Climate of India

The climate of India belongs to the 'Tropical monsoon type'. Although a sizeable part of the country lying north of the tropic of Cancer falls in the northern temperature zone but the shutting effects of the Himalayas and the existence of the Indian Ocean have played significant role in giving India a distinctive characteristics of Salient features of the Indian climate:
1. Seasonal Reversal of winds
Winter season Winds blow from NE to SW
Summer season Winds blow from SW to NE
2. Formation of Alternatively High and Low Pressure Areas over the land-
Winter season due to low temperature conditions high pressure area is formed. Summer season Intense heating of the land leads to the formation of thermally induced low pressure cell over NW part of the country.
3. Seasonal and Variable Rainfall
Over 80% of annual rainfall is obtained during the five month of the rainy season.
There is variability in rainfall so far time and place are considered.
There is considerable spatial variation in the general distribution of rainfall.
4. Plurality of Seasons That is constantly changing weather conditions
5. Characterized by National Calamities.-The word 'Monsoon' is derived from the Arabic word 'Mausim' Monsoon is flow pattern of the general atmosphere circulation over a wide geographical area, in which there is a clearly dominant wind in one direction in every port of the region concerned, but in which this prevailing direction is reversed (or almost reversed) from winter to summer and from summer to winter."

Concepts of the origin of Monsoon
1. Thermal concept: From Classical Theory of Hally (1686)
- Generated by the differential seasoned heating of continental and oceanic areas. High pressure is developed over the continent (near Lake Baikal and Peshawar), where low pressure over southern Indian ocean. Therefore outflow of air from the high pressure land areas to the low pressure areas resulting into NE Monsoon

2. Aerological Concept - Given by a German Meterologist R. Seherhag (1948)
According to him the changes in the direction of winds at all levels in the atmosphere are directly related to the temperature changes in the air above the friction layer.

3. Dynamic Concept-Propounded by Flohn (1951): Based on the dynamic origin of monsoons. According to him monsoon is the seasonal migration of planetary winds and presure belts following the sun. Over the land the annual temperature changes are relatively larger because of which the seasonal shifts of temperature and pressure belts amount to many degrees. 4 Due to the shifting the major part of the Indian subcontinent comes under impact of Equatorial Westerlies. During winter due to southward shifting of pressure and wind belt (he planetary system of northeast trade winds is established over the region. Hence this theory explains the existence of monsoon not by the temperature, contrasts between land and sea, but by the annual migration of thermally produced planetary winds and pressure belts.
4. Recent Concepts
(a) Jet Streams
Jet streams are high altitude geostrophic winds (i.e. blowing parallel to equator) blowing between middle latitude at high speed in a meandering course.
During winter season the upper air westerly jet streams are positioned in Asia. These are bifurcated in two branches due to Tibet Himalayan obstruction. North branch blows north of Himalayas and the Tibetan Plateau. Southern branch blows south of the mighty mountains.
The southern branch inscribes an anticyclone (Clockwise) arc across Afghanistan followed by a cyclonic (Anticlockwise) are along the southern flank of the Himalayas. A high pressure system gets formed south of the jet stream over Afghanistan and NW Pakistan where air tends to subside leading to atmospheric stability and dry conditions there by causing NE winter monsoons. The jet stream helps disturbances in the NW of the subcontinent, which tend to follow paths immediately beneath the jet stream. These disturbances move long the eastern Mediterranean and into NW India appearing here as waves rather then as well developed frontal cyclones.
During summer season as sun falls vertically over the Tropic of Cancer the polar surface high pressure is weakened and upper air circum polar whirl shift northward as a result of which the upper air westerly jet are also withdrawn from southern slopes of the Himalayas.
The removal of jet stream to north of the Tibetan plateau results in reversal of the curvature of How of free air to the north and north west of the subcontinent. This event may well be the trigger that sets off the 'burst' of the monsoon.
(b) Tibet Plateau
4 In 1973, the Monsoon Expedition (MONEX) was organized under the joint auspices of the erstwhile Soviet Union and India. Experiments concluded that summer time heating of Tibetan Highland plays a dominant role in the origin of Monsoon circulation. 4 Due to its protected height Tibetan plateau receives 23°C more insolation than the neighboring areas. The plateau affects the atmosphere in two ways (a) as a mechanical barrier and (b) as a high level heat sources. Infact the plateau accentuates the northland displacement of the jet stream. 4 The summer time heating of the Tibetan Plateau makes it a high level heat source, which produces thermal anticyclone over this region, thereby weakening the western subtropical jet stream south of the Himalayas and intensifying the move of S.W monsoon.
(c) Effect because of ocean
El Nino a warm ocean current appears along the Peru coast in December. It replaces the Peru or Humboldt Cold Ocean current flowing over this region during normal years. Under normal times the layer over the eastern Pacific is cool and shallow, while over the western Pacific it is warm and deep. Such conditions are helpful for strong southwest monsoons. The appearance of El Nino reverses the conditions (warm condition over eastern Pacific and cold in western Pacific). Since El Nino represents large atmospheric perturbations to which the ocean responds with warm of colder surface temperature, it lands to extreme events, such as drought, flood and poor monsoons. The Southern Oscillation is the name ascribed to a seesaw pattern of meteorological changes that are often observed between the Pacific, the pressures over the Indian Ocean tend to be low, and vice versa. The oscillation was discovered by Sir Gilber Walker and is therefore also known as "Walker circulation". The oscillation has a period varying from 27 years. The intensity of the Southern Oscillation is measured by the difference in sea level pressures of Tahiti and Port Darwin
El Nino Southern Oscillations
(d) The Somali Current: It is one of the few currents, which reverse its direction with the overlying wind. Summer Flows northward Winters Flows southward

Weather conditions
Cold Weather Season Southerly branch of the jet stream occupies its position south of the Himalayas, which is accompanied with the restoration of light northeast trade winds (monsoons) to the surface, withdrawal of the inter tropical convergence zone, formation of anti cyclonic cell over north western India and dry weather prevailing, over most of the areas in the country

Temperature conditions: General increase of temperature from North to South, Isotherms run almost parallel to the latitudes (in January the 21 °C isotherm runs through the middle of the country connecting Tapti estuary to the Mahanadi delta) in the east. West India Punjab, Haryana West U.P and Northern Rajasthan Less than 15°C.
In South India the isotherm, tend to bend southward and run parallel to the coast. The western coast is warmer than the eastern one by about LT C. This season is characterized by the inflow of depression from the west and the North West. These low pressure systems originate in West Asia near the Mediterranean Sea and are known as Western Disturbances. Their average frequency is four to five depressions per month and highly intensified Between December and February. (Rainfall due to these disturbances is highly helpful for RABI crops) Fine weather, clear skies, low humidity, absence of rainfall, low temperature and a large diurnal variation in it are the usual features of the winter season.

-North East parts of India also get some rainfall during this season.
A low pressure area occupies the northern parts of the Bay of Bengal during October, which moves southward and get deflecting towards, the coromandal coast thereby producing rains on this coast. The presence of inter tropical convergence and the easterly depression are responsible for these rains. Hot and Dry Weather is characterized by low pressure system high temperature, unstable pressure and wind circulation. -The dust storms of Punjab and Haryana, the Loos of UP, the Norwesters (Kalbaisakhis) of W. Bengal and cyclonic depressions of the eastern coast produce a stormy and turbulent weather.

-The rains caused by thunderstorm in Karnataka are called 'Cherry Blossoms'. These are beneficial for coffee plantation. Elsewhere in South India they are known as 'Mango Showers'
- Dry and dusty westerly winds flow in the northern western parts of the country which make the outdoor life difficult are known as Loo. The Wet Season: The southern branch of the western jet is withdrawn from south of the Himalaya thereby leading to the formation of a dynamic depression over the surface thermal low. The ITC shifts northwards allowing equatorial westerlies to in the subcontinent.

- Indian subcontinent receives bulk of its rainfall (around 80%) from the southwest
- The Arabian Sea current causes rainfall all along the Western Coast, Western Ghats, Gujarat, Maharashtra, parts of M.P and Rajasthan.
- While crossing the Sahyadris (Western Ghats), the monsoonal current produce heavy rainfall on the windward and scanty rainfall on the leeward side thereby producing a rain shadow area. The rainfall is also erratic on the leeward side, which results in frequent drought in Maharashtra and Karnataka

-The Tamil Nadu coast goes dry in this season.
- The Arabian Sea branch meets the Bay of Bengal branch over ChhotaNagpur Plateau producing copious rainfall.
-Absence of moutain barrier in Kutch, parallel position of the Aravalli, effect of the hot and dry air results in failure of Arabian Sea branch to produce adequate rainfall. The Bay of Bengal branch, obstructed by the eastern hill is deflected westward towards the Ganga Plain. Entrapped in the valleys of Meghalaya, the current produces very heavy rainfall [Cherapunji (1087cm) and Mawsynram (1141 cm)]
- The weather is also affected by a number of cyclonic depressions entering the country through the Bay of Bengal and the Arabian Sea. About 20 to 25 such depressions develop during monsoon period.
- With the exception of J & K and Parts of Tamil Nadu, most of the country receives heavy rainfall.

Season of Retreating Monsoon

- South West begins to retreat from the second or third week of September.
- Unlike the Sudden burst, the retreat is highly gradual.
-The southerly branch of the jet stream returns to its winter position by October and this is
accompanied by the restoration of light North East trade winds to the surface.
- Cloudiness and moisture are low except in the southern parts of the Peninsula.
- It is this retreating monsoon which brings rain to the Tamil Nadu coast as North East Monsoon.