Liquid precipitation measured in millimetres (mm). 1 mm equals 1 litre of water spread evenly over 1 square metre.
basicWhat is Rainfall?
Rainfall is the liquid form of precipitation — water droplets falling from clouds and reaching the ground. It is measured in millimetres (mm), where 1 mm represents the depth of water that would accumulate on a perfectly flat, impermeable surface if none of it ran off or evaporated.
A simple way to picture it: 1 mm of rain = 1 litre of water per square metre (or 10,000 litres per hectare; 1 million litres per square kilometre).
Rainfall is the single most important weather variable for South Asia. It supplies drinking water, irrigation, hydropower, groundwater recharge, river flow, ecology and agricultural prosperity. The seasonal pattern of rainfall — dominated by the southwest monsoon — shapes the calendar, economy and lifestyle of 1.8 billion people.
How rainfall is measured
The standard instrument is a rain gauge — typically a cylinder with a known funnel area that collects rain over a fixed interval (usually 24 hours, 8:30 AM to 8:30 AM local time in India). Modern automatic gauges record at 1-minute resolution.
India’s rain gauge network:
- ~5,000 IMD-operated gauges
- ~25,000 additional state-government gauges
- ~700 Automatic Weather Stations (AWS) for real-time data
- Satellite estimates fill the gaps between gauges
Doppler weather radar at major cities estimates rainfall in real time over a 200–400 km radius. Satellite-based estimates (GPM, IMERG, INSAT-3D) provide global coverage.
Rainfall intensity classification
The India Meteorological Department classifies 24-hour rainfall as follows:
| Category | 24-hour total |
|---|---|
| No rain | 0 mm |
| Very light rain | 0.1–2.4 mm |
| Light rain | 2.5–7.5 mm |
| Moderate rain | 7.6–35.5 mm |
| Rather heavy rain | 35.6–64.4 mm |
| Heavy rain | 64.5–124.4 mm |
| Very heavy rain | 124.5–244.4 mm |
| Extremely heavy rain | ≥ 244.5 mm |
For comparison, the 2005 Mumbai cloudburst delivered 944 mm in 24 hours — almost four times the “extremely heavy” threshold.
For hourly rainfall:
- Light: < 2.5 mm/hour
- Moderate: 2.5–10 mm/hour
- Heavy: 10–50 mm/hour
- Cloudburst: > 100 mm/hour
Rainfall in South Asia
South Asia receives an extraordinarily variable amount of rain:
Wettest places:
- Mawsynram, Meghalaya: 11,872 mm/year (world record)
- Cherrapunji, Meghalaya: 11,777 mm/year
- Agumbe, Karnataka: ~7,640 mm/year
- Mahabaleshwar, Maharashtra: ~6,000 mm/year
Driest places:
- Jaisalmer, Rajasthan: ~165 mm/year
- Leh, Ladakh: ~115 mm/year
- Quetta, Pakistan: ~250 mm/year
India’s regional averages (annual):
- Northeast: 1,500–3,000 mm
- Konkan/Western Ghats: 2,500–6,000 mm
- Indo-Gangetic plain: 700–1,500 mm
- Northwest (Rajasthan, Gujarat): 200–500 mm
- South interior (Karnataka, AP, TN): 600–1,000 mm
Monsoon and annual rainfall distribution
India’s average 1,170 mm/year breaks down by season:
| Season | Months | Share of annual rain |
|---|---|---|
| Winter | Jan–Feb | ~3% |
| Pre-monsoon | Mar–May | ~10% |
| Southwest monsoon | Jun–Sep | ~75% |
| Post-monsoon | Oct–Dec | ~12% |
The southwest monsoon’s dominance means water security depends almost entirely on June–September performance.
Why rainfall measurement matters
- Agriculture: Kharif crop sowing depends on monsoon onset and seasonal totals.
- Reservoirs: Catchment rainfall determines hydropower and irrigation availability.
- Floods: Heavy 1-hour and 24-hour rainfall triggers flash floods.
- Drought: Cumulative deficits over weeks trigger drought declarations.
- Cyclones: Total cyclone rainfall determines damage and inland flooding.
- Insurance: Crop insurance payouts trigger at defined rainfall thresholds.
Frequently asked questions
How is 1 mm of rain so small but still meaningful? 1 mm doesn’t sound like much, but 1 mm over a 1-hectare field = 10,000 litres of water. Over a small city catchment of 100 km², 1 mm = 100 million litres. Even small daily totals add up to massive water volumes.
What is “trace” rainfall? Less than 0.1 mm in 24 hours — recorded but too small to fill the gauge. Typically a light drizzle or fog drip.
What is the world rainfall record? The 24-hour world record is 1,825 mm at Foc-Foc, Réunion in January 1966 during Cyclone Denise. India’s record is 944 mm at Mumbai on 26 July 2005. Single-hour record: 401 mm at Holt, Missouri, 1947.
Why is rainfall so unevenly distributed in India? The Western Ghats and Himalayas force moist monsoon winds to rise (orographic lift), causing very heavy rain on windward slopes. Leeward sides — Maharashtra interior, Karnataka interior, Tibet — sit in “rain shadow” with much less rain.
Where can I see live rainfall for my city? Mausam Online displays current and 7-day rainfall on every city page. See Mumbai, Kolkata, Chennai, Bengaluru, Karachi, Dhaka.