Climate plays a decisive role in determining the types of vegetation and their zonation. Hence its study is necessary for a proper understanding of the ecology of forests. After dwelling briefly on the salient features of the climate of India in general we will take up the Western Ghats in particular, and analyze the variations of some of the major climatic parameters.
The large Eurasian (Europe and Asia) land mass and the equally large Indian Ocean result in differences between the heating capacities of the Eurasian landmass and the Indian Ocean. Because land heats up faster and cools down quicker than water, a seasonal reversal of winds occurs. This is called the monsoon from an Arabic word Mousim that describes a seasonal reversal of winds. Southwesterly winds [southwest monsoon] blow on shore in South Asia during the Northern Hemisphere summer while northeasterly winds [south dry monsoon] blow offshore during the Northern Hemisphere's winter.
As the sun begins its northward movement to its highest point (The Tropic of Cancer [wwtropic]) during the spring the land heats up rapidly. This intense heating creates lower pressure with rising air currents. Additional air blows in [southwest monsoon] from the ocean bringing precipitation. As the precipitation wave advances, first into the Western Ghats of India and then from the Bay of Bengal [southwest monsoon] into northern India, precipitation arrives and the dry fields become flooded soon. Some of the world's heaviest rains occur in monsoon climates. During one year at Cherripunji in northeastern India, 466 inches of rain was recorded-in one month! Eventually the rains stop as the season's change. Dry autumn gives way to a cool, dry winter when the huge Siberian High dominates all of the Eurasia's weather. Winds circulate from the northeast out of this high bringing South Asia a dry winter regime.
The monsoons determine the agriculture calendar of farmers throughout the region. Prior to the monsoon onset, rural life is slow as the earth is scorched under the intense sun with air temperatures often exceeding 115 degrees F. With the onset of rain, human activity resumes with soil preparation, weeding and other tasks associated with growing season accomplished into a very short time period. If the monsoon commences later than usual (the monsoon occurs on regular basis in South Asia), the consequences can be severe and can still result in famine to this densely populated region.
The winds from the northeast during the winter months are dry because they have lost their moisture on the Asian landmass. As these winds approach the southern tip of India, the location of the state of Tamil Nadu, they do pass over the Bay of Bengal and pick up moisture. TamilNadu then receives most of its rainfall during these months [rainfall-Madras]. Toward late spring and early summer the weather is hot and dry over most of the subcontinent.
September to March: Winds from the northeast dry winds because of rain shadow effect of Himalayas some rain in Tamil Nadu province of southeast India and wind pickup moisture over the Bay of Bengal.
As the land surface heats up air is drawn in from the Indian Ocean and Arabian Sea in the east. These winds pick up plenty of moisture and the rains fall first along India's western coast. Later the winds round the southern tip of India and are funneled up the Bay of Bengal into the delta area of the Ganges and Brahmaputra rivers. Later, the rains reach the upper Ganges valley and the capital city of New Delhi receives less moisture than the other areas mentioned and it arrives later in June and July. Locate the cities of Bombay, Calcutta, and New Delhi [sscities] and then check their monthly precipitation to confirm this pattern.
The Deccan Plateau to the east of the Western Ghats receives significantly less rainfall than the coasts. As the summer (wet) monsoons approach the West Coast of India, they rise up the western Ghats (mountains) and the air cools. This cool air is less able to hold moisture and it is released as rainfall. This is called OROGRAPHIC RAINFALL. Orographic means that it is related to mountains.
By the time the winds make it over the Western Ghats they have lost most of their moisture and very little falls on the Deccan Plateau to the east of the Ghats. This reduced rainfall on the leeward side (away from the wind) of mountains is called a RAIN SHADOW EFFECT.
June to September: Western Ghats produce OROGRAPHIC rainfall rain shadow effect on east side of Western Ghats little rain in the northwest because winds blow over little water due to the Arabian peninsula later (mid June) rains reach Ganges-Brahmaputra delta late June rains reach New Delhi.
The variations in temperature are also marked over the Indian sub-continent. During the winter seasons from November to February, due to the effect of continental winds over most of the country, the temperature decreases from South to North. The mean maximum temperature during the coldest months of December and January varies from 29 degree centigrade in some part of the peninsula to about 18 degree centigrade in the North, whereas the mean minimum varies from about 24 degree centigrade in the extreme South to below 5 degree centigrade in the North. From March to May is usually a period of continuous and rapid rise of temperature. The highest temperature occurs in North India, particularly in the desert regions of the North-West where the maximum may exceed 48 degree centigrade. With the advent of South West Monsoon in June, there is a rapid fall in the maximum temperature in the central portions of the country. The temperature is almost uniform over the area covering two thirds of the country which gets good rain. In August, there is a marked fall in temperature when the monsoon retreat from North Indian in September. In North-West India, in the month of November, the mean maximum temperature is below 38 degree centigrade and the mean minimum below 10 degree centigrade. In the extreme North, temperature drops below freezing point.
In the southern tip of the Indian peninsula rainfall is less than 2000 mm per year, which is not sufficient to support evergreen formations.The rain fall distribution shows two distinctive peaks -one around June and the other around October. A delayed arrival and an earlier withdrawal of the monsoon cuts short the rainy period in higher latitudes. Generally the maximum rainfall is in July, sometimes in August (Mahabaleshwar, 17 56N).Rainfall higher than 5000mm is found only to the north of 11o 15N,except for the western slopes of Anaimalai, Palni and Nilgiri ranges whose high elevations favour condensation.From the Wyanad region almost continuously up to Mahabaleshwar,the annual rainfall exceeds 5000mm over the entire region between the foot of the Ghats and the Western edge of the plateau..The amount of rainfall may be considered in some places: thus Agumbe, although situated at an altitude of only 645m receives a mean annual rainfall of 7640 mm concentrated over 128 days.This station received an average of 2427 mm in July and 2124 mm in August. The absolute maximum rainfall far a single month is 4508 mm (in August 1946). The Nilgiris shows that the increase in rainfall from 2400 mm to more than 6000 mm corresponds to a marked elevation of the hill range (here the Ghats rise up to 2500m). However after crossing this peak, the rainfall decreases abruptly and is only 2000 mm 5 Km from the plateaus edge. A major part of the high Nilgiri plateau receives scanty rainfall of 1500 to 1000 mm. Precipitation is much greater at the latitude of Agumbe already exceeding 4000 mm near the coast, it increases steadily and rapidly towards the interior. The Ghats, although not very high in this region (650m), has nevertheless considerable influence :precipitation increases from 5900 to 7500 mm in 6 km. The decrease over the plateau is also very rapid: from 7500 to 4000 mm in 15 km and 2000 mm in 50 km. It is remarkable that despite the low elevation of the Ghats and the high rainfall in this region. The transect at the latitude of Goa is very similar to that of Agumbe. The maximum rainfall is slightly less than 6000 mm isohyete is attained 25 Km.
In the western Ghats wind pays a major role in governing the climate as it determines the alteration of seasons. In summer they bring large masses of water, the condensation of which cause the monsoon rains. They are also responsible for the drying up and cooling of the peninsula in winter.
During the dry season (Dec to March), the dominant winds are from north -east /or east corresponding to the dry trade winds which blow over India by pushing back the inter-tropical front of convergence. These winds commence from November itself. Their main direction varies according to the stations. They are essentially from the east Belgaum, Hassan, Plaghat and Kozhikode. The NE winds are also frequent in Gadag, Shimoga, Mysore and Coimbatore and are dominant in Ootacamund and Agumbe.
In autumn, the direction is reversed once again and the winds are from northeast and east. This phenomenon is quite clear in Ootacamund and Agumbe from month corresponds more to a transitional period similar to the situation in March or April. However, from November onwards, the winds from the east and or north -east dominate everywhere.
The winds are never very violent in these regions. During the period of 10 years that have been taken into consideration, the frequency of speed exceeding 61 Km/h is nil. Even velocities of 20 -61 km/h are exceptional: their frequency does not exceed 10% except during the monsoon months (June -July - August) in some stations situated towards the interior such as Coimbatore, Mysore, Gadag where, however they do not exceed 30 %.
In the coastal stations it is slightly different, as the most violent winds blow in the afternoon in April, May, rather than in June-July. The average values are generally between 3.8 Km/h (Shimoga) and 8.8 Km/h (Mysore). The speed is a little higher (between 10 and 11 km/h) near the Palghat Gap (the only discontinuity in the Ghats) where the wind blows more violently between the Palni and Nilgiri horsts (Kozhikode, Palghat, and Coimbatore). The afternoon winds are stronger than the morning winds near the coast, as well as in the interior of the plateau.