climate notes

1. Define Climate and Weather

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).

Weather refers to the state of the atmosphere over an area at any point of time.

2. Describe Monsoon.

The word monsoon is derived from the Arabic word ‘mausim’ which literally means season. • ‘Monsoon’ refers to the seasonal reversal in the wind direction during a year.

3. Factors which control the climate

There are six major controls of the climate of any place. They are: latitude, altitude, pressure and wind system, distance from the sea (continentally), ocean currents and relief features.

Latitude Due to the curvature of the earth, the amount of solar energy received varies according to latitude.

As a result, air temperature decreases from the equator towards the poles.

Altitude As one goes from the surface of the earth to higher altitudes, the atmosphere becomes less dense and temperature decreases. The hills are therefore cooler during summers.

Pressure and wind it influences the temperature and rainfall pattern.

distance from the sea The sea exerts a moderating influence on climate: As the distance from the sea increases, its moderating influence decreases and the people experience extreme weather conditions. This condition is known as continentality (i.e. very hot during summers and very cold during winters)

Ocean currents any coastal area with warm or cold currents flowing past it, will be warmed or cooled.

Relief High Mountains act as barriers for cold or hot winds; they may also cause precipitation.

Factor affecting India’s climate

Latitude Almost half of the country, lying south of the Tropic of Cancer, belongs to the tropical area. All the remaining area, north of the Tropic, lies in the sub-tropics.

Altitude India has mountains to the north. The Himalayas prevent the cold winds from Central Asia from entering the subcontinent.

Pressure and Winds Pressure and surface winds; • Upper air circulation; and • Western cyclonic disturbances and tropical cyclones.

Coriolis force: An apparent force caused by the earth’s rotation. The Coriolis force is responsible for deflecting winds towards the right in the northern hemisphere and towards the left in the southern hemisphere. This is also known as ‘Ferrel’s Law’.

Western Cyclonic Disturbances: 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.

The mechanism of the monsoons.: (a) 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.

(b) The shift of the position of Inter Tropical Convergence Zone (ITCZ)- in summer, over the Ganga plain.

(c) The presence of the high-pressure area, east of Madagascar-

(d) The Tibetan plateau gets intensely heated- during summer the formation of high pressure over the plateau at about 9 km above sea level.

The movement of the westerly jet stream to the north of the Himalayas and tropical easterly jet stream over the Indian peninsula.

What is Inter Tropical Convergence Zone?

The Inter Tropical Convergence Zone (ITCZ,) is a broad trough of low pressure in equatorial latitudes. This is where the northeast and the southeast trade winds converge.

Describe Southern Oscillation.

Normally when the tropical eastern south Pacific Ocean experiences high pressure, the tropical eastern Indian Ocean experiences low pressure. But in certain years, there is a reversal in the pressure conditions and the eastern Pacific has lower pressure in comparison to the eastern Indian Ocean. This periodic change in pressure conditions is known as the Southern Oscillation.

From where the intensity of monsoon is predicted?

The difference in pressure over Tahiti (Pacific Ocean, 18°S/149°W) and Darwin in northern Australia (Indian Ocean, 12°30’S/131°E) is computed to predict the intensity of the monsoons. If the pressure differences were negative, it would mean below average and late monsoons.

El Nino: This is a name given to the periodic development of a warm ocean current along the coast of Peru as a temporary replacement of the cold Peruvian current. ‘El Nino’ is a Spanish word meaning ‘the child’, and refers to the baby Christ, as this current starts flowing during Christmas. The presence of the El Nino leads to an increase in sea-surface temperatures and weakening of the trade winds in the region.

What is monsoon burst?

 Around the time of monsoon arrival, the normal rainfall increases suddenly and continues constantly for several days. This is known as the ‘burst’ of the monsoon.

Delhi generally receives the monsoon showers from the Bay of Bengal branch by the end of June (tentative date is 29th of June).

Four main seasons can be identified in India – the cold weather season, the hot weather season, the advancing monsoon and the retreating monsoon.

The Cold Weather Season (Winter)- The cold weather season begins from midNovember in northern India and stays till February. During this season, the northeast trade winds prevail over the country. They blow from land to sea and hence, for most part of the country, it is a dry season.

Over the northern plains is the inflow of cyclonic disturbances from the west and the northwest. These low-pressure systems originate over the Mediterranean Sea and western Asia and move into India, along with the westerly flow. They cause the much-needed winter rains over the plains and snowfall in the mountains.

The Hot Weather Season (Summer) - from March to May, it is hot weather season in India. The summer months experience rising temperature and falling air pressure in the northern part of the country.

A striking feature of the hot weather season is the ‘loo’. These are strong, gusty, hot, dry winds blowing during the day over the north and northwestern India.

In West Bengal, these storms are known as the ‘Kaal Baisakhi’ calamity for the month of Baisakh.

mango showers- pre-monsoon showers are common especially, in Kerala and Karnataka. They help in the early ripening of mangoes, and are often referred to as ‘mango showers’.

Advancing Monsoon (The Rainy Season)- Start By early June. The trade winds of the southern hemisphere entering the Indian peninsula as the south-west monsoon.

The windward side of the Western Ghats receives very heavy rainfall, more than 250 cm.

Mawsynram in the southern ranges of the Khasi Hills receives the highest average rainfall in the world.

Monsoon has a ‘breaks’ in rainfall.

When the axis of the monsoon trough lies over the plains, rainfall is good in these parts. On the other hand, whenever the axis shifts closer to the Himalayas, there are longer dry spells in the plains, and widespread rain occur in the mountainous catchment areas of the Himalayan rivers.

The monsoon is known for its uncertainties. Justify.

It causes heavy floods one part; it may be responsible for droughts in the other. It is often irregular in its arrival and its retreat. Hence, it sometimes disturbs the farming schedule of millions of farmers all over the country.

Retreating Monsoon (The Transition Season) During October-November, with the apparent movement of the sun towards the south, the monsoon trough or the low-pressure trough over the northern plains becomes weaker. The south-west monsoon winds weaken and start withdrawing gradually. By the beginning of October, the monsoon withdraws from the Northern Plains.

The retreat of the monsoon is marked by clear skies and rise in temperature.

October heat-

Distribution of rainfall- The western coast and northeastern India receive over about 400 cm of rainfall annualy. It is less than 60 cm in western Rajasthan and adjoining parts of Gujarat, Haryana and Punjab.

Rainfall is equally low in the interior of the Deccan plateau, and east of the Sahyadris. A third area of low precipitation is around Leh in Jammu and Kashmir. The rest of the country receives moderate rainfall.