Precipitation

Any form of water — rain, drizzle, snow, sleet, hail — falling from clouds to the ground. Measured in millimetres of liquid-equivalent depth.

basic

What is Precipitation?

Precipitation is the broad meteorological term for all water that falls from clouds to the Earth’s surface. It covers six distinct forms:

Rain — Liquid water droplets larger than 0.5 mm.
Drizzle — Very small liquid droplets (< 0.5 mm).
Snow — Ice crystals or aggregates.
Sleet — Frozen raindrops or partly melted snow.
Graupel — Soft hail; rounded ice pellets.
Hail — Hard ice balls grown by repeated cycling through a thunderstorm updraft.

Precipitation is the delivery side of the water cycle. Water evaporates from oceans, lakes and plants, rises into the atmosphere, condenses on aerosol nuclei to form clouds, and eventually falls as precipitation. The global cycle moves approximately 505,000 cubic kilometres of water per year.

For South Asia, precipitation is dominated by rain — snow is significant only in the Himalayas, hail in pre-monsoon thunderstorms, and other forms are rare or absent.

How precipitation forms

The basic recipe:

Moist air rises — either by convection (thermal lift), orographic lift (terrain), or frontal lift (warm air over cold).
Air cools as it rises (about 6.5°C per kilometre).
Water vapour condenses onto microscopic aerosol nuclei (dust, pollen, soot, sea salt) when the air cools to its dew point.
Cloud droplets grow by collision-coalescence (warm clouds) or via the ice-crystal “Bergeron process” (cold clouds).
Droplets become heavy enough to fall — typically when they reach 100 micrometres or larger.
Falling droplets pass through warmer or cooler layers below, determining whether they reach the ground as rain, sleet, snow, or hail.

How precipitation is measured

All forms of precipitation are reported as liquid-equivalent depth in millimetres so they can be compared and aggregated:

1 mm of rain = 1 litre/m²
10 mm of snow ≈ 1 mm of rain (snow-to-water ratio ~10:1; varies)
Hail: weighed and converted to depth

Standard instruments:

Rain gauges — manual cylinders or tipping-bucket sensors.
Snow gauges — heated to melt snow and measure liquid equivalent.
Hail pads — Styrofoam squares that capture hail impact crater diameters.
Doppler radar — estimates precipitation rate by reflectivity.
Satellites — global coverage via passive microwave (GPM) and visible/IR sensors.

Precipitation across South Asia

The subcontinent shows extreme spatial variability:

Highest annual precipitation:

Mawsynram, Meghalaya, India: 11,872 mm — world record
Cherrapunji: 11,777 mm
Western Ghats stations: 4,000–7,000 mm
Coastal Bangladesh: 3,000–4,000 mm

Lowest:

Western Rajasthan: 100–300 mm
Ladakh / Tibet plateau: 50–150 mm
Balochistan, Pakistan: 100–250 mm
Northern Sindh: 100–200 mm

Seasonal pattern (Indian subcontinent):

75% of annual precipitation falls June–September (southwest monsoon).
12% in October–December (post-monsoon / northeast monsoon).
10% in pre-monsoon (March–May).
3% in winter (January–February).

Special regimes:

South India (Tamil Nadu): Northeast monsoon (October–December) dominates.
Northwest Himalaya: Winter snowfall from western disturbances is substantial.
Maldives, Sri Lanka: Receive precipitation from both monsoons.

Forms of precipitation in South Asia

Rain — overwhelmingly the most common form. Year-round in coastal cities; monsoon-concentrated inland.

Snow — Restricted to:

Western Himalayas (Kashmir, Himachal, Ladakh, Uttarakhand) in winter.
Eastern Himalayas (Sikkim, Arunachal) at high elevations.
Pakistan’s northern areas (Skardu, Gilgit, Murree).
Bhutan’s high passes.

Hail — Common during pre-monsoon Nor’westers in eastern India, Bangladesh, eastern Madhya Pradesh. Damages standing crops every year.

Sleet — Rare; occasional in Kashmir in late winter.

Graupel — Occasional during cold-season hill-station storms.

Freezing rain — Extremely rare in South Asia.

Precipitation forecasting

Modern forecasts use:

Numerical weather models (ECMWF, GFS, IMD’s NCMRWF) for 1–10 day forecasts.
Nowcasting (0–6 hours) from Doppler radar.
Subseasonal-to-seasonal (S2S) forecasts for 2–6 weeks.
Seasonal monsoon forecasts from IMD, ECMWF, NOAA each spring.

Forecast skill has improved dramatically. IMD’s 3-day rainfall forecasts now have ~70% skill vs ~50% in 2010.

Frequently asked questions

What is the difference between precipitation and rainfall? Precipitation includes all forms (rain, snow, sleet, hail, etc.). Rainfall refers specifically to liquid rain. In tropical South Asia they are nearly synonymous; in mountainous regions, “precipitation” is the correct term.

What is “probability of precipitation” on weather apps? The chance that measurable precipitation (≥ 0.1 mm) will fall at a specific location during a given period. 60% PoP for the next 6 hours means there is a 60% chance any given spot in the forecast area will see rain.

How does precipitation get into rivers and reservoirs? About 30–40% becomes surface runoff (rivers, lakes); 30–40% infiltrates the soil and recharges groundwater; the rest evaporates or is transpired by vegetation. The ratio varies hugely with soil, vegetation, slope and intensity.

Why does Cherrapunji get so much rain? Cherrapunji sits on the southern edge of the Khasi Hills, directly facing the moist Bay of Bengal monsoon winds. The winds are forced to rise sharply over the hills, dumping enormous quantities of precipitation in a narrow band. This orographic enhancement can multiply rainfall by 5–10× compared to nearby flatlands.

Where can I see precipitation forecast for my city? Mausam Online shows hourly and daily precipitation forecasts on every city page. See Mumbai, Mawsynram, Chennai, Karachi, Dhaka.