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The Cold Weather Season
1-Temperature : Usually, the cold weather season sets in by mid-November in northern India.
December and January are the coldest months in the northern plain.
There are three main reasons for the excessive cold in north India during this season :
(i) States like P unj ab, Haryana and Rajasthan being far away from the moderating influence of sea experience continental climate.
(ii) The snowfall in the nearby Himalayan ranges creates cold wave situation;
(iii) Around February, the cold winds coming from the Caspian Sea and Turkmenistan bring cold wave along with frost and fog over the northwestern parts of India
The Peninsular region of India, however, does not have any well-defined cold weather season. There is hardly any seasonal change in the distribution pattern of the temperature in coastal areas because of moderating influence of the sea and the proximity to equator.
2- Pressure and Winds : By the end of December (22nd December), the sun shines vertically over the Tropic of Capricorn in the southern hemisphere. The weather in this season is characterised by feeble high pressure conditions over the northern plain.
3-Rainfall : Winter monsoons do not cause rainfall as they move from land to the sea. It is because firstly, they have little humidity; and secondly, due to anti cyclonic circulation on land, the possibility of rainfall from them reduces. So, most parts of India do not have rainfall in the winter season.
However, there are some exceptions to it
(i) 1-In northwes tern India, some weak temperate cyclones from the Mediterranean sea cause rainfall in Punjab, Haryana, Delhi and western Uttar Pradesh. Although the amount is meagre, it is highly beneficial for rabi crops. The precipitation is in the form of snow fall in the lower Himalayas. It is this snow that sustains the flow of water in the Himalayan rivers during the summer months.
(ii) During October and November , northeast monsoon while crossing over the Bay of Bengal, picks up moisture and causes torrential rainfall over the Tamil Nadu coast, southern Andhra Pradesh, s outheast Karnataka and southeast Kerala.
The Hot Weather Season
1. Temperature: With the apparent northward movement of the sun towards the Tropic of Cancer in March, temperatures start rising in north India. April, May and June are the months of summer in north India.
In the coastal regions, the north-south extent ofisotherms parallel to the coast confirms that temperature does not decrease from north to south rather it increases from the coast to the interior.
2. Pressure and Winds: The summer months are a period of excessive heat and falling air pressure in the northern half of the country. Because of the heating of the subcontinent, the ITCZ moves northwards occupying a position centered at 25°N in July.
THE SOUTHWEST MONSOON SEASON
increase of temperature in May over the northwestern plains, the low pressure conditions over there get further intensified. By early June, they are powerful enough to attract the trade winds of Southern Hemisphere coming from the Indian Ocean. These southeast trade winds cross the equator and enter the Bay of Bengal and the Arabian Sea, . Passing over the equatorial warm currents, they bring with them moisture in abundance. After crossing the equator, they follow a southwesterly direction. That is why they are known as southwest monsoons.
The rain in the southwest monsoon season begins rather abruptly. One result of the first rain is that it brings down the temperature substantially. This sudden onset of the moisture-laden winds associated with violent thunder and lightening, is often termed as the “break” or “burst” of the monsoons.
The monsoon approaches the landmass in two branches:
(i) The Arabian Sea branch
(ii)The Bay of Bengal branch.
Monsoon Winds of the Arabian Sea
The monsoon winds originating over the Arabian Sea further split into three branches:
(i) Its one branch is obstructed by the Western Ghats. These winds climb the slopes of the Western Ghats from 900-1200 m. Soon, they become cool, and as a result, the windward side of the Sahyadris and Western Coastal Plain receive very heavy rainfall ranging between 250 cm and 400 cm. After crossing the Western Ghats, these winds descend and get heated up. This reduces humidity in the winds. As a result, these winds cause little rainfall east of the Western Ghats. This region of low rainfall is known as the rain-shadow area.
(ii) Another branch of the Arabian sea monsoon strikes the coast north of Mumbai. Moving along the Narmada and Tapi river valleys, these winds cause rainfall in extensive areas of central India. The Chotanagpur plateau gets 15 cm rainfall from this part of the branch. Thereafter, they enter the Ganga plains and mingle with the Bay of Bengal branch.
(iii) A third branch of this monsoon wind strikes the Saurashtra Peninsula and the Kachchh. It then passes over west Rajasthan and along the Aravalis, causing only a scanty rainfall.
Monsoon Winds of the Bay of Bengal
The Bay of Bengal branch strikes the coast of Myanmar and part of south east Bangladesh. But the Arakan Hills along the coast of Myanmar deflect a big portion of this branch towards the Indian subcontinent. The monsoon, therefore, enters West Bengal and Bang ladesh from south and south east instead of from the south-westerly direction From here, this branch splits into two under the influence of the Himalayas and the thermal low is northwest India. Its one branch moves westward along the Ganga plains reaching as far as the Punjab plains. The other branch moves up the Brahmaputra valley in the north and the northeast, causing widespread rains. Its subbranch strikes the Garo and Khasi hills of Meghalaya . Mawsynram, located on the crest of Khasi hills, receives the highest average annual rainfall in the world.
Here it is important to know why the Tamil Nadu coast remains dry during this season.There are two factors responsible for it:
(i) The Tamil Nadu coast is situated parallel to the Bay of Bengal branch of southwest monsoon.
(ii) It lies in the rainshadow area of the Arabian Sea branch of the south-west monsoon.
Characteristics of Monsoonal Rainfall
Season of Retreating Monsoon
Distribution of Rainfall
The average annual rainfall in India is about 125 cm, but it has great spatial variations
Climatic Regions of India
Major climatic types of India based on Koeppen‘s scheme: Koeppen based his scheme of Climatic classification on monthly values of temperature and precipitation. He identified five major climatic types, namely:
(i) Tropical climates, where mean monthly temperature throughout the year is over 18°C.
(ii) Dry climates, where precipitation is very low in comparison to temperature, and hence, dry. If dryness is less, it is semi arid (S); if it is more, the climate is arid(W).
(iii) Warm temperate climates, where mean temperature of the coldest month is between 18°C and minus 3°C.
(iv) Cool temperate climates, where mean temperature of the warmest month is over 10°C, and mean temperature of the coldest month is under minus 3°C.
(v) Ice climates, where mean temperature of the warmest month is under 10°C.