In simple words, climate of a place is known as the average of the weather
components including the frequency of extreme events. As per the current practice
weather parameters over a period of at least 30 years constitutes the climate. There are
continuous interactions between the components of climate system such as atmosphere,
biosphere, cryosphere, land and ocean. Natural variation in climate can occur due to
volcanic eruption, continental drift and built up of mountains, changes in tilt angle of
earth’s orbit, sun’s radiation intensity and the slow but large scale ocean circulation etc.
However, changes in greenhouse gas (GHG) concentrations, aerosol emissions and
urbanization give rise to the anthropogenic climate change. Today, climate change is one
of the major issues on the earth system and its impact has already been realized in several
geographical locations and sectors in the society (IPCC 2007). There is a worldwide
concern about the anthropogenic climate change, particularly during the last two decades.
Most important visible changes are those in temperature, snow/ice melt and precipitation
extremes. Before understanding any scientific study on Indian climate, it is essential to
know some of the important diversities in Indian weather and climate.
1.1 Weather and climate of India
India is a south Asian country which lies to the north of the equator between 8.4o
and 37.6o north latitude and 68.7o and 97.25o east longitude. This tropical country is
surrounded by water bodies on three sides, Arabian Sea towards the West, Bay of Bengal
towards the East and Indian Ocean towards the South. India is having unique
geographical features with complex topography. Mountain range such as the Himalayas
broadens in the north and northeast. The Vindhyas separate the Indo Gangetic plain from
the Deccan Plateau. The Satpura, Aravalli and Sahyadri cover the eastern fringe of the
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West Coast plains. The coasts of southern parts of India are known as Western and
Eastern Ghats. Eastern Ghats are irregularly scattered and forms the boundary of the East
Coast plains. Tibetan plateau towards the north of the foothills of Himalayas also
influences Indian summer monsoon. On the northwest part of India, Thar desert extends
from the edge of the Rann of Kachchh of Gujarat up to the frontier of Rajasthan.
Classification of climate for any region is the organization of climate information
for analysis and communication. India is a vast country with diversities in its weather and
climate. Wladimir Köppen, a German botanist and climatologist is most widely known
for the descriptive climate classification system which he first proposed in 1884. After
several modifications, world map of climatic classification by Köppen was introduced in
1936. It combines the average of annual and monthly temperature and precipitation, and
the seasonality of precipitation. According to the Köppen climate classification system,
Indian climate can be divided into six major subtypes. Desert or arid climate regions are
in the west and beyond that there is semi-arid climate. Semi-arid climate can also be
observed between Eastern and Western Ghats. Alpine tundra and glaciers are found in the
north and over the Himalayan ranges. Humid subtropical regions are in the north of
central and eastern India. Tropical wet climate regions are the Western Ghats and Island
territories. Parts of peninsular India experience tropical wet and dry climate.
In India there are four distinct seasons, pre-monsoon (April-May), summer
monsoon (June-July-August-September), post-monsoon (October-November) and winter
(December-January-February). India is a tropical country with hot weather conditions
that varies from region to region. In the pre-monsoon months vast land portion of India is
dominated by intense solar heating which leads to heat wave conditions. Similarly cold
wave conditions during winter season occur due to the intense high pressure cells and
passing of western disturbances. There is no absolute definition for heat or cold wave
events. The term is relative to the average weather condition of a region of study.
According to a study by Raghavan (1967), severe cold waves develop very often in situ
within the country itself and account for the higher incidence in certain isolated regions.
Irrespective of the intensity, severe cold wave are mostly confined to either eastern or the
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western half of the Ladakh in Jammu-Kashmir. Severe heat waves are also not so far
been observed to migrate from the neighboring countries Raghavan (1966). They develop
in situ within the country itself and expand from West Pakistan to affect Northwest India.
He further concluded that available statistics do not suggest any periodicity in the
incidence of extreme temperature conditions in any region or in the country as a whole.
He considered the persistency of the waves over different subdivisions where they
dissipate or wherefrom they migrate to adjacent regions. In India more than 70% of its
population relying on agriculture directly or indirectly and thus the impact of extreme
weather events is critical. In the last two decades India has been affected by successive
extreme temperature events, monsoons flooding and droughts (De et al. 2005). Snow is
also an important component of the hydrological cycle. Major contribution of many parts
of the world snow is in the form of precipitation or total annual water supply. The
allocation of limited water resources has significant economic and policy consequences.
Eurasian, Tibetan and Himalayan snow in winter and spring may affect the Indian
summer monsoon circulation and rainfall. It may influence both river basin runoff and
climate change dynamics. It has been revealed that snow cover/depth variations during
winter in Eurasia are not only associated with monsoon rainfall in Southeast Asia but also
recognized as an effective source of freshwater flowing to the Arctic Ocean (Rogers et al.
2001), and may thus be linked to the global thermohaline circulation (Walsh et al. 1998),
which is a major determinant of the global climate.
In India two major monsoon systems are observed, southwest/summer monsoon
from June to September and northeast/winter monsoon from October to November.
Monsoon is produced by differences between land and sea temperatures in eastern and
southern Asia. It is a seasonally varying wind system, e.g. in the Indian Ocean winds are
southwesterly with moist air and high precipitation in summer, northeasterly with dry air
and clear skies in winter. During the post-monsoon months northeast monsoon dominates
over the southern parts of Peninsular India particularly in Andhra Pradesh, Rayalaseema,
Tamilnadu and Pondicherry. The principal components of northeast monsoon system are
Siberian high pressure system, northeasterly lower tropospheric flow, wind surges carried
by the northwesterly monsoon flow along the western shores of South China Sea, the
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monsoon trough located near to the north of the equator in December and south of the
equator in January and February, the west Pacific high and the subtropical jet stream of
winter. At the beginning of October, a trough of low pressure develops over the south of
Bay of Bengal and equatorial maritime air moves towards the southern India that causes
northeast monsoon rainfall (Kripalani and Kumar, 2004). They found that Indian Ocean
dipole positive phase also enhances the rainfall activity of northeast monsoon and
negative phase suppresses. Dhar and Rakecha (1983) and Singh and Sontakke (1999)
identified enhancement in northeast monsoon rainfall during El Niño events.
1.2 Indian summer monsoon main features
The weather and climate of India are dominated by the summer monsoon, which
returns with remarkable regularity in each summer and provides the rainfall needed to
sustain over a billion of people. The vastness of Indian sub-continent and the unique
configuration of the east African highlands and the Tibetan plateau mean that the Indian
summer monsoon is the most vigorou