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| Class 9th Chapters | ||
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| 1. India – Size And Location | 2. Physical Features Of India | 3. Drainage |
| 4. Climate | 5. Natural Vegetation And Wildlife | 6. Population |
Chapter 4 Climate
Following the study of India's landforms and drainage systems, this chapter focuses on the third basic element of the natural environment: **atmospheric conditions**, or climate. Understanding the climate helps explain seasonal variations like wearing woollens in December, experiencing heat in May, or receiving rain in June-July.
**Climate** is defined as the sum total of **weather conditions and variations** observed over a large geographical area for a **long period of time** (typically more than thirty years).
**Weather**, in contrast, refers to the state of the atmosphere over a specific area at any particular point in time. The elements that define both weather and climate are identical: temperature, atmospheric pressure, wind, humidity, and precipitation. Weather conditions can fluctuate frequently, even within a single day, but they often exhibit consistent patterns over weeks or months, leading to the concept of seasons.
Based on generalised monthly atmospheric conditions, the year is divided into distinct **seasons**, such as winter, summer, and the rainy season. The world is divided into various climatic regions. India's climate is known as the **'monsoon' type**, a term derived from the Arabic word 'mausim', meaning season. 'Monsoon' specifically refers to the **seasonal reversal in wind direction** throughout the year, a defining characteristic of the Indian climate.
While India's climate has an overall unity due to the monsoon influence, there are noticeable **regional variations** in climatic conditions across the country. These variations are evident when examining key elements like temperature and precipitation:
- **Temperature:** In summer, temperatures can reach 50°C in parts of the Rajasthan desert, while remaining around 20°C in mountainous regions like Pahalgam (Jammu and Kashmir). In winter, temperatures can drop to minus 45°C in places like Drass (Jammu and Kashmir), while Thiruvananthapuram in the south maintains a temperature of around 22°C. There are also variations in diurnal (day and night) temperature ranges; the Thar Desert can experience a difference of 35°C (50°C day to 15°C night), while coastal areas like Andaman and Nicobar or Kerala show minimal diurnal variation.
- **Precipitation:** Variations exist in the form, type, amount, and seasonal distribution of precipitation. Precipitation in the upper Himalayas is mostly snowfall, while the rest of the country receives rain. Annual precipitation varies significantly, from over 400 cm in Meghalaya to less than 10 cm in Ladakh and western Rajasthan. Most parts of India receive rainfall during the monsoon season (June to September), but some regions, like the Tamil Nadu coast, receive a large portion of their rain during October and November (retreating monsoon).
Generally, coastal areas experience less extreme temperatures due to the moderating influence of the sea, while interior regions have more pronounced seasonal temperature contrasts. Rainfall tends to decrease from east to west in the Northern Plains. These regional climatic variations contribute to the diversity in people's lives, influencing their food, clothing, and housing styles.
Houses in Rajasthan often have thick walls and flat roofs to provide insulation against extreme heat and facilitate rainwater harvesting on the roof. Houses in the Terai region, Goa, and Mangalore (areas receiving heavy rainfall) have sloping roofs to allow rainwater to drain off easily. Houses in Assam, a region prone to floods and high rainfall, are often built on stilts to protect against inundation.
Climatic Controls
The climate of any place is influenced by six major controlling factors:
- **Latitude:** Due to the Earth's curvature, the amount of solar energy received varies with latitude. Temperature generally decreases as one moves from the equator towards the poles.
- **Altitude:** As elevation increases, the atmosphere becomes less dense, and temperature decreases. Higher altitudes are therefore cooler, especially during summers.
- **Pressure and Wind System:** These are influenced by latitude and altitude. They determine the circulation of air, bringing or taking away moisture, thus affecting temperature and rainfall patterns.
- **Distance from the Sea (Continentality):** The sea has a moderating effect on temperature. Coastal areas experience less extreme temperatures (equable climate) compared to interior regions, which have a continental climate with very hot summers and very cold winters.
- **Ocean Currents:** Warm or cold ocean currents flowing past a coastal area can affect its climate, particularly if accompanied by onshore winds.
- **Relief Features:** Mountains and other landforms influence climate. High mountains can act as barriers to winds (cold or hot) and can cause precipitation if they are high enough and lie in the path of rain-bearing winds. Areas on the leeward side of mountains (away from the wind) often receive less rainfall and are relatively dry (rain shadow effect).
Most of the world's deserts are located in the western margins of continents in the subtropics because of the presence of permanent high-pressure belts in these latitudes, which cause descending dry air. Additionally, prevailing trade winds blow away from the western coasts, preventing moisture-laden air from reaching these regions. Cold ocean currents along some western coasts also contribute to arid conditions by stabilising the air and reducing evaporation.
Factors Affecting India’s Climate
Several factors specifically influence the climate of India, shaping its monsoon pattern and regional variations.
Latitude
The **Tropic of Cancer** (23°30'N) passes through the central part of India. This divides the country into two climatic zones: the area south of the Tropic falls within the **tropical zone**, experiencing high temperatures throughout the year, while the area north of the Tropic lies in the **sub-tropical zone**, which experiences both warm and cold seasons. Therefore, India's climate exhibits characteristics of both tropical and subtropical climates.
Altitude
India has formidable mountains in the north, the Himalayas, with an average height of around 6,000 metres. These high mountains act as a barrier, **preventing the cold winds from Central Asia** from entering the Indian subcontinent. This protective function of the Himalayas results in India experiencing comparatively **milder winters** than the regions in Central Asia situated at similar latitudes. Additionally, India has extensive coastal areas with much lower elevations (maximum around 30 metres), contributing to temperature variations.
Pressure And Winds
The atmospheric pressure and wind conditions over India are unique and play a decisive role in determining its climate, particularly the monsoon. India lies in the zone of **north-easterly trade winds**. These winds originate from the subtropical high-pressure belt in the northern hemisphere, blow southwards, and due to the Coriolis force, deflect to the right, moving towards the equatorial low-pressure area.
**Coriolis force** is an apparent force caused by the Earth's rotation that deflects moving objects (like winds) to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is also known as 'Ferrel's Law'.
Generally, north-easterly trade winds blow over land and carry little moisture, leading to dry conditions. However, India's climate is not arid, primarily due to the unique seasonal changes in pressure and wind systems. During winter, a high-pressure area develops north of the Himalayas, causing cold, dry winds to blow towards the low-pressure areas over the southern oceans. In summer, as the sun shifts north, an intense low-pressure area develops over interior Asia, including northwestern India. This creates a complete reversal of wind direction. Air from the high-pressure area over the southern Indian Ocean blows in a southeasterly direction, crosses the equator, is deflected to the right by the Coriolis force, and flows towards the Indian subcontinent as the **Southwest Monsoon winds**. These winds blow over warm oceans, collecting abundant moisture, and bring widespread rainfall to the mainland of India, characteristic of the monsoon season.
The word **monsoon** is derived from the Arabic word 'mausim', literally meaning season. It refers to the **seasonal reversal in wind direction** during the year, which is the defining feature of India's climate.
The Seasons
The monsoon climate in India is marked by a distinct pattern of seasons, with significant changes in weather conditions from one season to the next. These seasonal changes are most noticeable in the interior parts of the country, while coastal areas experience less variation in temperature, although rainfall patterns differ seasonally. India experiences four main seasons:
- The Cold Weather Season (Winter)
- The Hot Weather Season (Summer)
- The Advancing Monsoon (The Rainy Season)
- The Retreating Monsoon (The Transition Season)
The Cold Weather Season (Winter)
The cold weather season in northern India spans from **mid-November to February**. December and January are the coldest months. Temperatures decrease from south to north. While coastal cities like Chennai have average temperatures of 24°–25°C, the Northern Plains experience temperatures ranging from 10°C to 15°C. Days are relatively warm, but nights are cold. Frost is common in the north, and higher altitudes in the Himalayas receive snowfall.
During winter, **northeast trade winds** prevail. As they blow from land to sea, they generally bring **dry conditions** to most parts of the country. However, the **Tamil Nadu coast** receives some rainfall from these winds because here they blow from sea to land. In northern India, a weak high-pressure area forms, leading to light winds flowing outwards, generally from west and northwest through the Ganga valley. The weather is typically characterised by clear skies, low temperatures, low humidity, and weak, variable winds.
A notable feature of the winter season in the Northern Plains is the inflow of **cyclonic disturbances from the west and northwest**. These low-pressure systems originate over the Mediterranean Sea and Western Asia and travel into India, moving along with the westerly winds (jet streams). They cause welcome **winter rains over the plains** (locally known as ‘mahawat’), which, though small in amount, are highly important for the cultivation of rabi (winter) crops. These disturbances also bring snowfall to the mountains.
The peninsular region generally does not experience a distinct or severe cold season due to the moderating influence of the sea on temperatures during winters.
The Hot Weather Season (Summer)
The hot weather season, or summer, lasts from **March to May**. Due to the sun's apparent movement northward, the heat belt also shifts northwards, leading to rising temperatures across India. Temperatures recorded in March on the Deccan plateau can reach around 38°C. In April, temperatures in Gujarat and Madhya Pradesh may rise to around 42°C, and by May, 45°C is common in the northwestern parts of the country. Coastal areas of peninsular India experience relatively lower temperatures during summer due to the moderating effect of the oceans.
In northern India, summer months are marked by rising temperatures and falling air pressure. By the end of May, an elongated low-pressure area forms, stretching from the Thar Desert in the northwest to areas like Patna and the Chotanagpur plateau in the east. Air circulation develops around this low-pressure trough.
A striking characteristic of the hot weather season in north and northwestern India is the occurrence of the **‘loo’** – strong, gusty, hot, and dry winds that blow during the day and sometimes continue into the evening. Direct exposure to these winds can be dangerous, even fatal. **Dust storms** are frequent in May in northern India, providing temporary relief by lowering temperature and sometimes bringing light rain and cool breezes. This season also sees localised thunderstorms associated with violent winds, heavy rainfall, and sometimes hail. In West Bengal, these storms are called the **‘Kaal Baisakhi’**.
Towards the end of summer, **pre-monsoon showers** are common, particularly in Kerala and Karnataka. These showers help ripen mangoes early and are often referred to as **‘mango showers’**.
Advancing Monsoon (The Rainy Season)
By **early June**, the low-pressure conditions over the northern plains intensify. This attracts the **south-east trade winds** from the southern hemisphere. These winds originate over the warm subtropical areas of the southern oceans, cross the equator, and influenced by the Coriolis force, turn right and enter the Indian Peninsula as the **South-West Monsoon**. As these winds travel over warm oceans, they carry significant moisture to the subcontinent.
The South-West Monsoon winds are strong, blowing at an average speed of 30 km per hour, and cover most of the country (except the extreme north-west) within approximately a month. Their arrival brings about a dramatic change in weather conditions across India.
Figure 4.1 is a map showing the direction of the Advancing Monsoon (Southwest Monsoon) winds over India, indicating how they bring moisture from the Arabian Sea and Bay of Bengal to the subcontinent during the rainy season.
Key characteristics of the Advancing Monsoon:
- The **windward side of the Western Ghats** receives very heavy rainfall early in the season, often exceeding 250 cm.
- The Deccan Plateau and parts of Madhya Pradesh receive some rainfall, despite lying in the rain shadow area of the Western Ghats.
- The **north-eastern part of the country** receives the maximum rainfall during this season. **Mawsynram** in the southern ranges of the Khasi Hills (Meghalaya) is renowned for receiving the highest average rainfall in the world.
- Rainfall in the Ganga valley generally decreases as one moves from east to west. Rajasthan and parts of Gujarat receive scanty rainfall.
A notable feature of the monsoon rainfall is its tendency for **‘breaks’**, alternating between wet and dry spells. Rainfall occurs for a few days at a time, interspersed with rainless periods. These breaks are linked to the movement of the monsoon trough. When the trough's axis is over the plains, these areas receive good rainfall. When it shifts closer to the Himalayas, the plains experience longer dry spells, while mountainous catchment areas receive heavy rains, leading to floods in the plains. The amount and duration of monsoon rains are also influenced by the frequency and intensity of tropical depressions forming in the Bay of Bengal.
The monsoon is known for its **uncertainties** – variations in intensity, frequency, and duration of wet and dry spells. This can lead to devastating floods in some areas while causing droughts in others. The irregular arrival and retreat of the monsoon can disrupt the farming schedules of millions of farmers.
Retreating/Post Monsoons (The Transition Season)
The period of **October-November** marks the transition from the hot rainy season to the dry winter. During these months, as the sun appears to move southwards, the monsoon trough (low-pressure area) over the northern plains weakens and is gradually replaced by a **high-pressure system**. Consequently, the south-west monsoon winds weaken and start withdrawing from the Northern Plains by early October.
Figure 4.2 is a map showing the direction of the Retreating Monsoon (Northeast Monsoon) winds over India, indicating how they withdraw from the Northern Plains but pick up moisture over the Bay of Bengal to bring rain to the southeastern coast.
The retreat of the monsoon is accompanied by clear skies and a rise in temperature during the day. Nights become cool and pleasant, but high temperature and humidity during the day make the weather rather oppressive, a condition commonly known as **‘October heat’**. In the latter half of October, temperatures begin to fall rapidly in northern India.
By early November, the low-pressure conditions over northwestern India transfer to the **Bay of Bengal**. This shift is associated with the formation of **tropical cyclones** originating over the Andaman Sea. These cyclones typically cross the eastern coasts of India, bringing heavy and widespread rain. They are often very destructive, frequently impacting the densely populated deltas of the Godavari, Krishna, and Kaveri rivers, as well as the coasts of Odisha, West Bengal, and Bangladesh, causing significant damage.
A large portion of the rainfall received by the **Coromandel Coast** (Tamil Nadu coast) occurs during this season, derived from these tropical depressions and cyclones forming in the Bay of Bengal.
Distribution Of Rainfall
The amount and distribution of rainfall vary significantly across India due to the nature of the monsoon and relief features (Figure 4.3).
Areas receiving **very high rainfall (over 400 cm)** annually include parts of the western coast (especially the windward side of Western Ghats) and northeastern India (like Meghalaya, home to Mawsynram).
Areas receiving **very low rainfall (less than 60 cm)** include western Rajasthan and adjoining parts of Gujarat, Haryana, and Punjab. Rainfall is also low in the interior of the Deccan plateau, particularly east of the Sahyadris (Western Ghats), which lies in the rain shadow. A third region of low precipitation is around Leh in Jammu and Kashmir (cold desert).
Figure 4.3 is a map showing the annual rainfall distribution across India. It uses different colours or patterns to indicate regions receiving high, moderate, and low rainfall, illustrating the geographical variations in precipitation due to the monsoon and relief features.
These regions receive low rainfall primarily due to their location relative to the rain-bearing monsoon winds and relief features. Western Rajasthan and adjoining areas are too far from the monsoon's main moisture source and also affected by the direction of the Aravalli range. The interior Deccan plateau is in the rain shadow of the Western Ghats. Leh is a cold desert region with very low precipitation.
The rest of the country receives moderate rainfall. Snowfall is confined to the Himalayan region.
The annual rainfall in India is highly **variable** from year to year due to the monsoon's uncertainties. Variability is particularly high in regions that already receive low rainfall, making these areas prone to droughts. Conversely, areas of high rainfall can be susceptible to floods.
Monsoon As A Unifying Bond
Despite the regional variations in climate and temperature conditions (e.g., cold in the Himalayas vs. moderate in the Peninsula), the **monsoon acts as a strong unifying bond** for the Indian subcontinent.
The Himalayas protect India from the extreme cold winds of Central Asia, ensuring relatively uniformly higher temperatures in northern India compared to other regions at similar latitudes. The peninsular plateau benefits from the moderating influence of the sea on three sides, resulting in moderate temperatures.
The seasonal reversal of wind systems, characteristic of the monsoon, provides a distinct rhythmic cycle of seasons across the country. Even the uncertainties, uneven distribution of rain, and occasional breaks in the monsoon are typical aspects of this phenomenon that are experienced nationwide.
The influence of the monsoon is deeply ingrained in the Indian landscape, plant and animal life, and human activities. The entire agricultural calendar, from sowing to harvesting, revolves around the monsoon. Even cultural aspects like festivals are often linked to the monsoon cycle. Year after year, people across India eagerly anticipate the arrival of the monsoon.
These monsoon winds effectively **bind the entire country together** by providing the essential water needed to initiate agricultural activities. The river valleys, which carry the monsoon water, also contribute to the sense of unity by forming interconnected river valley units across the subcontinent.
Table 1 in the exercise section provides monthly average temperature and rainfall data for various representative stations across India. Studying this data allows for comparison of climatic patterns in different regions, demonstrating the diversity within the overall monsoon framework. For instance, comparing data for Chennai (south-east coast, winter rain from retreating monsoon) and Mumbai (west coast, heavy summer rain from advancing monsoon) highlights regional differences in rainfall patterns.
The question about Chennai and Mumbai rainfall differences points to the two branches of the monsoon. Mumbai receives heavy rain from the Arabian Sea branch of the southwest monsoon in summer. Chennai receives most of its rain from the retreating monsoon winds that pick up moisture over the Bay of Bengal in autumn/winter, not the southwest monsoon directly. Similarly, Shillong (Meghalaya) receives very heavy rain from the Bay of Bengal branch of the southwest monsoon due to its location and hilly terrain (orographic rainfall). Kolkata also receives rain from this branch, but less than Shillong. Thiruvananthapuram receives rain from the Arabian Sea branch but is also influenced by proximity to the equator and potentially earlier arrival of monsoon currents.