Weather describes the day-to-day meteorological conditions such as wind, temperature, cloudiness, moisture, rainfall, etc. affecting a place. Climate is the average weather usually taken over a 30-year period for a particular region and time. The basic elements of weather are wind, temperature, air pressure, precipitation and moisture.
‘Monsoon’ refers to the seasonal reversal in the wind direction during the year. The two important elements of climate are temperature and precipitation.
Some parts of Rajasthan desert, the temperature in summers is 50 degrees Celsius, whereas summer temperature in Jammu and Kashmir is 20 degrees Celsius. During winters, the temperature in Jammu and Kashmir may be -45 degrees Celsius. Drass in Jammu and Kashmir is the second coldest inhabited place in the world.
In India, the Tropic of Cancer passes through the central part of the country, from the Rann of Kutch in the west to Mizoram in the east. India has both tropical and subtropical climates. Altitude refers to the height a place above sea level.
Contrasts in temperature are experienced more in the interior of the country. The rainfall in India varies in its form, types, amount and seasonal distribution. The upper parts of the Himalayas, precipitation is mostly in the form of snowfall, whereas the remaining parts of the country receive rains. There is a decrease in the rainfall generally from east to west in the Northern Plains. Climatic variations also affect the way people live i.e. depends on the food, the clothes and the kind of houses they live in.
In India, the elevation of land ranges from 30 metres to 6,000 metres. The Himalayan mountains to the north of India have an average height of about 6,000 metres. The average summer temperature on the Himalayas can vary from zero degrees Celsius to 14 degrees Celsius, while winters can see the temperature dipping below freezing point with heavy snowfall. The Himalayas prevent the cold winds from Central Asia from entering the subcontinent. The rainfall in India is governed mainly by pressure and surface winds, upper air circulation, and western cyclonic disturbances and tropical cyclones.
Due to the Coriolis force, these winds move on towards the equatorial low-pressure area. The Coriolis force also known as ‘Ferrel’s Law,’ is an apparent force caused by the earth’s rotation. This force deflects winds towards the right in the northern hemisphere and towards the left in the southern hemisphere.
The north-easterly winds are land-bearing winds; hence they carry very little moisture and bring little or no rain in India.
During winter, a high-pressure area is created in the north of the Himalayas. In summer, a low-pressure area develops over interior Asia as well as over north-western India. This causes a complete reversal of the direction of winds during summer.
Winds move from the high-pressure area over the southern Indian Ocean, cross the equator and turn right towards the low-pressure areas over the Indian subcontinent. These winds are known as the south-west monsoon winds.
An important component of the flow is the jet stream. Jet streams are a narrow belt of high altitude westerly winds that blow in the troposphere. Their speed varies from about 110 kilometres per hour in summer to about 184 kilometres per hour in winter.
A number of separate jet streams have been identified. The most constant are the mid-latitude and the sub-tropical jet streams. They originate from the Mediterranean region and are known as subtropical westerly jet streams. An easterly jet stream, called the tropical easterly jet stream, blows over peninsular India, approximately over 14°N during the summer months. The movement of water in the oceans is called currents.