INTRODUCTION
- Weather is central to our existence. Weather and climate are two different terms. Weather is the condition of the atmosphere at a particular place and time while climate is the average weather condition over a long period of time. If weather information deals with a specific event, then climate represents the generalisation of the weather.
- Weather is the state of atmosphere as determined by the meteorological phenomena that are occuring at any one place and time.
- The meteorological phenomena include temperature, precipitation, winds, clouds, sunshine, pressure and visibility.
- Weather conditions over a specific length of time, usually a period of at least 30 year, are taken into consideration.
- The main elements of climate temperature, atmosphere pressure, wind and humidity (including precipitation).
INFLUENCES ON CLIMATE
On a large scale, the climate of a particular region is determined by :
- Latitude and tilt of the earth’s axis, which determines the amount of solar radiation received by the area.
- The distribution of land and sea and proximity of ocean currents.
- The altitude and topography of the area.
- The location of the area in relation to the main circulation belts of the earth.
CLASSIFICATION OF CLIMATE
Climate can be classified on the basis of temperature, rainfall, evaporation, evapotranspiration and water balance.
One of the universally accepted climate classifications is by Koppen.
The Climate of India
- The Indian climate can be described as the tropical monsoon type. it is greatly influenced by presence of the Himalaya in the north and the India Ocean in the South. Regional variations do, however, occur.
- The Tropic of Cancer divides India into two almost equal climatic zones, namely, the northern zone and the southern zone.
- The warm temperate or sub-typical climate of the northern zone gives cold winter seasons and hot summer season. The southern tropical climatic zone of India is warmer than the north and uses not have a clear-cut winter season.
- While the northern zone does not have the midday sun vertically overhead during any part of the year, the southern zone has the mid-day sun almost vertically overhead at least twice every year.
FACTORS INFLUENCING INDIAN CLIMATE
The major factors affecting India’s climate are :
(a) Latitude
- The Tropic of Cancer (23½°N) passes through the middle part of the country. The Great Plains also lie within 32°N latitude.
- It keeps the temperature of the region substantially high.
(b) The Monsoon Winds
- The monsoon winds which blow from south-west during summer and from the north-easterly direction during winter influence Indian climate, byaffecting rainfall, humidity and temperature.
- Onset and instensity of monsoon depends upon :
(i) Thermal contrast
(ii) Inter-tropical Convergence (ITC)-ITC shifts northward and it results in the formation of Equatorial Westernlies which blow towards (ITC).
(iii) Upper air currents or jet-streams.
(c) Relief
- Relief features, particularly the Himalayas, the Western Ghats and the Purvanchal Hills, influence the climate by affecting temperature and rainfall.
- The Himalayas check the cold and dry polar air coming from Antarctica region and direct the summer monsoons towards north west.
SEASONS IN INDIA
Four main season can be identified in India -
(a) The Cold Weather Season (Winter) :
- The cold weather season begins from mid-November in northern India and stays till February. December and January are the coldest months in the northern part of India.
- 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.
- Inflow of cyclonic distubrances 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 pains and snowfall in the mountains. Locally known as ‘mahawat’ are of immense importance for the cultivation of ‘rabi’ crops.
- The northwest trade winds cause fair amount of rainfall in Chennai or the Coromandel Coast in winter.
(b) Hot Weather Season (Summer) :
- Due to the apparent movement of the sun, the global heat belt shifts northward. As such, from March to May, it is hot weather season in India.
- Temperature increases from south to north. In peninsular India, temperature remain lower. High temperature between 38°C and 48°C prevails in the plains.
- Local dust storms accompanied with light rains. Hot dry winds, ‘loo’ is common in May and June.
(c) Advancing Monsoon (The Rainy Season) :
The climate of India is described as of monsoon type. Derived from an Arabic word ‘mausim’ mansoons refer to the seasonal reversal in the wind direction through the year.
(i) Mechanism of the Monsoon :
- The word monsoon denotes a season in which the wind regime is completely reversed.
- The southeast winds, after crossing the equator in the Indian Ocean, take a southwesterly direction. The dry and hot land bearing trades are thus completely replaced by sea bearing winds full of moisture.
- This phenomenon of complete reversal of winds is confined to tropical lands lying between 20°N and 20°S.
- This phenomenon account for 75 to 90 percent of the annual rainfall just from June to September
(ii) Characteristics of the Monsoon :
- Almost all over the country, the rain occur from June to September.
- 75% to 90% of the total annual rainfall is concentrated over this period.
- There is great variation in the advance and withdrawal dates of the monsoons.
- The monsoons occur in wet spells, interspersed by dry spells.
- The amount of rainfall also varies, causing floods are drought conditions.
(d) Retreating Monsoon :
- During October-November, the monsoon trough of low pressure becomes weaker and is gradually replaced by high pressure.
- The outreach of the monsoon becomes unsustainable and it starts withdrawing gradually. This is known as the retreat of monsoon.
(a) Areas the Heavy rainfall :
Areas which get rainfall of 200 cms and above are Assam. The Ganga Delta, the Western Ghats and the Western Coastal regions and the mountainous regions of Himachal Pradesh.
(b) Areas of Moderate Rainfall :
Areas which get annual rainfall between 100 cms to 200 cms are Madhye Pradesh, Orissa, Chhota Nagpur, Western Bengal, Bihar, Eastern U.P., North-Eastern Punjab, Eastern parts to Tamil Nadu and Eastern slopes of Western Ghats receive moderate rainfall.
(c) Areas with Low Rainfall :
Areas receiving annual rainfall between 50 cms to 100 cms are the Deccan Plateau, Western U.P., South-Eastern Punjab, Eastern Rajasthan and parts of Kashmir get low rainfall.
(d) Areas with Scanty Rainfall :
Areas which get less than 50 cms rainfall annually are western Rajasthan, Kutch, Southeastern parts of Haryana, Northeastern Kashmir get scanty rainfall.
