Climate — NCERT Solutions

Given: A question about the onset (bursting) of monsoon and the place of highest rainfall in India.

Bursting of Monsoon: The sudden onset of the monsoon, marked by a dramatic increase in rainfall accompanied by thunder and lightning, is called the 'bursting' or 'onset' of the monsoon. When the South-West monsoon arrives over the Kerala coast (usually around 1st June), the rainfall increases so suddenly and so heavily — compared to the pre-monsoon showers — that it appears as if the sky has 'burst' open. This sudden transition from dry conditions to heavy, continuous rainfall is what is described as the 'bursting of monsoon.'

Place of Highest Rainfall: Mawsynram (in the Khasi Hills of Meghalaya) receives the highest annual rainfall in India and is also considered one of the wettest places in the world. It receives an average annual rainfall of about 1,141 cm. (Cherrapunji, also in Meghalaya, is another place that receives extremely high rainfall.)

Given: A question about the type of cyclones causing winter rainfall in north-western India and their origin.

During winter, north-western India (including Punjab, Haryana, Delhi, and western Uttar Pradesh) receives rainfall from Western Disturbances (also called temperate or extra-tropical cyclones).

Origin: These cyclones originate over the Mediterranean Sea (and sometimes the Caspian Sea or the Black Sea region). They travel eastward, carried by the westerly jet stream, and enter the Indian subcontinent from the west and north-west. As they pass over north-western India, they bring moderate rainfall to the plains and snowfall to the Himalayan region. This winter rainfall is very beneficial for the rabi crops (especially wheat) grown in north-western India.

Given: India is broadly classified as having a 'monsoon' type of climate, yet there are significant regional variations.

Introduction: Although India has a broad climatic unity — dominated by the monsoon — the country's vast size, varied relief, and location lead to remarkable regional differences in temperature, rainfall, and seasons. The following points elaborate this:

1. Temperature Variations:

• In Rajasthan (Thar Desert), temperatures may exceed $50°C$ in summer, while in Drass (Ladakh), temperatures can fall to $-45°C$ in winter.
• South India has a small annual range of temperature (equable climate) due to its peninsular location, while North India experiences extreme temperatures — very hot summers and very cold winters.
• Example: Thiruvananthapuram has an annual range of about $3°C$, whereas Delhi has a range of about $25°C$.

2. Rainfall Variations:

• Mawsynram (Meghalaya) receives over 1,000 cm of annual rainfall, making it one of the wettest places on Earth.
• Jaisalmer (Rajasthan) receives less than 10 cm of annual rainfall, making it one of the driest regions.
• The Western Ghats receive very heavy rainfall on the windward (western) side, while the Deccan Plateau lies in the rain-shadow and receives scanty rainfall.

3. Seasonal Variations:

• Tamil Nadu receives most of its rainfall in winter (October–December) from the North-East monsoon, while the rest of India receives rainfall in summer from the South-West monsoon.
• North-western India receives winter rainfall from Western Disturbances, which is absent in most other parts of India.

4. Humidity and Wind:

• Coastal areas like Mumbai and Chennai have high humidity throughout the year, while Delhi and Jodhpur experience very low humidity in summer.

Conclusion: Thus, while the monsoon provides a broad climatic unity to India, the regional variations in temperature, rainfall, and seasons are so significant that India's climate cannot be described in a single uniform manner.

Given: A question about the number of seasons in India and detailed weather conditions of one season.

Number of Seasons: According to the Indian Meteorological Department (IMD), India has four distinct seasons:

1. Winter Season (December to February)
2. Summer / Pre-Monsoon Season (March to May)
3. South-West Monsoon Season / Rainy Season (June to September)
4. Retreating Monsoon / Post-Monsoon Season (October to November)

Detailed Discussion: South-West Monsoon Season (June to September)

This is the most important season for India as it brings about 75–90% of India's total annual rainfall.

Onset:

• The monsoon arrives at the Kerala coast around 1st June and 'bursts' suddenly with heavy rainfall, thunder, and lightning.
• It advances in two branches: the Arabian Sea branch and the Bay of Bengal branch.

Temperature and Pressure:

• By June, the ITCZ (Inter-Tropical Convergence Zone) shifts northward to about $20°–25°$ N over the Thar Desert, creating an intense low pressure area.
• This draws in moisture-laden winds from the Indian Ocean.
• Temperatures are high but the arrival of rain brings some relief.

Rainfall:

• The Western Ghats receive very heavy rainfall on the windward side (over 250 cm), while the leeward side (Deccan Plateau) remains dry — the rain-shadow effect.
• North-eastern India (Meghalaya, Assam) receives extremely heavy rainfall due to the funnelling effect of the Bay of Bengal branch.
• The Gangetic Plain receives moderate to heavy rainfall.
• Rajasthan and parts of Gujarat receive scanty rainfall as the monsoon weakens by the time it reaches there.

Characteristics:

• Rainfall is not continuous but comes in spells separated by dry periods (called breaks in monsoon).
• The monsoon is highly variable — both in time of arrival and in the amount of rainfall received.
• Floods are common in low-lying areas of Assam, Bihar, and West Bengal.
• Droughts may occur in years when the monsoon is weak.

Withdrawal:

• The monsoon begins to withdraw from north-western India by early September and retreats completely from the mainland by mid-October.

Significance:

• This season is the lifeline of Indian agriculture, especially for kharif crops like rice, cotton, jowar, and bajra.
• It replenishes rivers, lakes, and groundwater reserves.

Conclusion: The South-West Monsoon season is the most significant climatic event in India, shaping agriculture, water resources, and the lives of millions of people.

This is a map-based activity. The following descriptions guide you on what to mark on the outline map of India:

(i) Areas of Winter Rain:

• North-western India (Punjab, Haryana, Delhi, western Uttar Pradesh, Himachal Pradesh, Jammu & Kashmir): Receives winter rainfall from Western Disturbances originating in the Mediterranean Sea.
• Tamil Nadu and the Coromandel Coast (south-eastern India): Receives winter rainfall from the North-East (retreating) monsoon during October–December.

Mark these two regions with a distinct shading or hatching on the map.

(ii) Wind Direction during the Summer Season (March–May / June–September):

• Draw arrows showing winds blowing from the south-west (Arabian Sea) towards the Indian landmass — i.e., from south-west to north-east over the Arabian Sea branch.
• Draw arrows showing winds blowing from the Bay of Bengal (south/south-east) towards north-eastern and northern India — i.e., the Bay of Bengal branch moving northward and then westward along the Gangetic Plain.
• These are the South-West Monsoon winds that dominate the summer/rainy season.

(iii) Areas having less than 15°C temperature in January:

• The following regions experience temperatures below $15°C$ in January:
• Jammu & Kashmir, Ladakh, Himachal Pradesh, Uttarakhand (entire Himalayan region)
• Punjab, Haryana, Delhi, western Uttar Pradesh, Rajasthan (north-western plains)
• Parts of Bihar and eastern Uttar Pradesh
• Mark these areas (roughly the entire northern and north-western India, north of approximately the Tropic of Cancer) with appropriate shading.

(iv) Isohyet of 100 cm:

• An isohyet is a line joining places of equal rainfall.
• The 100 cm isohyet roughly passes through:
• The Western Ghats (separating the heavy-rainfall western coast from the drier Deccan interior)
• Parts of Madhya Pradesh and Chhattisgarh
• Through the Gangetic Plain (roughly along the foothills of the Himalayas)
• Through West Bengal and Odisha
• Along the north-eastern states
• Draw a bold line on the map representing this isohyet, separating areas receiving more than 100 cm (to the east and along the coasts) from areas receiving less than 100 cm (interior Deccan, Rajasthan, north-western India).

Note: Students should use an atlas (NCERT Atlas or Survey of India maps) to accurately draw these features on the outline map.