The mean maximum temperature over most of Northern India remains quite high in September and October.
June is the hottest month in India. With the onset of monsoon rains, temperature starts falling. Rains Lower the temperature by 6°C in Northern India. The temperature begins to rise steadily after the rainy season. In September-October, the S.W. monsoons begin to retreat. During these retreating monsoons, dry spells are observed. The Southern India experiences high temperatures as the sun shines vertically over the equator. The northern India, gets a second maxima of temperature. The mean maximum temperature in September and October at Delhi is above 33°C.
Cherrapunji receives the highest amount of rainfall in the World.
Cherrapunji is the rainiest place of the world with an average annual rainfall of 1080 cms. It is located on the Southern slopes of Khasi Hills (Meghalaya) at an height of 1500 metres above Sea level. This place is enclosed by hills on three sides. The relief features gives this place a funnel shaped location. The bay of Bengal Branch of monsoons is trapped in these hills. These winds try to get out of it. These are forced to rise again and again resulting in heavy rainfall. In the year 1861, an amount of 2262 cms of rainfall was recorded here.
Tamil Nadu Coastal region receives most of the rain in winter.
Tamil Nadu lies on the Coromandel coast on the eastern costal plain. This region receives rainfall in winter as well as summer. It receives most of rainfall in winter.
(i) In summer, Tamil Nadu remains dry as this plain lies in the Rain shadow of Western Ghats.
(ii) In winter, N.E. Monsoons pick up moisture as they cross Bay of Bengal. These retreating monsoons are on-shore winds in Tamil Nadu coastal plain. The Eastern Ghat forces these winds to give moderate rainfall in this area.
The general circulation of winds system is disturbed by some atmospheric disturbances. Western disturbances are low pressure systems which originate in west Asia and the regions near Mediterranean Sea. These travel Eastwards across Iran and Pakistan and reach India during the winter season. The westerly Jet stream steers these disturbances towards India. These are active in winter in Northern India. On an average, four or five such depressions visit India each month. These give rainfall in areas of J. & K., Punjab, Himachal Pradesh, Haryana and Rajasthan. These give snowfall in N.W. Himalayas. The amount of rainfall decreases towards the East. This rainfall is of great importance of the rabi crops especially wheat. The average rainfall varies from 20 mm. to 50 mm.
Hottest parts of India
The highest temperatures are found in the western parts of Rajasthan. Barmer is the hottest place where summer temperatures of 50°C are recorded. The high temperatures are due to:—
(i) This place has an inland location and is located away from the India Ocean. The Continental effect raises the Summer temperature.
(ii) Hot dusty winds—Loo raises the temperature of this area.
(iii) The temperatures remain high due to sandy soils and lack of humidity.
main characteristics of summer monsoon rainfall
(i) It is a seasonal rainfall. Most of the rain comes in summer.
(ii) The rainfall is uncertain and variable.
(iii) The amount of rain decreases sway from the coast. In Northern plain, it decreases westward up the Ganges valley for example-Calcutta 119 cms, Patna 105 cms, Allahabad 100 cms, Delhi 65 cms.
(iv) S.W. monsoon have ‘breaks’ or spells of dry weather. The rain does not fall continuously.
(v) The intensity and amount of rain is increased by jet stream and depressions.
(vi) Tamilnadu and Deccan plateau remain dry as these are rain shadow areas.
The monsoon winds blow in South-west direction on the west coast. These are highly charged with water vapour. It advances rapidly over West coast. The rains begin rather suddenly in the first week of june. This sudden onset of rain is often termed as ‘Monsoon Burst’. The rainfall is heavy accompanied by thunder and lightning. The rain is so sudden and heavy that it appears as it a balloon of water has been burst.
‘Loo’ is a local wind. It is very lot and dry wind which blows during the day time in northern India. Loo increases the dry temperatures ranging between 40°C to 50°C. The hot and dry winds are unbearable. These result in “heat wave” causing many deaths.
Coldest parts of India
The trans-Himalayan region consisting of Jammu and Kashmir, Himachal Pradesh are the coldest parts of India. The minimum temperatures of – 40°C are recorded at Dras or Kargil (Ladakh). This is due to the fact that this place lies at a great altitude. These regions get snowfall during winter and temperature remains below freezing point.
Intertropical Convergence Zone
Intertropical convergence zone is a narrow Equatorial low pressure belt found near the surface of the equator. It changes its position with the shifting of the sun. I.T.C.Z. shifts northward to 25°N in summer. It forms a zone of contact of winds and results in the attraction of S.W. Monsoons. In winter, I.T.C.Z. shifts southwards.
Western part of India is drought prone, while eastern part is flood prone.
The distribution of rainfall is uneven in India. In Western parts, the average annual rainfall is less than 50 cms. Therefore Rajasthan state is drought prone. In the Eastern Parts, the average annual rainfall is 200 cms. Therefore, the North Eastern States are flood prone.
Monsoons exercise an all embracing and unifying influence on the weather conditions of India.
India has primarily a monsoon type of climate. Monsoon system of winds exercises a uniform influence throughout the country. Despite many regional variations of climate, there is a broad unity of monsoon type of climate having same rhythm of seasons. The monsoon climate prevails as much over Thar desert as on Assam and Kerala. Monsoons have a unifying influence on weather conditions of India. Throughout the country due to a seasonal flow of Land to Sea and Sea to Land winds, different seasons are found. Monsoon make India as a whole a single meteorological unit. Agriculture in each part of the country depends upon monsoons. The enclosed character of the country by the unbroken chain of Himalayas in the north has given a distinct character to Indian monsoon. Of course, there are many regional variation; Depressions give rain in the North West, Retreating monsoons give rainfall on the East coast, but these are the resulting facets of monsoons. Thus monsoons create a unity of climate over India.
Distribution of annual rainfall in India
The average annual rainfall of India is 110 cms. Regional variations in the distribution are found due to differences in relief of the country. Rainfall is unevenly distributed throughout the country. India can be divided into the following rainfall regions:
(1) Areas of heavy rainfall. These areas get more than 200 cms of annual rainfall. These include Western Coast and Western Ghats, Sub-Himalayas, and the N.E. Parts of India.
(2) Areas of Moderate Rainfall. These areas get annual rainfall of 100-200 cms. These include West Bengal. Orissa, Eastern parts of U.P. and Madhya Pradesh; coastal plains of Tamilnadu.
(3) Areas of Low Rainfall. These areas experience an annual Rainfall of 50-100 cms. These include western part of U.P., Haryana, Punjab, Gujarat, peninsular plateau and Eastern Rajastan.
(4) Areas of Scanty Rainfall. These areas get less than 50 cms of Annual rainfall. These include Ladakh, S.W. Punjab, Southern Haryana, Western Rajasthan,
Kutch and Thar Desert.
The term “cyclone” is used by India meteorologists for an area of low pressure where the velocity of the incoming winds, blowing in a spiral, exceeds 64 km per hour. A low pressure area develops when the stagnant moist air over a place becomes warmer and rise to upper atmospheric heights. The rising air is replaced by the air flowing in from all directions. The earth’s rotation prevents the incoming air currents converging at the centre of the low pressure. And instead exerts a deflecting force, causing the wind spiral. The system derives its energy from the latent heat released by the moisturisation of vapour. In the tropics the vertical winds current is generally weak enough to allow the flush-out of the liberated heat which remain trapped, incessantly intensifying the cyclone as long as it is over the warm sea.
Anti-cyclone measures: Coastal cities face greater risk from cyclones because the population growth has been greater there. Measures to minimise cyclone damage have been classified into three categories. Long-term measures, which require 10 to 20 years, include building house strong enough to withstand cyclones—this is cheaper than rebuilding destroyed houses, land planning to keep the most vulnerable areas as green areas; scientific research to minimise or neutralise the cyclone effects. Medium-term measure consist of improvement in the communication network and the warming systems, and establishment of emergency centres and cyclone shelters. Short-term measures come into play the moment a cyclone alert is received and include evacuation from vulnerable area, and public health and sanitation precautions. The INSAT and other weather satellites are able to give a 48-hour notice of cyclones. Thus timely warnings may be given to the people
Monex. The mechanism of the Indian monsoon is now explained as due to the shifting of the I.T.C., the northward movement of the Westerly Jet Stream and its replacement by the Easterly jet Stream and the Upper air circulation over Tibet. The World Metereological Organisation conducted a monsoon experiment known as MONEX over the Arabian sea and the Bay of Bengal to unravel the mysteries of the monsoon. However, the working of the monsoon is still not fully understood. This is partly due to difficulties involved in recording and measuring upper air observations over vast stretches of the seas around India.
El Nino Effect. The El Nino Effect has a bearing on the monsoon pattern.
The El Nino, a warm Ocean current on the Peru Coast which appears before Christmas affects the Southern Oscillation, i.e., the see-saw movement of weather between the Pacific and Indian Oceans. By measurements of the Southern Oscillation Index (SOI) it is possible to predict whether the monsoon will be weak or strong.