Climate

Have you ever wondered why you wear woollens in December, feel hot and sticky in May, and see it rain for weeks in June or July? The answer to all these questions lies in understanding the climate of India.

Climate refers to the average weather conditions and their variations over a very large area for a long period of time, typically more than thirty years. In contrast, weather is the state of the atmosphere over a specific area at any single point in time. While weather can change in a matter of hours, climate is the long-term pattern.

The basic elements of both weather and climate are the same:

Temperature
Atmospheric pressure
Wind
Humidity
Precipitation (rain, snow, etc.)

Based on these generalised monthly conditions, we divide the year into seasons like winter, summer, or rainy season.

The climate of India is described as the ‘monsoon’ type. This type of climate is common in South and Southeast Asia. The word monsoon comes from the Arabic word ‘mausim’, which means ‘season’. It refers to the seasonal reversal in the direction of the wind during the year.

While India has an overall unity in its monsoon climate, there are significant regional variations. Let's look at two key elements: temperature and precipitation.

Temperature Variations: In summer, the temperature in the Rajasthan desert can soar to 50°C, while in Pahalgam (Jammu and Kashmir), it might be a pleasant 20°C. In winter, Drass (Jammu and Kashmir) can experience bone-chilling temperatures as low as -45°C, while Thiruvananthapuram in the south might have a comfortable 22°C.

Example

Think about the difference between day and night temperatures. In the Thar Desert, the day can be 50°C, but the night can cool down to 15°C. This is a huge difference! On the other hand, in coastal areas like Kerala or the Andaman and Nicobar Islands, there is hardly any difference between day and night temperatures.

Precipitation Variations: Precipitation also varies greatly.
- Form: In the upper parts of the Himalayas, it is mostly snowfall, while the rest of the country receives rain.
- Amount: Annual precipitation is over 400 cm in Meghalaya but less than 10 cm in Ladakh and western Rajasthan.
- Timing: Most of the country gets rain from June to September. However, the Tamil Nadu coast receives most of its rainfall during October and November.

Generally, coastal areas have less extreme temperatures, while the interior of the country experiences more seasonal contrasts. In the Northern Plains, rainfall tends to decrease as you move from east to west. These climatic variations influence the lives of people, affecting the food they eat, the clothes they wear, and the types of houses they build.

Climatic Controls

There are six major factors, or controls, that determine the climate of any place in the world.

Latitude: Due to the Earth's curve, places closer to the equator receive more direct solar energy and are warmer. As you move towards the poles, temperature generally decreases.
Altitude: As you go higher up from the Earth's surface, the atmosphere becomes less dense, and the temperature drops. This is why hills are cooler than plains, especially in summer.
Pressure and Wind System: The pressure and wind system of an area is influenced by its latitude and altitude. It plays a major role in determining temperature and rainfall patterns.
Distance from the Sea (Continentality): The sea has a moderating effect on climate. Coastal areas have less extreme temperatures (not too hot, not too cold). As you move away from the sea, this influence decreases, leading to extreme weather conditions. This is known as continentality (very hot summers and very cold winters).
Ocean Currents: Ocean currents, along with winds blowing from the sea to the land (onshore winds), can affect the climate of coastal areas. For example, a warm current will warm up a coastal area if the winds blow onshore.
Relief Features: The physical features of the land, like mountains, play a significant role. High mountains can act as barriers, blocking cold or hot winds. They can also cause precipitation by forcing rain-bearing winds to rise. The side of the mountain that faces away from the wind, known as the leeward side, remains relatively dry.

Factors Affecting India's Climate

Let's see how these controls specifically apply to India.

Latitude

The Tropic of Cancer passes right through the middle of India, from the Rann of Kuchchh in the west to Mizoram in the east. This divides the country into two climatic zones:

The area south of the Tropic of Cancer is in the tropical area.
The area north of it is in the sub-tropics. Because of this, India's climate has characteristics of both tropical and subtropical climates.

Altitude

India has the towering Himalayas to the north, with an average height of about 6,000 metres. These mountains act as a massive barrier, preventing the intensely cold winds from Central Asia from entering the subcontinent. It is because of the Himalayas that India experiences comparatively milder winters than Central Asia.

Pressure and Winds

The pressure and wind conditions over India are unique and are governed by three main atmospheric conditions:

Pressure and surface winds
Upper air circulation
Western cyclonic disturbances and tropical cyclones

Normally, India lies in the region of north-easterly winds. These winds originate over the northern hemisphere's subtropical high-pressure belt and blow south. Due to the Coriolis force (an apparent force caused by the Earth's rotation that deflects winds), they turn right and move towards the equatorial low-pressure area. Since these winds originate over land, they carry very little moisture and should have made India an arid country. But that's not the case.

Note

The Coriolis force deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is also known as 'Ferrel's Law'.

The pressure and wind system over India undergoes a complete reversal between winter and summer.

In Winter: A high-pressure area develops north of the Himalayas. Cold, dry winds blow from this region towards the low-pressure areas over the oceans to the south.
In Summer: A low-pressure area develops over interior Asia and northwestern India. This causes a complete reversal of wind direction. Air from the high-pressure area over the southern Indian Ocean crosses the equator, gets deflected to the right, and blows towards the Indian subcontinent. These are known as the Southwest Monsoon winds. As they blow over warm oceans, they gather moisture and bring widespread rainfall to the mainland of India.

The Seasons

The monsoon climate is defined by a distinct seasonal pattern. India experiences four main seasons:

The Cold Weather Season (Winter)
The Hot Weather Season (Summer)
The Advancing Monsoon (The Rainy Season)
The Retreating/Post Monsoons (The Transition Season)

The Cold Weather Season (Winter)

This season begins in mid-November in northern India and lasts until February.

Temperature: December and January are the coldest months. Temperatures decrease as you move from south to north. Chennai, on the eastern coast, has an average temperature of 24°-25°C, while in the northern plains, it ranges between 10°C and 15°C.
Weather: Days are typically warm and nights are cold. Frost is common in the north, and the Himalayas experience snowfall. The weather is usually marked by clear skies, low temperatures, and low humidity.
Winds and Rainfall: The northeast trade winds blow from land to sea, so most of the country remains dry. However, these winds blow from sea to land over the Tamil Nadu coast, bringing some rainfall there.
Cyclonic Disturbances: A key feature of this season is the inflow of cyclonic disturbances from the west. These low-pressure systems originate over the Mediterranean Sea and move into India. They cause much-needed winter rains over the plains and snowfall in the mountains. This winter rainfall, locally known as ‘mahawat’, is small in amount but is very important for the cultivation of ‘rabi’ crops.
Peninsular Region: The peninsular region does not have a well-defined cold season due to the moderating influence of the sea.

The Hot Weather Season (Summer)

From March to May, India experiences its hot weather season.

Temperature: Due to the northward movement of the sun, the global heat belt shifts north. Temperatures rise across the country. In May, it is common for temperatures in northwestern India to reach 45°C. In peninsular India, temperatures remain lower because of the moderating influence of the oceans.
Pressure: As temperatures rise, air pressure falls in the northern part of the country. By the end of May, a low-pressure trough develops, stretching from the Thar Desert to the Chotanagpur plateau.
Local Storms and Winds:
- ‘Loo’: These are strong, gusty, hot, dry winds that blow during the day over north and northwestern India. Direct exposure can be dangerous.
- Dust Storms: Common in northern India during May, these storms can bring temporary relief by lowering temperatures and sometimes bringing light rain.
- ‘Kaal Baisakhi’: These are localised thunderstorms with violent winds and heavy rain, often with hail, that occur in West Bengal.
- ‘Mango showers’: Towards the end of summer, pre-monsoon showers are common in Kerala and Karnataka. They are called 'mango showers' because they help in the early ripening of mangoes.

Advancing Monsoon (The Rainy Season)

By early June, the low-pressure condition over the northern plains intensifies and attracts the trade winds from the southern hemisphere.

Onset: These winds, originating over the southern oceans, cross the equator and enter the Indian peninsula as the south-west monsoon. They blow at an average speed of 30 km per hour and cover the country in about a month.
Rainfall Distribution:
- The windward side of the Western Ghats receives very heavy rainfall (more than 250 cm).
- The Deccan Plateau and parts of Madhya Pradesh, which lie in the rain shadow area, receive less rain.
- The highest rainfall is received in the north-eastern part of the country. Mawsynram in the Khasi Hills gets the highest average rainfall in the world.
- In the Ganga valley, rainfall decreases from east to west. Rajasthan and parts of Gujarat get very little rain.
‘Breaks’ in Rainfall: The monsoon tends to have wet and dry spells. These rainless intervals are known as ‘breaks’ in the monsoon. They are related to the movement of the monsoon trough. When the trough's axis is over the plains, there is good rainfall. When it shifts closer to the Himalayas, the plains experience dry spells, while the mountains get widespread rain, which can cause devastating floods in the plains.
Uncertainty: The monsoon is known for its uncertainties. Its arrival and retreat can be irregular, and the distribution of rain can be uneven, causing floods in one part of the country and droughts in another. This often disrupts farming schedules for millions.

Retreating/Post Monsoons (The Transition Season)

During October and November, the sun's apparent movement towards the south causes the low-pressure trough over the northern plains to weaken and be replaced by a high-pressure system.

Withdrawal: The south-west monsoon winds weaken and start withdrawing. By the beginning of October, the monsoon withdraws from the Northern Plains.
‘October Heat’: This period is a transition from the hot rainy season to dry winter conditions. The retreat is marked by clear skies and a rise in temperature. While days are hot, nights are cool and pleasant. The combination of high temperature and humidity makes the weather oppressive during the day, a condition commonly known as ‘October heat’.
Cyclonic Depressions: In early November, the low-pressure conditions shift to the Bay of Bengal. This leads to the formation of cyclonic depressions, which originate over the Andaman Sea. These cyclones often cross the eastern coasts of India, causing heavy and widespread rain. The deltas of the Godavari, Krishna, and Kaveri rivers are frequently struck by these destructive cyclones. The bulk of the rainfall on the Coromandel Coast comes from these depressions and cyclones.

Distribution of Rainfall

Rainfall in India is highly uneven.

High Rainfall Areas (over 400 cm annually): Parts of the western coast and northeastern India.
Low Rainfall Areas (less than 60 cm annually): Western Rajasthan, adjoining parts of Gujarat, Haryana, and Punjab. Rainfall is also low in the interior of the Deccan plateau (east of the Sahyadris) and around Leh in Jammu and Kashmir.
Moderate Rainfall Areas: The rest of the country receives moderate rainfall.
Snowfall: This is restricted to the Himalayan region.