Mechanisms and Factors Affecting Indian Climate
There are certain factors which affect the climate of India. These factors can be broadly divided into two groups: 1) factors related to location and relief and 2) factors related to air pressure and winds.
|Factors related to location and relief||Factors related to air pressure and winds|
Factors Related to Location and Relief
The Tropic of Cancer passes through the middle of India and extends from Mizoram in the east and Rann of Kutch in the west; and considerably affects the climate of the country. To the south of the Tropic of Cancer lies the southern part of the country
which belongs to the tropical area and to its north lies the northern half of India which belongs to the subtropical and temperate zone. Therefore, India experiences both subtropical and tropical climates.
The tropical zone being nearer to the equator, experiences high temperatures throughout the year with small daily and annual range. Area north of the Tropic of Cancer being away from the equator, experiences extreme climate with high daily and annual range of temperature.
Distribution of Land and Water
India is flanked by the Indian Ocean on three sides in the south and girdled by a high and continuous mountain-wall in the north. As compared to the landmass, water heats up or cools down slowly. This differential heating of land and sea creates different
air pressure zones in different seasons in and around the Indian subcontinent. Difference in air pressure causes reversal in the direction of monsoon winds.
Distance From the Sea
The coastal areas have an equable or stable climate. The interior parts of India are far away from the moderating influence of the sea. Such areas have extremes of climate. That is why, the people of Mumbai and the Konkan coast have hardly any idea of
extremes of temperature and the seasonal rhythm of weather. On the other hand, the seasonal contrasts in weather at places in the interior of the country such as Delhi, Kanpur and Amritsar affect the entire sphere of life.
In the north, India is bounded by mountains with an average height of 6,000 metres and in the south, has a vast coastline with maximum elevation of about 30 metres.
The Himalayas act as a barrier against the cold winds from Central Asia (which originate near the Arctic circle and blow across Central and Eastern Asia). Therefore due to the altitude of these mountains, the Indian subcontinent experiences milder winters
than Central Asia. So, the Himalayas along with its extensions act as an effective climatic divide. The Himalayas also trap the monsoon winds, forcing them to shed their moisture within the subcontinent.
An another way to understand the how altitude affects the climate is by understanding the relation between the temperature and altitude. Temperature decreases with height. Due to thin air, places in the mountains are cooler than places on the plains.
For example, Agra and Darjeeling are located on the same latitude, but temperature of January in Agra is 16°C whereas it is only 4°C in Darjeeling.
The physiography or relief of India affects the temperature, air pressure, direction and speed of wind and the amount and distribution of rainfall.
As explained above, the Himalayas act as a protective shield against the cold winds from Central Asia. They also trap the monsoons and force them to rain in India. The windward sides of Western Ghats and Assam receive high rainfall during June-September whereas the southern plateau remains dry due to its leeward situation along the Western Ghats.
Factors Related to Air Pressure and Winds
These factors are responsible for the differences in local climates of India. To understand the mechanism of monsoon, we need to understand these factors in detail.
The three most important factors in this category are:
- Surface Pressure and Winds: Distribution of air pressure and winds on the surface of the earth
- Jet Stream and Upper Air Circulation: Upper air circulation caused by factors controlling global weather and the inflow of different air masses and jet streams
- Western Disturbances and Tropical Cyclones: Inflow of western cyclones generally known as disturbances during the winter season and tropical depressions during the south-west monsoon period into India, creating weather conditions favourable to rainfall
The mechanism of these three factors can be understood with reference to winter and summer seasons of the year separately.
Mechanism of Weather in the Winter Season
Surface Pressure and Winds
The pattern of air circulation discussed here is witnessed only at the lower level of the atmosphere near the surface of the earth.
The weather conditions of winters in India are influenced by a high pressure zone developed in Central and Western Asia during winter. This gives rise to the flow of dry continental air mass at the low level from the north towards the Indian subcontinent, south of the Himalayan range. These continental winds come in contact with trade winds over northwestern India. The position of this contact zone is not, however, stable. Occasionally, it may shift its position as far east as the middle Ganga valley with the result that the whole of the northwestern and northern India up to the middle Ganga valley comes under the influence of dry northwestern winds.
Jet Stream and Upper Air Circulation
The variations in the atmospheric pressure closer to the surface of the earth have no role to play in the making of upper air circulation. In the lower troposphere at about three km and above, a pattern of air circulation different from that of the surface winds is observed.
At the altitude of 9-13 km, Westerly jet streams blow parallel to the Tibetan highlands. These winds blow across the Asian continent at latitudes north of the Himalayas. Tibetan highlands act as a barrier to these jet streams as a result of which they get bifurcated. One of its branches blows to the north of the Tibetan highlands, while the southern branch blows in an eastward direction, south of the Himalayas. It has its mean position at 25°N in February at 200-300 mb level. It is believed that this southern branch of the jet stream exercises an important influence on the winter weather in India.
Western Cyclonic Disturbances and Tropical Cyclones
The western cyclonic disturbances enter the Indian subcontinent from the west and the northwest during the winter months. They have their origin over the Mediterranean Sea and are brought into India by the westerly jet stream. An increase in the prevailing night temperature generally indicates an advance in the arrival of these cyclones disturbances.
Tropical cyclones originate over the Bay of Bengal and the Indian ocean. These tropical cyclones have very high wind velocity and heavy rainfall (hence very destructive) and hit the Tamil Nadu, Andhra Pradesh and Orissa coast.
Mechanism of Weather in the Summer Season
Surface Pressure and Winds
The pattern of pressure and winds as mentioned here is formed only at the lower level of the troposphere. During the summer, the wind circulation over the subcontinent undergoes a complete reversal compared to that of winter. This reversal happens at both, the lower as well as the upper levels. By the middle of July, ITCZ shifts northwards (roughly parallel to the Himalayas between 20° N and 25° N) and the westerly jet stream withdraws from the Indian region. There is a cause and effect relationship between the northward shift of ITCZ and the withdrawal of the westerly jet stream from over the North Indian Plain.
The ITCZ being a zone of low pressure, attracts inflow of winds from different directions. The maritime tropical air mass (mT) from the southern hemisphere, after crossing the equator, rushes to the low pressure area in the general southwesterly direction. It is this moist air current which is popularly known as the southwest monsoon.
The Intertropical Convergence Zone (ITCZ), known by sailors as the doldrums or the calms because of its monotonous, windless weather, is the area where the northeast and southeast trade winds converge. At ITCZ, the trade winds converge and air tends to ascend.
It encircles Earth near the thermal equator, though its specific position varies seasonally. When it lies near the geographic Equator, it is called the near-equatorial trough. Where the ITCZ is drawn into and merges with a monsoonal circulation, it is sometimes referred to as a monsoon trough, a usage more common in parts of Asia and Australia.In India, in July, the ITCZ is located around 20°N-25°N latitudes (over the Gangetic plain), sometimes called the monsoon trough. This monsoon trough encourages the development of thermal low over north and northwest India. Due to the shift of ITCZ, the trade winds of the southern hemisphere cross the equator between 40° and 60°E longitudes and start blowing from southwest to northeast due to the Coriolis force. It becomes southwest monsoon. In winter, the ITCZ moves southward, and so the reversal of winds from northeast to south and southwest, takes place. They are called northeast monsoons.
Jet Streams and Upper Air Circulation
An easterly jet stream flows over the southern part of the Peninsula in June, and has a maximum speed of 90 km per hour. In August, it is confined to 15o N latitude, and in September up to 22o
N latitudes. The easterlies normally do not extend to the north of 30o N latitude in the upper atmosphere.
Easterly Jet Stream and Tropical Cyclones
The easterly jet stream steers the tropical depressions into India. These depressions distribute monsoon rainfall over the considerable part of the Indian subcontinent. The tracks of these depressions are the areas of highest rainfall in India. The frequency at which these depressions visit India, their direction and intensity, all go a long way in determining the rainfall pattern during the southwest monsoon period.
|Surface Pressure and Winds||Flow of dry continental air mass at the low level from the north towards the Indian subcontinent
||ITCZ shifts northwards (between 20° N and 25° N) and the westerly jet stream withdraws from the Indian region.|
|Jet Streams and Upper Air Circulation
||The southern branch of Westerly jet stream blows in an eastward direction, south of the Himalayas. It exercises an important influence on the winter weather in India.
||Easterly jet stream flows over the Indian Peninsula between 15oN to 30oN latitude.|
|Cyclones||The western cyclonic disturbances enter the Indian subcontinent.
Tropical cyclones originate over the Bay of Bengal and the Indian ocean.
|Tropical cyclones distribute monsoon rainfall over the considerable part of the India.|