1. Factors affecting India’s climate
- Latitude: Tropic of Cancer divides India into a tropical south and sub-tropical north.
- Altitude: temperature falls with height (hill stations cool).
- Distance from the sea: coastal places equable (small range, e.g. Mumbai), interior places extreme (large range, e.g. Delhi).
- Monsoon winds: seasonal reversal → wet summers, dry winters.
- Himalayas: block cold Central Asian winds; make monsoon winds rise and rain over the plains.
- Jet streams: westerly jet in winter steers western disturbances; changes in the jets help the monsoon set in.
- El Niño: warm eastern Pacific → often a weak monsoon and drought in India.
| Factor | Effect |
|---|---|
| Distance from sea | Range of temperature |
| Himalayas | Barrier and relief rain |
| El Niño | Weak monsoon |
Key terms
- Equable climate
A climate with a small difference between summer and winter temperatures, typical of coastal places.
- El Niño
An occasional warming of the eastern Pacific Ocean that tends to weaken the Indian monsoon.
2. Summer and winter patterns
| Feature | Summer (Mar–May) | Winter (Dec–Feb) |
|---|---|---|
| Temperature | Rises northward; highest in NW | Falls northward; coldest in NW |
| Pressure over NW India | Low | High |
| Winds | Hot dry winds (loo); onshore later | Dry offshore NE winds |
| Rain | Pre-monsoon showers | Western disturbances (NW); Tamil Nadu coast |
| Season | Coldest or hottest area |
|---|---|
| Summer | North-west plains (hottest) |
| Winter | North-west and hills (coldest) |
Key terms
- Loo
A hot, dry, strong afternoon wind of the northern plains in May and June.
3. The monsoon and its mechanism
- Monsoon: seasonal reversal of winds (Arabic mausim, season).
- Mechanism: differential heating in summer → low pressure over NW India, high pressure over the Indian Ocean; ITCZ shifts north; SE trades cross the Equator, are deflected right (Coriolis) → moist SW monsoon.
- Onset: Kerala about 1 June (“burst”); covers India by mid-July.
- Arabian Sea branch: heavy rain on windward Western Ghats; rain-shadow on the leeward Deccan.
- Bay of Bengal branch: heavy rain in the north-east (Meghalaya); deflected west along the Ganga plain, rain decreasing westward.
- Winter: pressure reverses; dry NE monsoon from land to sea.
| Branch | Heavy rain |
|---|---|
| Arabian Sea | Western Ghats, west coast |
| Bay of Bengal | North-east India |
Key terms
- Rain-shadow area
An area on the leeward side of mountains that receives little rain.
- Burst of the monsoon
The sudden onset of heavy monsoon rain, usually on the Kerala coast around 1 June.
4. The four seasons
- Hot weather (Mar–May): rising temperatures, low pressure in NW, local storms.
- SW monsoon (Jun–Sep): 75–90% of annual rain; uncertain, uneven, with breaks.
- Retreating monsoon (Oct–Nov): withdrawal; “October heat”; NE winds over the Bay bring heavy rain to the Tamil Nadu coast; cyclones on the east coast.
- Cold weather (Dec–Feb): clear, cool, dry; frost in NW; snow in the Himalayas; western disturbances.
The Tamil Nadu coast lies in the rain-shadow of the SW monsoon and gets most rain from the retreating/NE monsoon.
| Season | Months |
|---|---|
| Hot weather | Mar–May |
| SW monsoon | Jun–Sep |
| Retreating monsoon | Oct–Nov |
| Cold weather | Dec–Feb |
Key terms
- Retreating monsoon
The withdrawal of the south-west monsoon in October and November, when north-east winds over the Bay of Bengal bring rain to the Tamil Nadu coast.
5. Local winds and seasonal weather
| Name | When | Where | Effect |
|---|---|---|---|
| Loo | May–June | Northern plains | Hot, dry; heatstroke |
| Mango showers | Apr–May | Kerala, coastal Karnataka | Ripening of mangoes |
| Blossom showers | Apr–May | Karnataka | Coffee flowering |
| Kalbaisakhi | Apr–May | West Bengal, Assam | Violent storms; helps tea, jute, rice |
| Western disturbances | Winter | NW India | Rain for wheat; Himalayan snow |
| Local weather | Region |
|---|---|
| Loo | Northern plains |
| Kalbaisakhi | Bengal, Assam |
Key terms
- Western disturbances
Low-pressure systems from the Mediterranean region that bring winter rain to north-west India.
- Kalbaisakhi
Violent evening thunderstorms of West Bengal and Assam in April and May.
6. Interpreting climate data
- Annual range = highest monthly mean temperature − lowest monthly mean temperature.
- Total rainfall = sum of the 12 monthly totals.
- Small range (under about 8 °C) → coastal or southern; large range (over about 15 °C) → interior, northern.
- Rain peak Jun–Sep → SW monsoon region; peak Oct–Dec → Tamil Nadu coast; some Jan–Feb rain → NW India (western disturbances).
- Very heavy Jun–Sep rain with a small range → west coast or north-east hills.
| Clue | Meaning |
|---|---|
| Small range | Coastal |
| Large range | Interior |
| Winter rain in SE | Tamil Nadu coast |
Key terms
- Annual range of temperature
The difference between the mean temperatures of the hottest and coldest months.
Exam tips
Common mistakes
- Saying winds blow from low to high pressure.
- Placing the rain-shadow on the windward side.
- Attributing Tamil Nadu’s winter rain to the south-west monsoon.
- Computing annual range from rainfall figures.
How marks are usually awarded
- Reasons (1 mark each): name the factor and how it acts, with a named place.
- Monsoon mechanism (3 marks): differential heating; pressure difference; wind direction and deflection.
- Climate data (2–3 marks): working shown for range and total; inference with a reason.