Chapter Notes

Climate

Have you ever noticed that you drink more water and wear lighter clothes in the summer than in the winter? In northern India, people wear heavy woollen clothes during winter, but in southern India, these are rarely needed. This is because of variations in weather conditions across different seasons and regions. These changes are driven by the elements of weather, such as temperature, pressure, wind, humidity, and precipitation.

It's important to understand the difference between weather and climate:

• Weather is the state of the atmosphere at a specific moment. It can change quickly, within a day or even an hour.
• Climate is the average of weather conditions over a very long period, often 50 years or more. Climate changes are slow and almost unnoticeable over short periods.

India's climate is known as a hot monsoonal climate, which is common in south and southeast Asia. The word monsoon refers to a climate pattern with a seasonal reversal in the direction of winds.

Unity and Diversity in the Monsoon Climate

The monsoon system creates a broad climatic unity across India and Southeast Asia. However, within India, there are significant regional variations. The climate in Kerala is very different from that in Uttar Pradesh, yet both are considered monsoon climates. These regional diversities are like sub-types of the main monsoon climate and are seen in patterns of temperature, wind, and rainfall.

Temperature Variations:

• Seasonal Extremes: In summer, western Rajasthan can reach 55°C, while in winter, Leh can drop to -45°C.
• Same-Day Differences: On a June day, Churu (Rajasthan) might be 50°C, while Tawang (Arunachal Pradesh) is only 19°C.
• Night and Day Differences (Diurnal Range): In the Thar desert, a day temperature of 50°C can fall to 15°-20°C at night. In contrast, in coastal areas like Kerala or the Andaman Islands, the difference between day and night temperature is barely seven or eight degrees.

Precipitation Variations:

• Type of Precipitation: The Himalayas receive snowfall, while the rest of the country gets rain.
• Amount of Rainfall: Mawsynram in the Khasi Hills (Meghalaya) receives over 1,080 cm of rain annually, whereas Jaisalmer (Rajasthan) gets less than 9 cm.
• Intensity: Tura in the Garo Hills (Meghalaya) can receive as much rain in a single day as Jaisalmer gets in 10 years.
• Seasonal Distribution: Most of India gets rain from June to September. However, the coastal areas of Tamil Nadu receive rain at the beginning of winter.

Factors Determining the Climate of India

Several factors control India's climate.

Latitude

The Tropic of Cancer passes through the middle of India.

• South of the Tropic of Cancer: This area is in the tropical zone, close to the equator. It experiences high temperatures year-round with a small daily and annual range of temperature.
• North of the Tropic of Cancer: This area lies in the sub-tropical and temperate zones. It is further from the equator and experiences an extreme climate with high daily and annual temperature ranges.

The Himalayan Mountains

The Himalayas act as a massive climatic divide.

• Protection from Cold Winds: They form a shield that protects the subcontinent from the intensely cold, chilly winds that originate near the Arctic circle and blow across Asia.
• Trapping Monsoon Winds: The mountains also trap the moisture-laden monsoon winds, forcing them to release their rainfall within the Indian subcontinent.

Distribution of Land and Water

India is surrounded by the Indian Ocean on three sides and a high mountain wall in the north. Land heats up and cools down faster than water. This differential heating of land and sea creates different air pressure zones in different seasons, which causes the seasonal reversal of monsoon winds.

Distance from the Sea

• Coastal Areas: Places with a long coastline, like Mumbai and the Konkan coast, have an equable climate. The sea has a moderating influence, meaning there are no extreme temperature variations.
• Interior Areas: Places far from the sea, like Delhi, Kanpur, and Amritsar, experience extremes of climate (very hot summers and very cold winters).

Altitude

Temperature decreases as altitude increases. Because the air is thinner at higher elevations, mountain places are cooler than places on the plains.

Relief

The physical features (physiography) of India also affect climate.

• Windward vs. Leeward Side: The windward sides of the Western Ghats and Assam receive heavy rainfall because they face the moisture-carrying winds. The southern plateau, which is on the leeward side (the side sheltered from the wind), remains dry.

Inter Tropical Convergence Zone (ITCZ)

The Inter Tropical Convergence Zone (ITCZ) is a low-pressure zone near the equator where trade winds meet.

• In Summer (July): The ITCZ shifts northward and is located over the Gangetic plain (around 20°N-25°N). This is often called the monsoon trough. This trough helps develop a thermal low over northwest India.
• Effect on Winds: Because of this shift, the trade winds from the southern hemisphere cross the equator and, due to the Coriolis force, start blowing from the southwest to the northeast. This becomes the southwest monsoon.
• In Winter: The ITCZ moves southward. This causes a reversal of winds, which then blow from the northeast to the south and southwest, becoming the northeast monsoons.

The Nature of Indian Monsoon

The monsoon is a well-known but complex climatic phenomenon that scientists are still studying. A real breakthrough in understanding it came when it was studied at a global level, not just a regional one. Key aspects to understand are the onset and the breaks in the monsoon.

Onset of the Monsoon

The traditional belief is that the differential heating of land and sea sets the stage for the monsoon.

1. During April and May, the sun is vertically over the Tropic of Cancer, intensely heating the landmass of northern India.
2. This creates an intense low-pressure area in the northwestern part of the subcontinent.
3. Meanwhile, the Indian Ocean to the south remains cooler and has a high-pressure zone.
4. This pressure difference attracts the southeast trade winds from the southern hemisphere. After crossing the equator, these winds are deflected towards the Indian subcontinent, becoming the southwest monsoon.

The onset is also linked to upper-air circulation:

• Westerly Jet Stream: The withdrawal of the westerly jet stream from its position over the north Indian plain is crucial.
• Easterly Jet Stream: Only after the westerly jet stream withdraws does the easterly jet stream set in along 15°N latitude. This easterly jet stream is considered responsible for the burst of the monsoon in India.

Entry of Monsoon into India:

• The southwest monsoon arrives at the Kerala coast around 1st June.
• It moves quickly, reaching Mumbai and Kolkata between 10th and 13th June.
• By mid-July, the monsoon covers the entire subcontinent.

Break in the Monsoon

During the monsoon season, if rain fails to occur for one or more weeks, it is known as a break in the monsoon. These dry spells are common and happen for different reasons:

• In Northern India: Breaks occur if the rain-bearing storms are not frequent along the monsoon trough (ITCZ).
• On the West Coast: Dry spells happen when the winds blow parallel to the coast instead of hitting it.

The Rhythm of Seasons

Indian meteorologists recognize four distinct seasons.

The Cold Weather Season

This season typically begins in mid-November in northern India, with December and January being the coldest months.

• Temperature:
  • In northern India, the mean daily temperature stays below 21°C. Night temperatures can drop below freezing in Punjab and Rajasthan.
  • The Peninsular region does not have a well-defined cold season due to the moderating influence of the sea and its proximity to the equator.
• Reasons for Cold in North India:
  • Continental Climate: States like Punjab, Haryana, and Rajasthan are far from the sea.
  • Himalayan Snowfall: Snowfall in the nearby Himalayas creates cold waves.
  • Cold Winds: Around February, cold winds from the Caspian Sea and Turkmenistan bring cold waves, frost, and fog to northwestern India.
• Pressure and Winds:
  • By late December, the sun is over the Tropic of Capricorn (southern hemisphere), leading to feeble high-pressure conditions over the northern plains of India.
  • Winds start blowing from the high-pressure zone in the northwest towards the low-pressure zone over the Indian Ocean. These are the northeast monsoons.
  • These winds are light, with speeds of about 3-5 km per hour.
• Rainfall:
  • Winter monsoons are generally dry as they move from land to sea.
  • Exception 1 (Western Disturbances): Weak temperate cyclones originating from the Mediterranean Sea travel eastwards and cause winter rainfall in Punjab, Haryana, Delhi, and western Uttar Pradesh. This rain is vital for rabi crops. In the Himalayas, this precipitation falls as snow.
  • Exception 2 (Northeast Monsoon): During October and November, the northeast monsoon picks up moisture while crossing the Bay of Bengal and causes heavy rainfall over the Tamil Nadu coast, southern Andhra Pradesh, and southeast Kerala.

The Hot Weather Season

With the sun's northward movement towards the Tropic of Cancer in March, temperatures begin to rise in north India. April, May, and June are the summer months.

• Temperature:
  • In May, temperatures in northwest India can reach around 48°C.
  • South India experiences a milder summer due to the moderating effect of the oceans, with temperatures between 26°C and 32°C.
• Pressure and Winds:
  • The intense heat creates a strong low-pressure trough (the ITCZ) extending from the Thar desert to the Chotanagpur plateau.
  • Hot, dry, and oppressive winds called the 'Loo' blow in the northern plains, especially between Delhi and Patna.
  • Dust storms are common in the evening in Punjab, Haryana, and Uttar Pradesh, bringing light rain and a cool breeze.

Some Famous Local Storms of Hot Weather Season

• Mango Shower: Pre-monsoon showers in Kerala and coastal Karnataka that help in the early ripening of mangoes.
• Blossom Shower: Showers in Kerala and nearby areas that cause coffee flowers to blossom.
• Nor Westers: Dreaded evening thunderstorms in Bengal and Assam, locally known as 'Kalbaisakhi' (a calamity of the month of Baisakh). These are useful for tea, jute, and rice cultivation. In Assam, they are called "Bardoisila".
• Loo: The hot, dry winds of the Northern plains.

The Southwest Monsoon Season

By early June, the low-pressure conditions over the northwestern plains are strong enough to attract the moisture-laden trade winds from the Southern Hemisphere.

• The "Burst" of the Monsoon: The sudden arrival of these moisture-laden winds, accompanied by violent thunder and lightning, is called the "break" or "burst" of the monsoons. This brings a significant drop in temperature.
• Two Branches of the Monsoon: The monsoon approaches India in two branches.
  1. The Arabian Sea branch
  2. The Bay of Bengal branch

Monsoon Winds of the Arabian Sea

This branch splits into three parts:

1. First Branch: Obstructed by the Western Ghats. It brings very heavy rain (250 cm to 400 cm) to the windward side. After crossing the Ghats, the winds descend, get heated, and cause little rainfall on the leeward side, which is known as a rain-shadow area.
2. Second Branch: Strikes the coast north of Mumbai and travels along the Narmada and Tapi river valleys, causing rainfall in central India before merging with the Bay of Bengal branch.
3. Third Branch: Strikes Saurashtra and Kachchh, passes over Rajasthan along the Aravalis (causing scanty rain), and joins the Bay of Bengal branch in Punjab and Haryana.

Monsoon Winds of the Bay of Bengal

• This branch strikes the coast of Myanmar and is deflected by the Arakan Hills towards the Indian subcontinent.
• It enters West Bengal and Bangladesh from the south and southeast.
• It then splits into two:
  1. One branch moves west along the Ganga plains towards Punjab.
  2. Another branch moves up the Brahmaputra valley in the northeast. A sub-branch strikes the Garo and Khasi hills, where Mawsynram receives the highest average annual rainfall in the world.

Season of Retreating Monsoon

The months of October and November are known for the retreating monsoon.

• Withdrawal Process: As the sun marches south, the low-pressure trough over the Ganga plain weakens and starts moving southward. The monsoon withdraws from western Rajasthan by the first week of September and most of north and central India by the end of the month.
• Weather Conditions: The retreat is marked by clear skies and a rise in temperature. The land is still moist, and the combination of high temperature and humidity creates oppressive weather known as ‘October heat’.
• Rainfall:
  • North India becomes dry.
  • The eastern part of the Peninsula receives rain. The low-pressure center shifts to the Bay of Bengal, leading to cyclonic depressions.
  • These tropical cyclones originate over the Andaman Sea, cross the eastern coast, and cause destructive, widespread rain on the Coromandel coast (the deltas of the Godavari, Krishna, and Kaveri).

El-Nino and the Indian Monsoon

• El-Nino is a complex weather system involving the appearance of a warm ocean current off the coast of Peru in the Eastern Pacific, which occurs every three to seven years.
• It causes weather extremes like droughts and floods worldwide.
• In India, El-Nino is used to forecast long-range monsoon rainfall. An El-Nino event is often linked to delayed or weaker monsoons. In 1990-91, a strong El-Nino event delayed the onset of the monsoon by five to twelve days across the country.

Traditional Indian Seasons

In north and central India, a year is traditionally divided into six seasons, each lasting two months. This system is based on practical experience but does not match the climate of south India, where there is little seasonal variation.

Distribution of Rainfall

The average annual rainfall in India is about 125 cm, but it varies greatly from place to place.

• Areas of High Rainfall (over 200 cm): The west coast, Western Ghats, sub-Himalayan areas in the northeast, and the hills of Meghalaya. Some parts of the Khasi and Jaintia hills receive over 1,000 cm.
• Areas of Medium Rainfall (100-200 cm): Southern Gujarat, east Tamil Nadu, Odisha, Jharkhand, Bihar, eastern Madhya Pradesh, and the northern Ganga plain.
• Areas of Low Rainfall (50-100 cm): Western Uttar Pradesh, Delhi, Haryana, Punjab, Jammu and Kashmir, eastern Rajasthan, Gujarat, and the Deccan Plateau.
• Areas of Inadequate Rainfall (below 50 cm): Parts of the Peninsula (in Andhra Pradesh, Karnataka, Maharashtra), Ladakh, and most of western Rajasthan.

Monsoons and the Economic Life in India

The monsoon is central to India's economy and culture.

• Agriculture: The entire agricultural cycle revolves around the monsoon. About 64% of people depend on agriculture, which is itself based on the southwest monsoon.
• Crop Diversity: Regional variations in the monsoon climate allow for a wide variety of crops to be grown.
• Water Availability: Winter rainfall from temperate cyclones is crucial for rabi crops in north India. Snowfall in the Himalayas sustains the flow of water in rivers during the summer.
• Economic Impact: The success of agriculture depends on timely and well-distributed rainfall. Failure of the monsoon can lead to droughts, while excessive rain can cause floods and soil erosion.
• Cultural Reflection: The climatic variation is reflected in the diversity of food, clothing, and house types across India.

Global Warming

Climate is always changing, but human activities, especially large-scale industrialization, are accelerating this change through global warming.

• Greenhouse Effect: Gases like carbon dioxide (CO2), methane, chlorofluorocarbons, and nitrous oxide are known as greenhouse gases. They are released by burning fossil fuels and other activities. These gases are very effective at trapping long-wave radiation, which enhances the greenhouse effect and warms the planet.
• Impacts of Global Warming:
  • The mean annual surface temperature of the Earth has increased over the last 150 years. It is projected to increase by about 2°C by the year 2100.
  • This rise in temperature will cause polar ice caps and mountain glaciers to melt.
  • As a result, sea levels are predicted to rise by an average of 48 cm by the end of this century, increasing the risk of annual flooding for coastal areas.
  • It could also lead to shifts in climatic boundaries, changes in agricultural patterns, and the spread of insect-borne diseases like malaria.