Precipitation: forms and measurement

Detail

What precipitation is

• Precipitation is moisture that falls from clouds to the ground, in liquid or solid form.
• How it starts:
• Air cools and condensation (water vapour turning into tiny water drops or ice) begins on small dust or salt particles.
• Condensation keeps going in the free air, so the cloud particles keep growing.
• At some point they become too heavy. The resistance of the air can no longer hold them up against gravity.

• They fall. This is precipitation.

• A cloud is not the same as rain. Most cloud droplets are far too small to fall. A raindrop holds the water of about a million cloud droplets, so the droplets must grow first.

How droplets grow: two routes (beyond NCERT)

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

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

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

• This 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 is still liquid below 0 °C because it has nothing to freeze on).
• Ice crystals and supercooled droplets sit side by side. Air that is "full" (saturated) for liquid water is more than full for ice, because air over ice needs less vapour to be saturated.
• So water evaporates from the droplets and deposits 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, or 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
Growth method	Droplets hit and join	Ice crystals take vapour from supercooled droplets
Product	Rain (often drizzle or showers)	Snow, or rain after melting

What controls precipitation

• How much precipitation a place gets depends on prevailing winds (the winds that blow most often), mountains and the seasons.
• Winds from the sea carry moisture. Winds from land are dry.
• Mountains force moist air up. The side facing the wind (windward side) gets rain, and the far side (leeward side or rain-shadow area) stays dry. Examples: the Western Ghats windward slopes against the Deccan interior, and Meghalaya against the Brahmaputra valley.

• Seasons decide timing. 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.

Forms of precipitation

Rainfall

• Rainfall is precipitation in the form of liquid water drops.
• It is the most common form, especially in the tropics.
• It is measured as the depth of water in millimetres (mm) or centimetres (cm), as if no water ran off, soaked in or evaporated.

Snowfall

• Snowfall happens when condensation takes place below 0 °C.
• Process:
• The water vapour changes directly into ice. This is called deposition.
• The ice forms hexagonal (six-sided) ice crystals.

• The crystals stick together into snowflakes.

• Where: in high latitudes (Canada, Siberia, Scandinavia, Antarctica) and on high mountains (Himalaya, Alps, Andes, Rockies).

• Why it matters:
• Snow builds up and feeds glaciers.

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

• India: western disturbances bring winter snow to the Himalaya, in Jammu & Kashmir, Ladakh, Himachal Pradesh and Uttarakhand. Western disturbances are low-pressure systems that start over the Mediterranean region and travel east with the westerly winds. The same systems give winter rain to the plains of Punjab, Haryana and Delhi, which helps rabi crops.

Sleet

• Sleet is frozen raindrops and refrozen melted snow-water.
• Process (cause → process → result):
• 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 air below.
• 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, no bigger than the raindrops they came from. They bounce when they hit the ground.

• Note on usage: in Britain, "sleet" means rain and snow falling together.

• Freezing rain is different. The cold layer near the ground is too thin, so the drops do not freeze in the air. They fall as supercooled rain and freeze on contact with roads, wires and trees, making a smooth coat of ice called glaze.

Hail

• Hail falls as rounded lumps of ice called hailstones. Inside they have concentric layers (one around the other, like an onion).
• Process (cause → process → result):
• Cause: tall cumulonimbus clouds (thunder clouds that can reach 12–15 km) have very strong updrafts (rising air currents).
• Process: a raindrop or small ice bit is pushed up past the freezing level and freezes. It falls, gathers a coat of water and is pushed up again, so a new ice layer 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.

• India: hailstorms in March–April damage rabi crops (wheat, mustard, gram) and orchards (apple, mango blossoms) in north and central India: Punjab, Haryana, Uttar Pradesh, Madhya Pradesh, Rajasthan and Maharashtra.

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

Form	Where it forms	Temperature setting	Shape and size	Typical season or place
Rainfall	Warm or cold cloud	Above 0 °C below the cloud	Liquid drops	Everywhere; monsoon India
Snowfall	Cold cloud	Below 0 °C all the way down	Hexagonal crystals into flakes	High latitudes, high mountains; Himalayan winter
Sleet	Falling through two layers	Warm layer above, freezing layer below	Small ice pellets, size of drops	Winter, mid-latitudes
Freezing rain	Falling through a thin freezing layer	Supercooled drops, freeze on contact	Ice glaze on surfaces	Winter, mid-latitudes
Hail	Inside cumulonimbus	Repeated trips through the freezing level	Rounded, layered lumps; can be large	Pre-monsoon thunderstorms (March–May) in India

Measuring precipitation: the rain gauge

• A rain gauge is the instrument that measures how much rain has fallen.
• Parts and working:
• A funnel at the top catches the rain.
• The water flows into a collecting cylinder (bottle) below.
• The water is poured into a measuring jar with a scale in millimetres.

• If 5 mm is collected, the rainfall is 5 mm.

• Snow: it is melted first, and the depth of water is recorded.

• Where to place it:
• On open, level ground, raised a little above the surface.

• Away from buildings, walls and trees, so nothing blocks the rain or splashes extra water into the funnel.

• Reporting time in India:

• IMD (India Meteorological Department) reads daily rainfall at 0830 hours IST across the country. The daily figure is the total for the 24 hours ending at 0830 IST on that date [3].
• Monthly rainfall is the sum of daily rainfall for that month [3].

• Weather tables, for example for Jammu & Kashmir, give separate rainfall and snowfall columns.

• Rainfall categories used by IMD:

• All-India monsoon (June–September): "Normal" if within ±10% of the long period average (LPA) (the average over many years). "Excess" if above 110% of LPA. "Deficient" if below 90% of LPA [3].

• Sub-divisions and districts: Excess is +20% of normal or more. Normal is +19% to −19%. Deficient is −20% to −59%. Scanty is −60% or less [3].

• The network:

• IMD started the District-wise Rainfall Monitoring Scheme (DRMS) in 1992 with about 2,000 rain gauges [2].
• By 2025 the network had grown to more than 6,700 rain gauges, giving daily, monthly, seasonal, annual and decadal rainfall figures for 738 districts [2].
• Daily rainfall records of about 9,000 rain-gauge stations are kept in the IMD archive, 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 administrative blocks. That is a tenfold rise in spatial detail over district-level data [2].

• Modern tools: Automatic rain gauges (ARGs) and automatic weather stations (AWS) record rain every hour without a human reader. Doppler weather radars track rain and hail inside clouds in real time. They now add to the manual gauges.

Prelims Hooks

• Condensation below 0 °C gives snowfall, made of hexagonal ice crystals.
• Sleet means a warm layer above a freezing layer near the ground, giving ice pellets no bigger than raindrops. In British usage, sleet is rain and snow together.
• Hail has concentric layers and forms in cumulonimbus clouds. Sleet and hail are sporadic in time and space.
• Trap: freezing rain falls as liquid and freezes on contact. Sleet freezes before reaching the ground.
• Bergeron-Findeisen process: ice crystals grow at the expense of supercooled droplets in cold clouds. Collision-coalescence works in warm tropical clouds.
• IMD daily rainfall is the 24 hours ending at 0830 IST [3].
• BRMS (launched July 2025) covers 7,200 blocks. The older DRMS (since 1992) covers 738 districts with 6,700+ rain gauges [2].
• Monsoon categories: Normal is within ±10% of LPA. Excess is above 110%. Deficient is below 90% [3].
• Western disturbances bring winter snow to the Himalaya and winter rain for rabi crops in north-west India.

Mains Points

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

• Response: crop insurance under PMFBY covers hail as a localised risk. IMD nowcasts (short warnings for the next few hours) use Doppler radar. Anti-hail nets are used in orchards.

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

• Himalayan snow feeds glaciers and perennial rivers (rivers that flow all year).

• Weak western disturbances mean less winter snow. That leads to lower summer river flow, which hurts irrigation and hydropower in the Indus–Ganga–Brahmaputra basins.

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

• Rainfall varies a lot over short distances.

• The move from district-level DRMS to block-level BRMS (7,200 blocks, 2025) helps local drought and flood declarations, crop advisories, insurance claims and water-budgeting [2].

• Spatial pattern of forms:

• Rain dominates the tropics and monsoon India.
• Snow falls at high latitudes and high altitudes (the Himalaya, above roughly 2,000–3,000 m in winter).
• Sleet and freezing rain mostly hit mid-latitude winters (North America, Europe).
• Hail comes with pre-monsoon convective storms (storms driven by strong rising air) in India.