure and Winds:

• In winter months, the weather conditions over India are generally influenced by the distribution of pressure in Central and Western Asia.
• A high pressure centre in the region lying to the north of the Himalayas develops during winter.
• This centre of high pressure gives rise to the flow of air at the low level from the north towards the Indian subcontinent, south of the mountain range.
• The surface winds blowing out of the high pressure centre over Central Asia reach India in the form of a dry continental air mass.
• These continental winds come in contact with trade winds over north-western India.

Jet Stream and Upper Air Circulation:

• Higher up in the lower troposphere, about three km above the surface of the earth, a different pattern of air circulation is observed.
• The variations in the atmospheric pressure closer to the surface of the earth have no role to play in the making of upper air circulation.
• All of Western and Central Asia remains under the influence of westerly winds along the altitude of 9-13 km from west to east.
• These winds blow across the Asian continent at latitudes north of the Himalayas roughly parallel to the Tibetan highlands.
• These are known as jet streams.
• Jet streams are fast-moving winds in the upper atmosphere which influences the global atmospheric phenomenon. Jets streams play a key role in determining the weather because they usually separate colder air and warmer air.
• Jet streams generally push air masses around, moving weather systems to new areas and even causing them to stall if they have moved too far away.
• Climatologists say that changes in the jet streams are closely tied to global warming, especially the polar jet streams, because there is a great deal of evidence that the North and South poles are warming faster than the remainder of the planet.
• When the jets streams are warmer, their ups and downs become more extreme, bringing different types of weather to areas that are not accustomed to climate variations.
• Tibetian highlands acts a barrier In the path of Jet streams as a result it get bifurcated.
• One of its branch blow north of tibetian highland and southern branch blows in an eastward direction, south of the Himalayas there by influencing winter weather in India. 

Western Cyclonic Disturbance and Tropical Cyclones:

• The western cyclonic disturbances which enter the Indian subcontinent from the west and the northwest during the winter months, originate over the Mediterranean Sea and are brought into India by the westerly jet stream.
• An increase in the prevailing night temperature generally indicates an advance in the arrival of these cyclones disturbances.
• Tropical cyclones originate over the Bay of Bengal and the Indian ocean.
• These tropical cyclones have very high wind velocity and heavy rainfall and hit the Tamil Nadu, Andhra Pradesh and Orissa coast.
• Most of these cyclones are very destructive due to high wind velocity and torrential rain that accompanies.

Mechanism of Weather in the Summer Season

Surface Pressure and Winds:

• As the summer sets in and the sun shifts northwards, the wind circulation over the subcontinent undergoes a complete reversal at both, the lower as well as the upper levels.
• By the middle of July, the low pressure belt nearer the surface [termed as Inter Tropical Convergence Zone (ITCZ)] shifts northwards, By this time, the westerly jet stream withdraws from the Indian region.
• It is generally believed that there is a cause and effect relationship between the ITCZ and jet stream.
• The ITCZ being a zone of low pressure, attracts inflow of winds from different directions.
• The maritime tropical airmass (mT) from the southern hemisphere, after crossing the equator, rushes to the low pressure area in the general south-westerly direction.
• It is this moist air current which is popularly known as the southwest monsoon.

Jet Streams and Upper Air Circulation:

• An easterly jet stream flows over the southern part of the Peninsula in June, The easterlies normally do not extend to the north of 30 degree N latitude in the upper atmosphere. 

Easterly Jet Stream and Tropical Cyclones:

• The easterly jet stream steers the tropical depressions into India.
• These depressions play a significant role in the distribution of monsoon rainfall over the Indian subcontinent.
• The tracks of these depressions are the areas of highest rainfall in India.
• The frequency at which these depressions visit India, their direction and intensity, all go a long way in determining the rainfall pattern during the southwest monsoon period.

The Inter Tropical Convergence Zone (ITCZ):

• A low pressure zone located at the equator where trade winds converge, and so, it is a zone where air tends to ascend.
• In July, the ITCZ is located around 20°N-25°N latitudes (over the Gangetic plain), sometimes called the monsoon trough.
• This monsoon trough encourages the development of thermal low over north and northwest India.
• Due to the shift of ITCZ, the trade winds of the southern hemisphere cross the equator between 40° and 60°E longitudes and start blowing from southwest to northeast due to the Coriolis force.
• It becomes southwest monsoon.
• In winter, the ITCZ moves southward, and so the reversal of winds from northeast to south and southwest, takes place.
• They are called northeast monsoons.