?? PART 1: Factors Affecting India's Climate
- Latitude: India lies between 8�N and 37�N. The Tropic of Cancer (23�30'N) divides India
� south of it is tropical (hot year-round); north of it is subtropical (distinct seasons).
- Altitude: Temperature decreases ~6.5�C per 1000 m rise. Himalayan regions (Shimla,
Darjeeling) are cooler than the plains despite being at the same latitude.
- Distance from the Sea (Continentality): Coastal areas (Mumbai, Chennai) have moderate,
equable climate � cool summers, warm winters. Inland areas (Delhi, Jodhpur) have extreme continental
climate � very hot summers, cold winters.
- Himalayas: Act as a barrier � (a) prevent cold Siberian winds from entering India (keep
winters mild in the plains); (b) force the SW Monsoon to rise and shed rain over India.
- Jet Streams: High-altitude fast-moving air currents (around 12 km altitude). Two
important ones for India:
- Western Jet Stream (subtropical): Brings Western Disturbances (winter rains)
across Punjab and northern India.
- Easterly Jet Stream (tropical): Helps sustain the SW Monsoon over India in
summer.
- El Ni�o: An abnormal warming of the central and eastern Pacific Ocean (every 2�7
years). Results in weaker, delayed, or deficient SW Monsoon over India ? drought
conditions. E.g., droughts of 1982�83, 2002, 2009 were linked to El Ni�o.
?? El Ni�o vs La Ni�a
El Ni�o: Warm Pacific ? Weak/poor Indian Monsoon ? Drought in India.
La Ni�a: Cool Pacific ? Stronger-than-normal Monsoon ? Floods possible in India.
??? PART 2: Monsoon � Mechanism
The Monsoon is a seasonal reversal of wind direction, bringing heavy rainfall to South and
Southeast Asia. The word "monsoon" comes from the Arabic word mausam (season).
Mechanism of the Southwest Monsoon
- Differential Heating: In summer (May-June), the land (especially the Thar Desert and
northwestern India) heats up rapidly. A strong low pressure develops over the Indian
subcontinent.
- The Indian Ocean remains relatively cooler ? high pressure over the ocean.
- Winds blow from high pressure (ocean) to low pressure (land) � bringing moisture-laden sea air inland.
This is the SW Monsoon.
- The Intertropical Convergence Zone (ITCZ) shifts northward in summer to approximately
25�N over the Thar Desert � this acts as the "monsoon trough" pulling in moisture-laden winds.
Two Branches of SW Monsoon
| Branch |
Entry Point |
Areas Covered |
Key Feature |
| Arabian Sea Branch |
Kerala (around June 1st) |
Western Ghats, West Coast, Mumbai, Gujarat, Western MP |
Brings very heavy rainfall to Western Ghats (windward side); Deccan Plateau is rain-shadow (dry)
|
| Bay of Bengal Branch |
Northeast India (Assam, Meghalaya) around June |
Northeast India, Bengal, UP, Delhi, Punjab |
Mawsynram/Cherrapunji (Meghalaya) is wettest place � orographic rainfall on Khasi Hills |
??? PART 3: Seasons of India
| Season |
Months |
Key Characteristics |
| Hot Weather / Pre-Monsoon (Summer) |
March � May |
Very hot; Loo � hot dry winds in North India; Dust storms in Rajasthan; temperature up
to 48�C in Thar. Mango Showers in Kerala/Karnataka. Nor'westers / Kalbaisakhi
in West Bengal/Assam. |
| Monsoon (Rainy Season) |
June � September |
SW Monsoon brings 70�90% of India's annual rainfall. Kerala first to receive (June 1). Highest
rainfall: Mawsynram (~11,871 mm/year). Lowest: Jaisalmer (~10 mm/year). |
| Retreating Monsoon / NE Monsoon |
October � November |
SW Monsoon withdraws northward, then southeast. Bay of Bengal low-pressure systems bring heavy
rain to Coromandel Coast (Tamil Nadu). Cyclones common in Bay of Bengal (Oct�Nov). |
| Cold Weather (Winter) |
December � February |
Cold and dry in North India. Western Disturbances (Mediterranean origin) bring winter rainfall
to Punjab, Haryana, UP � essential for wheat (rabi crop). South India remains warm and dry. |
Key Local Winds and Weather Phenomena
| Local Wind / Phenomenon |
Description |
Region |
| Loo |
Hot, dry, scorching wind blowing in afternoon |
Punjab, Haryana, UP, Rajasthan (May�June) |
| Kalbaisakhi / Nor'westers |
Violent thunder-squalls in late afternoon |
West Bengal, Assam (April�May). Good for jute & tea. |
| Mango Showers |
Pre-monsoon showers that ripen mangoes |
Kerala, Karnataka, Andhra Pradesh |
| Western Disturbances |
Extratropical cyclones from the Mediterranean |
Punjab, Haryana, HP, J&K (Dec�Feb); bring winter rain |
Distribution of Rainfall in India
- Very heavy (above 200 cm): Western Ghats, Meghalaya, Northeast India, Andaman &
Nicobar Islands.
- Heavy (100�200 cm): North Bengal, Coastal Andhra/Odisha, most of Assam.
- Moderate (60�100 cm): Eastern MP, Deccan plateau (interior).
- Low (below 60 cm): Northwest India, Rajasthan, parts of Gujarat, Ladakh.
??? PART 4: Reading and Interpreting Climate Data
- Climate data is presented in graphs (rainfall bar chart + temperature line graph) called
climographs or separate bar/line graphs.
- From a temperature graph: identify seasonal pattern; note the month of highest and lowest temperature.
- From a rainfall graph: identify monsoon months; calculate annual total; identify dry and wet months.
- Temperature Range = Maximum Temperature - Minimum Temperature. Large range =
continental climate. Small range = equable/maritime climate.
?? Quick Revision � Key Facts
| Topic |
Key Fact |
| Wettest place in India |
Mawsynram (Meghalaya) � avg ~11,871 mm/year |
| Driest place in India |
Jaisalmer / Leh area (~10 cm/year) |
| Monsoon onset (Kerala) |
Around June 1st |
| NE Monsoon area |
Tamil Nadu Coromandel Coast (Oct�Nov) |
| El Ni�o effect |
Weaker monsoon ? drought |
| Loo wind |
Hot dry wind; North India; May�June |
| Kalbaisakhi |
Nor'westers; West Bengal; good for jute and tea |
| Western Disturbances |
Mediterranean origin; winter rains Punjab/Haryana; vital for wheat |
?? Chapter Summary
- India's climate is basically Tropical Monsoon � seasonal reversal of winds.
- Factors: Latitude, Altitude, Himalayas (barrier), Distance from sea, Jet Streams, El Ni�o.
- SW Monsoon: June�Sept; two branches (Arabian Sea + Bay of Bengal); 70�90% of annual rain.
- Four seasons: Summer (March�May); Monsoon (June�Sept); Retreating Monsoon (Oct�Nov); Winter
(Dec�Feb).
- Key winds: Loo (North India summer), Kalbaisakhi (Bengal/Assam spring squalls), Western Disturbances
(winter rains, NW India).