Climate Notes on chapter 4 Geography Climate ० Climate refers to the sum total of weather conditions and variations over a large area for a ...
Climate
Notes on chapter 4 Geography
Climate
० Climate refers to the sum total of weather conditions and variations over a large area for a long period of time (more than thirty years).
० Weather refers to the state of the atmosphere over an area at any point of time.
० The elements of weather and climate are temperature, atmospheric pressure, wind, humidity and precipitation.
* Precipitation is any form of moisture which falls to the earth. It includes rain, snow, hail and sleet.
० The climate depends upon
1) Latitude - temperature generally decreases from the equator towards the poles due to angle of sunlight.
2) Altitude - The atmosphere becomes less dense and temperature decreases when we go to the higher altitude.
3) Pressure and wind system - depends on altitude and latitude, it influences the temperature and rainfall pattern.
4) Distance from the Sea - The sea exerts a moderating influence on the climate.so coastal areas have moderate climate.
5) Ocean Currents - Along with onshore winds, the ocean currents (warm or cold) affect the climate of coastal areas.
6) Relief features - Relief plays a major role in determining the climate of a place. High mountains act as barriers for cold or hot winds.
० Factors affecting India's climate:
1) Latitude - The Tropic of Cancer passes through the middle of the country, so India’s climate has characteristics of tropical as well as subtropical climates.
2) Altitude - The Indian sub-continent experiences milder winters as compared to Central Asia because of the Himalayas which prevent the cold winds from entering the sub-continent.
3) Pressure and Winds
The climate and weather conditions in India are governed by the following atmospheric conditions:
० Pressure and surface winds- India lies in the region of North-Easterly surface winds. These winds originate during winter from the sub-tropical high-pressure belt of the Northern hemisphere.
In summer, due to high temperature, low-pressure area develops over interior Asia and over North-Western India. Air from high-pressure areas blow towards this region resulting in complete reversal of wind direction.
० Upper air circulation - The upper air circulation of the Indian subcontinent is dominated by a westerly flow which is governed by Jet stream (Jet streams are fast flowing, narrow, meandering air currents in the atmosphere) Due to their location over 27°-30° N latitude, these jet streams are known as sub-tropical westerly jet streams. They blow South of the Himalayas, throughout the year except in summer.
० Western cyclonic disturbances and tropical cyclones - 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.
The Indian Monsoon
The climate of India is strongly influenced by monsoon winds. The seasonal reversal of the wind system is known as “monsoon”. The monsoons are experienced in the tropical area roughly between 20° N and 20° S.
Mechanism of Monsoon
Factors that affects the mechanism of India's monsoon
० 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.
० The shift of the position of Inter-Tropical Convergence Zone (ITCZ) in summer, over the Ganga plain, is also known as the monsoon-trough during the monsoon season.
० The presence of the high-pressure area, east of Madagascar, approximately at 20°S over the Indian Ocean affects the Indian Monsoon.
० The Tibetan plateau gets intensely heated during summer, which results in strong vertical air currents and the formation of low pressure over the plateau at about 9 km above sea level.
० 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 also impact the Indian Monsoon.
० The changes in the pressure conditions over the southern oceans also affect the monsoons. The irregularly periodic variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean that affects the climate of the tropics and subtropics is known as Southern Oscillation or SO.
EL Nino Southern Oscillations (ENSO)
The EL Nino phenomenon is a feature connected with the Southern Oscillation. In this, a warm ocean current flows past the Peruvian Coast, in place of the cold Peruvian current. It occurs at the interval of 2 to 5 years.The changes in pressure conditions are connected to the EL Nino. Hence, the phenomenon is referred to as ENSO (EL Nino Southern Oscillations).
It is computed to predict the intensity of the monsoons.If the pressure differences were negative, it would mean below average and late monsoons.
The Onset of the Monsoon and Withdrawal
The duration of the monsoon is between 100-120 days from early June to mid-September. Around the time of its arrival, the normal rainfall increases suddenly and continues constantly for several days, which is known as the “burst” of the monsoon.
The monsoon arrives at the southern tip of the Indian peninsula generally by the 1st week of June. Subsequently, it proceeds into two – the Arabian Sea branch and the Bay of Bengal branch.
The Arabian Sea branch reaches Mumbai on approximately the 10th of June.The Bay of Bengal branch arrives in Assam in the 1st week of June.By mid-June, the Arabian Sea branch of the monsoon arrives over Saurashtra-Kuchchh and the central part of the country. The Arabian Sea and the Bay of Bengal branches of the monsoon merge over the northwestern part of the Ganga plains. Delhi receives the monsoon showers from the Bay of Bengal branch by the end of June. By the first week of July, western Uttar Pradesh, Punjab, Haryana and eastern Rajasthan experience the monsoon.By mid-July, the monsoon reaches Himachal Pradesh and the rest of the country.
Withdrawal of Monsoon
Withdrawal or the retreat of the monsoon is a more gradual process. The process begins by early September in North-Western states. By mid-October, it withdraws completely from the Northern half of the peninsula.The withdrawal from the Southern half of the peninsula is fairly rapid. By early December, the monsoon has withdrawn from the rest of the country.
The Seasons
There are basically four seasons identified in India.
1. The Cold Weather Season (Winter)
begins from mid-November and stays till February in Northern parts of India with December and January as the coldest months. Frost occurs in the Northern plains and snow falls in the high mountainous regions of Himalayas.
As the North-East trade winds blow rain occurs in Tamil Nadu and Southern Andhra Pradesh.
2. The Hot Weather Season(Summer)
starts in March and lasts upto the end of May.
The temperature of the Northern part of India goes up and the atmospheric pressure comes down.
Loo blows during this season over the North and North-Western India and can cause even death if persons are exposed to it for a long time.
*Loo is the strong, gusty, hot, dry winds blowing during the day over the north and northwestern India.
Dust storms and light rain are very common in North India in the month of May. Localised thunderstorms also occur during summer, which may have high speed winds and even precipitate hail. Such thunderstorms are called Kaal Baisakhi in West Bengal.
Near the end of summer, there may be pre-monsoon showers. These are called Mango Showers in Kerala and Karnataka, as they help in the early ripening of the mango fruit.
3. Advancing Monsoon (The Rainy Season)
South-east trade winds originate over the warm subtropical areas of the southern oceans. They cross the equator and blow in a southwesterly direction entering the Indian peninsula as the south-west monsoon. The monsoon winds cover the country in about a month. Mawsynram in the southern ranges of the Khasi Hills receives the highest average rainfall in the world.
A phenomenon associated with the monsoon is its tendency to have ‘breaks’ in rainfall. These breaks in monsoon are related to the movement of the monsoon trough.The monsoon is known for its uncertainties. The alternation of dry and wet spells vary in intensity, frequency and duration.
4. Retreating/Post Monsoons (The Transition Season)
The months of October-November form a period of transition from hot rainy season to dry winter conditions.During this time, the low pressure trough over the Northern plains weakens and is replaced gradually by a high-pressure system. This is followed by the South-West monsoon winds. The retreat of the monsoon is marked by clear skies and rise in temperature. While day temperatures are high, nights are cool and pleasant. Owing to the conditions of high temperature and humidity, the weather becomes rather oppressive during the day. This is commonly known as ‘October heat’.
Monsoon as a Unifying Bond
The vegetation, animal life and agricultural activities are all revolving around the effects of the monsoon.The life of the people, their celebration of festivals and other activities all are geared to the monsoon. The monsoon provides the water(to various water bodies) to set agricultural activities in motion and hence, the arrival of the monsoon is awaited eagerly.
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