Detailed Chapter Notes (Part - 1) - Climate - Social Studies (SST) Class 9

We study climate to understand the atmospheric conditions prevailing over a particular area, which helps us anticipate seasonal changes and adapt accordingly. This knowledge allows us to prepare for different weather conditions, such as wearing woolen in cold months or expecting rain during certain periods. 

By studying climate, we can better understand and predict weather patterns, ensuring our comfort and safety throughout the year.

What is the Difference between Weather and Climate?

Weather: It is the state of atmosphere at any point in time and space, it changes every moment.

Example: Cloudy, dry, windy, wet weather

Climate : It refers to the sum total of weather conditions and variations over a large area for a long period of time. It is the sum total of average weather conditions of 30 years.

Example: Monsoon, equatorial desert, cold climate etc.
Climate and Weather Differences

India has Diverse Climatic Conditions

We can take two important elements, Temperature, and Precipitation, and examine how they vary from place to place and season to season.

Temperature variations:

• Rajasthan desert: up to 50°C in summer
• Pahalgam, Jammu and Kashmir: around 20°C in summer
• Drass, Jammu and Kashmir: as low as -45°C in winter
• Thiruvananthapuram: around 20°C in winter
• Thar Desert: 50°C in the day, 15°C at night
• Andaman and Nicobar Islands, Kerala: minimal temperature difference between day and night

Different Climatic Zones in India

Precipitation variations:

• Snowfall in the upper parts of the Himalayas
• Rainfall in the rest of the country
• Annual precipitation: over 400 cm in Meghalaya, less than 10 cm in Ladakh and western Rajasthan
• Rainy season: June to September for most parts, October and November for the Tamil Nadu coast
• Coastal areas: less contrast in temperature and seasonal conditions compared to interiors

Climatic Controls

The climate of a place is influenced by various factors that interact to determine its characteristics. These factors include:

• Latitude: The amount of solar energy received varies with latitude, resulting in temperature variations from the equator to the poles. The closer a location is to the equator, the warmer it tends to be.

Latitudes

• Altitude: Higher altitudes have a less dense atmosphere, leading to lower temperatures. As elevation increases, the air becomes thinner, causing a decrease in temperature.
• Pressure and Wind Systems: The distribution of pressure and wind systems across the Earth influences temperature and rainfall patterns. These systems are influenced by factors such as latitude and altitude.
• Distance from the Sea (Continentality): Locations closer to the sea tend to experience more moderate climates due to the moderating effect of the ocean. Greater distance from the sea can result in more extreme weather conditions, with greater temperature variations.
• Ocean Currents: Ocean currents play a role in shaping coastal climates, particularly when combined with onshore winds. Warm ocean currents can lead to milder temperatures, while cold currents can result in cooler climates.
• Relief Features: The presence of relief features, such as mountains, can impact local climates. High mountains act as barriers to winds, forcing them to rise and cool, which can lead to increased precipitation on the windward side. In contrast, the leeward side, sheltered from prevailing winds, tends to be drier.

These factors work together to shape the climate of a region, with each playing a unique role in determining the temperature, precipitation patterns, and overall weather conditions experienced in a particular area.

Factors Affecting India’s Climate

Latitude

• India is situated roughly between 8^οN and  37^ο N latitudes. 
• India is divided into almost two equal parts by the tropic of cancer. 
• The southern half lies in the tropical zone, and the western half is in the subtropical zone. 
• Therefore, India’s climate has characteristics of tropical as well as subtropical climates.

Altitude

• India has mountains to the north, with an average height of about 6000 meters.
• The Himalayas prevent the cold winds from Central Asia from entering the subcontinent.
• It is because of these mountains that this subcontinent experiences comparatively milder winters as compared to Central Asia.

Pressure and Winds

• India lies in the subtropical high-pressure belt; thus, the winds originate from the land and move outwards towards the equatorial low-pressure belt. 
• These winds are known as northeast trade winds and are devoid of any moisture. However, due to unequal heating of land and water in the summer, low pressure develops over the interior of landmasses. 
• This low pressure attracts the winds from the south of the equator. After crossing the equator, the southeast trade winds get deflected and are known as southwest monsoons. The climate of India is also affected by jet streams. 
• This is a fast-flowing wind blowing in a narrow zone in the upper atmosphere. The jet streams are responsible for the sudden outbreak of monsoons in Northern India. A subtropical westerly jet stream brings in the western disturbances in winter. These disturbances cause heavy snowfall in the mountains and light rains in the northwestern part of India.

The Seasons

Four main seasons can be identified in India:

• Cold weather season  --  December to February
• Hot weather season  --  March to May
• Advancing monsoon season  --  June to September
• Retreating monsoon season  -- October and November

The Cold Weather Season (Winter)

• The cold weather season begins from mid-November in India and stays till February.
• December and January are the coldest months in the northern part of India. The temperature decreases as one moves from the south to the north.
• Days are warm, and the nights are cold. Frost is common in the north, and the higher slopes of the Himalayas experience snowfall.
• The northeast trade winds prevail over the country. They blow from land to sea, and hence, for the most part of the country, it is a dry season.
• In the northern part of the country, a feeble high-pressure region develops, with light winds moving outwards from this area.
• The weather is normally marked by a clear sky, low temperatures, and low humidity, and feeble variable winds.
• The inflow of cyclonic disturbances from the west and the northwest. These low-pressure systems originate over the Mediterranean Sea and western Asia and move into India, along with the westerly flow. 
• They cause the much-needed winter rains over the plains and snowfall in the mountains. Locally known as ‘mahawat’ they are of immense importance for the cultivation of ‘rabi’ crops.
• The northeast trade winds cause a fair amount of rainfall in Chennai or the Coromandel Coast in winter.

Hot weather season (Summer)

Atmospheric Conditions over the Indian Subcontinent in the Month of June

• Due to the apparent movement of the sun, the global heat belt shifts northward. As such, from March to May, it is not weather season in India.
• Temperature increases from south to north. In peninsular India, temperatures remain lower.
• High temperature between 38^οC and 48^ο C in the plains.
• Local dust storms are accompanied by light rains.
• Hot, dry winds ‘loo’ are common in May and June.
• Kerala and Karnataka coast receivers pre-monsoon showers (Mango showers)
• West Bengal and Assam are affected by northwesterly winds (Kalbaisakhi).

Additional Information (Old Syllabus)

The Indian Monsoon

The climate of India is strongly influenced by monsoon winds. The Arabs, who had come to India as traders, benefited from the reversal of the wind system as they came by sailing ships at the mercy of winds; they named this seasonal reversal of the wind system ‘monsoon’.

The direction of Monsoon Winds

The monsoons are experienced in the tropical area roughly between 20⁰N - 20⁰ S. 

To understand the mechanism of the monsoons, the following facts are important:

• The differential heating and cooling of land and water create a low pressure on the landmass of India while the seas around experience comparatively high pressure.
• The shift of the position of the Inter-Tropical Convergence Zone (ITCZ) in summer, over the Ganga plain (this is the equatorial trough normally positioned about 5⁰ N of the equator – also known as the monsoon-trough during the monsoon season).
• The presence of the high-pressure area, east of Madagascar, approximately at 20⁰ S over the Indian Ocean. The intensity and position of this high-pressure area affect the Indian Monsoon.
• The Tibetan plateau gets intensely heated during summer, which results in strong vertical air currents and the formation of high pressure. The plateau is about 9 km above sea level.
• The movement of the westerly jet stream to the north of the Himalayas and the presence of the tropical easterly jet stream over the Indian peninsula during summer.
• Changes in the pressure conditions over the southern oceans also affect the monsoons. 
• Normally, when the tropical eastern South Pacific Ocean experiences high pressure, the tropical eastern Indian Ocean experiences low pressure. 
• The difference in pressure over Tahiti (the Pacific Ocean, 18⁰ S/149⁰ W) and Darwin in northern Australia (the Indian Ocean, 12⁰ 30’S/ 131⁰ E) are computed to predict the intensity of the monsoons. If the pressure differences are negative, it means below average and late monsoons.

The Onset of The Monsoon And Withdrawal

• The Monsoon, unlike the trades, does not have steady winds but is pulsating in nature, affected by different atmospheric conditions encountered by it, on its way over the warm tropical seas. 
• The duration of the monsoon is between 100-120 days from early June to mid-September. The monsoon arrives at the southern tip of the Indian peninsula generally by the first week of June. 
• Subsequently, it was divided into two – the Arabian Sea branch and the Bay of Bengal branch. The Arabian Sea branch reaches Mumbai about ten days later, on approximately the 10^th of June. 
• The Bay of Bengal branch arrives in Assam in the first week of June. The lofty mountains cause the monsoon winds to deflect towards the west over the Ganga Plains. By mid-June, the Arabian Sea branch of the monsoon arrives over Saurashtra-Kachchh and the central part of the country. 

• The Arabian Sea and the Bay of Bengal branches of the monsoon merge over the northwestern part of the Ganga plains. Delhi generally receives the monsoon showers from the Bay of Bengal branch by the end of June. 
• By the first week of July, western Uttar Pradesh, Punjab, Haryana, and eastern Rajasthan will experience the monsoon. By mid-July, the monsoon reaches Himachal Pradesh and the rest of the country. Withdrawal or the retreat of the monsoon is a more gradual process. 
• The withdrawal of the monsoon begins in the northwestern states of India by early September. 
• By mid-October, it withdraws completely from the northern half of the peninsula. 
• The withdrawal from the southern half of the peninsula is fairly rapid. By early December, the monsoon had withdrawn from the rest of the country.