Precipitation

Topic: Water in the Atmosphere: Humidity, Condensation, Clouds and Precipitation · NCERT: Class 6, Ch 3 "Landforms and Life"; Class 7, Ch 2 "Understanding the Weather"; Class 9, Ch 3 "Atmosphere and Climate"; Class 11, Ch 10 "Water in the Atmosphere"

Meaning

Precipitation is water that falls from clouds to the ground, either as liquid (rain) or as solid ice (snow, sleet, hail). It falls once cloud particles grow so heavy that the air can no longer hold them up against gravity.

• It is the main way water in the air comes back to the land. It feeds rivers, glaciers and crops.
• Its amount and form shape a place's climate, its farming and its disaster risk.

Explanation

How precipitation forms (cause → process → result)

• Cause: air cools.
• Water vapour turns into tiny water drops or ice. This is condensation.

• It starts on small dust or salt particles in the air.

• Process: the particles grow.

• Condensation keeps going in the free air, so the cloud particles keep getting bigger.
• A cloud is not rain yet. Most cloud droplets are far too small to fall.

• One raindrop holds the water of about a million cloud droplets.

• Result: they fall.

• At some point the particles become too heavy.
• The resistance of the air can no longer hold them up against gravity, so they fall to the ground.

Two ways droplets grow (beyond NCERT)

• Collision and coalescence (warm-cloud process):
• It happens in warm clouds (the whole cloud is above 0 °C), mostly in the tropics.
• Big drops and small drops fall at different speeds, so they bump into each other (collision).
• Drops that hit each other join into one bigger drop (coalescence).
• Bigger drops fall faster and hit even more droplets. So they grow quickly and fall as rain.

• This explains the heavy showers from low warm clouds over tropical oceans and monsoon India.

• Bergeron-Findeisen process (ice-crystal or cold-cloud process):

• It happens in cold clouds in middle and high latitudes, and in the tall upper parts of tropical clouds.
• Above the freezing level, many droplets stay liquid below 0 °C. These are supercooled droplets (water that stays liquid below 0 °C because it has nothing to freeze on).
• Air that is saturated ("full") for liquid water is more than full for ice. This is because air over ice needs less water vapour to become saturated.
• So water evaporates from the droplets and settles on the ice crystals. The ice crystals grow at the expense of the droplets.
• The heavy crystals fall. They reach the ground as snow if the air below is cold. They melt into rain if they pass through warm air.

Feature	Collision and coalescence	Bergeron-Findeisen
Cloud type	Warm cloud (above 0 °C)	Cold cloud (below 0 °C, mixed ice and water)
Main region	Tropics, low clouds	Middle and high latitudes; tops of tall clouds
How particles grow	Droplets hit and join	Ice crystals take vapour from supercooled droplets
Product	Rain (often drizzle or showers)	Snow, or rain after melting

Forms of precipitation

• Rainfall: water falls as liquid drops.
• It is the most common form, especially in the tropics.

• It is measured as the depth of water in mm or cm. The depth is counted as if no water ran off, soaked into the ground or evaporated.

• Snowfall: condensation happens below 0 °C.

• Water vapour turns straight into ice. This is called deposition.
• The ice forms hexagonal (six-sided) crystals.

• The crystals stick together into snowflakes.

• Sleet: frozen raindrops and melted snow-water that has frozen again.

• Cause: a layer of air above freezing lies over a layer below freezing near the ground. This is a temperature inversion (air higher up is warmer than the air below it).
• Process: snow melts into raindrops in the warm upper layer. The drops then fall into the cold layer near the ground and freeze again.
• Result: small, clear ice pellets that are no bigger than the raindrops they came from. They bounce when they hit the ground.

• In Britain, "sleet" means rain and snow falling together.

• Hail: rounded lumps of ice called hailstones. Inside, they have concentric layers (one layer around another, like an onion).

• Cause: tall cumulonimbus clouds (thunder clouds that can reach 12–15 km) have very strong updrafts (rising air currents).
• Process: a raindrop or a small bit of ice is pushed up above the freezing level, where it freezes. It falls, picks up a coat of water, and is pushed up again. A new layer of ice freezes on. This repeats many times.

• Result: layered hailstones. A stronger updraft keeps the stone up longer, so it grows bigger. It falls when the updraft can no longer hold it.

• Sleet and hail are sporadic. They come rarely and do not last long (patchy in time). They hit small patches, not wide areas (patchy in space).

What controls how much falls, and how it is measured

• Prevailing winds (the winds that blow most often):
• Winds from the sea carry moisture and bring rain.

• Winds from the land are dry.

• Mountains:

• Moist wind hits a mountain and is pushed up. The rising air cools, and its water vapour condenses.
• It rains on the side facing the wind (the windward side).

• The far side (the leeward side or rain-shadow area) stays dry.

• Seasons decide when the rain falls. Most of India gets its rain from the south-west monsoon (June–September).

• Effects:
• Rain lowers the temperature.

• A long period without rain gives a place a dry climate.

• The rain gauge (the instrument that measures rainfall):

• A funnel catches the rain.
• The water flows into a collecting cylinder below.
• The water is then poured into a measuring jar marked in mm.
• Snow is melted first, and the depth of the water is recorded.

• The gauge stands on open, level ground, raised a little above the surface and away from buildings, walls and trees. This stops anything from blocking the rain or splashing extra water into the funnel.

• Modern tools:

• Automatic rain gauges (ARGs) and automatic weather stations (AWS) record rain every hour without a person reading them.
• Doppler weather radars track rain and hail inside clouds in real time.

Where it occurs

• Rain:
• It is most common in the tropics and in monsoon India.

• Windward slopes get heavy rain. Examples: the Western Ghats windward slopes (compared with the dry Deccan interior in their rain shadow), and Meghalaya (compared with the Brahmaputra valley).

• Snow:

• It falls in high latitudes: Canada, Siberia, Scandinavia and Antarctica.
• It falls on high mountains: the Himalaya, Alps, Andes and Rockies. In winter, Himalayan snow falls above roughly 2,000–3,000 m.
• India: western disturbances bring winter snow to Jammu & Kashmir, Ladakh, Himachal Pradesh and Uttarakhand.
  • Western disturbances are low-pressure systems. They start over the Mediterranean region and travel east with the westerly winds.
  • The same systems give winter rain to Punjab, Haryana and Delhi, which helps rabi crops.

• Melting snow feeds snow-fed (perennial) rivers such as the Ganga, Yamuna, Indus and Brahmaputra. These rivers flow even in the dry season.

• Sleet and freezing rain: they mostly fall in mid-latitude winters, in North America and Europe.

• Hail: it comes with pre-monsoon thunderstorms (March–May) in India. The March–April storms hit Punjab, Haryana, Uttar Pradesh, Madhya Pradesh, Rajasthan and Maharashtra.
• How India measures rainfall (IMD, the India Meteorological Department):
• IMD reads daily rainfall at 0830 hours IST. The daily figure is the total for the 24 hours ending at 0830 IST [3].
• Monthly rainfall is the sum of the daily rainfall for that month [3].
• The District-wise Rainfall Monitoring Scheme (DRMS) began in 1992 with about 2,000 rain gauges [2].
• By 2025, the DRMS had more than 6,700 rain gauges covering 738 districts [2].
• The IMD archive keeps the daily rainfall records of about 9,000 stations, going back to 1875 (as of the late 2000s) [3].
• On 30 July 2025, IMD launched the Block-Wise Rainfall Monitoring Scheme (BRMS). It is an Indian-built system that gives real-time rainfall data for 7,200 blocks. That is ten times more detailed than district-level data [2].

Don't confuse with

• Condensation vs precipitation: condensation only makes tiny cloud droplets that stay in the air. It becomes precipitation only when the particles grow heavy enough to fall.
• Sleet vs freezing rain: sleet freezes in the air, before it reaches the ground, and lands as ice pellets. Freezing rain falls as supercooled liquid and freezes on contact with roads, wires and trees. It forms a smooth coat of ice called glaze.
• Sleet vs hail: sleet forms by falling once through a warm layer and then a cold layer. Its pellets are no bigger than raindrops. Hail forms inside cumulonimbus clouds through many up-and-down trips past the freezing level. Its stones have concentric layers and can be large.
• Collision-coalescence vs Bergeron-Findeisen: collision-coalescence is the warm-cloud route in the tropics, where droplets join. Bergeron-Findeisen is the cold-cloud route, where ice crystals grow by taking water from supercooled droplets.

Prelims Hooks

• Condensation below 0 °C gives snowfall, which is made of hexagonal ice crystals.
• Sleet = a warm layer above a freezing layer near the ground → ice pellets no bigger than raindrops. In British usage, sleet means rain and snow falling together.
• Hail has concentric layers and forms in cumulonimbus clouds. Sleet and hail are sporadic in both time and space.
• IMD daily rainfall is the 24 hours ending at 0830 IST [3]. Monsoon categories: "Normal" is within ±10% of the LPA (long period average, the average over many years). "Excess" is above 110% of LPA. "Deficient" is below 90% of LPA [3].
• Sub-division and district categories: Excess is +20% of normal or more. Normal is +19% to −19%. Deficient is −20% to −59%. Scanty is −60% or less [3].
• BRMS (launched July 2025) covers 7,200 blocks. The older DRMS (running since 1992) covers 738 districts with 6,700+ rain gauges [2].

Mains Points

• Hailstorms and rabi crops (GS-III, disasters and agriculture):
• Cause: in March–April, heat is rising and western disturbances still bring moisture. Together they build tall cumulonimbus clouds.
• Effect: wheat, mustard and gram near harvest, and apple and mango blossoms in orchards, are damaged in Punjab, Haryana, Uttar Pradesh, Madhya Pradesh, Rajasthan and Maharashtra.

• Response: PMFBY crop insurance covers hail as a local risk. IMD issues nowcasts (short warnings for the next few hours) using Doppler radar. Orchards use anti-hail nets.

• Snowfall and water security (GS-I and III):

• Himalayan snow feeds glaciers and perennial rivers.
• When western disturbances are weak, less snow falls in winter. Rivers then carry less water in summer.

• This hurts irrigation and hydropower in the Indus–Ganga–Brahmaputra basins.

• Finer rainfall data for planning (GS-III):

• Rainfall changes a lot over short distances.
• The move from district-level DRMS to block-level BRMS (7,200 blocks, 2025) supports local drought and flood declarations, crop advice, insurance claims and water budgeting [2].

Related concepts

• Rainfall
• Snowfall
• Sleet
• Hail
• Rain gauge

Read more

• Precipitation: forms and measurement