Maarg Manthan · Topic 5.17

Types of Precipitation: Convectional, Orographic, Cyclonic, Frontal, Monsoonal

Geography › Climatology · Topic 5.17

Precipitation is any form of water, liquid or solid, that falls from clouds to the ground: rain, drizzle, snow, sleet and hail. Droplets grow by the collision-coalescence process and the ice-crystal process until they are too heavy to stay aloft. Rainfall is of three types by cause: convectional, orographic or relief rainfall, and cyclonic or frontal rainfall, and it varies widely over the world.

6 min read · UPSC Prelims · Geography · Climatology

At a Glance

Forms Rain, drizzle, snow, sleet, hail
Convectional Heating and rising of moist air
Orographic Air forced up mountains
Cyclonic Air rising at fronts or cyclones
Wettest place Mawsynram, Meghalaya
Exam link UPSC Prelims, Geography: Climatology

Where Does It Fit?

This topic follows clouds and shows how they release water. It completes the part of the macro on atmospheric moisture and links to the monsoon, the cyclones and the climate types that follow. Previous topic: Types of Clouds: Cirrus, Cumulus, Stratus, Nimbus and Their Combinations. Next topic: Tropical Cyclones: Formation Conditions, Paths, Naming Conventions, Early Warning Systems.

What Is Precipitation and How Does It Form?

In short: when cloud droplets grow too big to float, they fall as precipitation.

Precipitation is water that falls from clouds to the surface of the Earth in liquid or solid form. Cloud droplets are very small and float, but they can grow. In warm clouds, droplets collide and merge, a process called collision-coalescence, until they are heavy enough to fall. In cold clouds, as in the temperate and high clouds, the Bergeron process operates: ice crystals grow at the cost of supercooled water droplets because the saturation vapour pressure over ice is lower, and they become snowflakes that may melt on the way down to form rain. The vapour comes from the processes in Humidity, Saturation, Dew Point, Evaporation and Condensation.

Form	Description
Rain	Liquid drops greater than 0.5 mm in diameter
Drizzle	Very fine drops less than 0.5 mm, from low stratus clouds
Snow	Ice crystals that fall as flakes when the temperature is below freezing throughout the fall
Sleet	Frozen raindrops, or a mixture of snow and rain, formed when rain falls through a layer of freezing air
Hail	Balls of ice made in strong thunderclouds, in layers, as updrafts carry them up repeatedly; can damage crops

Hail is discussed further in Thunderstorms and Tornadoes.

What Are the Types of Rainfall?

Type	Process	Features	Examples
Convectional rainfall	Surface heating makes moist air rise, expand, cool and condense into cumulonimbus clouds	Short, heavy showers, often with thunder, usually in the afternoon; common in the equatorial belt and in the summer of continental interiors	Equatorial regions, such as the Amazon and Congo basins; the pre-monsoon showers of India
Orographic (relief) rainfall	Moist air is forced to rise over a mountain barrier, cooling and condensing	Heavy rain on the windward slope, and a dry rain shadow area on the leeward side	Western Ghats and the windward Himalayan slopes in India
Cyclonic (frontal) rainfall	Air rises at a low pressure centre, or warm air rises over cold air at a front	Widespread, prolonged and steady; moderate intensity	Mid-latitude westerlies, western Europe; western disturbances in northwest India

Orographic rainfall is the main reason why the windward slopes of mountains are wetter than the leeward slopes. In a rain shadow the descending air warms and dries and little rain falls. The Deccan interior, east of the Western Ghats, is in the rain shadow of the southwest monsoon. In cyclonic rain, a tropical cyclone can bring extreme rain, as discussed in Tropical Cyclones: Formation Conditions, Paths, Naming Conventions, Early Warning Systems.

How Is Rainfall Distributed Over the World?

Equatorial belt: heavy convectional rain nearly all year, often over 2,000 mm, in the ITCZ.
Trade wind coasts: heavy rain on the east coasts of continents in the tropics, such as eastern Brazil and the Philippines.
Subtropical highs and western continental margins: very low rainfall, creating hot deserts such as the Sahara, Arabia, Thar and Atacama.
Mid-latitudes: cyclonic rainfall from westerlies on western coasts, with rain all year round in places such as Britain, and winter rain in the Mediterranean.
Polar regions: little precipitation because the cold air holds little moisture; they are cold deserts.

The wettest places on Earth by annual average are in India: Mawsynram in Meghalaya, with an annual average of about 11,870 mm, and Sohra (Cherrapunji) nearby, where a record of about 26,460 mm fell in the twelve months from August 1860 to July 1861. The driest is the Atacama Desert in Chile, where some stations have gone years without rain. The general circulation behind this pattern is in General Circulation of the Atmosphere: Hadley Cell, Ferrel Cell, Polar Cell.

How Is Rainfall Measured and What Defines It in India?

Rainfall is measured with a rain gauge in millimetres or centimetres; a depth of 1 mm over a hectare is 10,000 litres. The India Meteorological Department uses categories such as light, moderate, heavy, very heavy and extremely heavy rain, and colour-coded warnings, which are green, yellow, orange and red alerts. The long period average (LPA) of all-India monsoon rainfall from June to September is about 87 cm, and the monsoon is described as normal if rain is 96 to 104 per cent of the LPA, as discussed in Monsoon Mechanism: Traditional and Contemporary Theories (Air Mass Theory, Jet Stream Theory). As an analytical point, total rainfall is less important than its distribution: the same annual total can be a flood or a drought, depending on whether it arrives in a few days or over a season, and a warming climate is increasing the share of rainfall that falls in extreme events.

Why Is This Topic Important?

Types Convectional, orographic, cyclonic
India Rain shadow of the Western Ghats
Records Mawsynram and Cherrapunji

What Came Next?

The next topic describes tropical cyclones: their formation, paths, naming and warning systems. Next topic: Tropical Cyclones: Formation Conditions, Paths, Naming Conventions, Early Warning Systems.

Key People and Terms

Precipitation Water falling from clouds
Rain shadow Dry leeward area
Collision-coalescence Droplets merging to grow
Bergeron process Ice crystals growing in cold clouds
Sleet Frozen rain or rain-snow mix
LPA Long period average of rainfall

Exam Corner

Points to Remember

Convectional rain is heavy and afternoon-based.
Orographic rain falls on the windward slope.
Mawsynram is the wettest place on average.
Rain drops are over 0.5 mm; drizzle is smaller.
Monsoon LPA is about 87 cm.

Do Not Confuse With

Rain, drizzle and sleet Large drops, tiny drops and frozen rain
Orographic and cyclonic rainfall Mountains versus lows and fronts
Windward and leeward Wet versus rain shadow

Memory Hook

Heat lifts, hills block, fronts lift: convection, orography, cyclone.

Mains Angle

Questions ask for types and distribution. These points add depth.

Cause Lifting of moist air
Pattern Windward and rain shadow
Trend More extreme rain

A Question You May Face

An original practice question, not a past paper question.

“Describe the types of rainfall and explain how relief influences the distribution of rain in India.”

How to Answer

Formation Droplet growth.
Types Three types.
Relief Western Ghats.
Trend Extremes.

GS Relevance

Prelims: types of rainfall, forms of precipitation, rain shadow, wettest places. GS Paper 1: climatology and Indian climate.

Frequently Asked Questions

What is precipitation?

Precipitation is any form of water, liquid or solid, that falls from clouds to the Earth's surface. Its forms include rain, drizzle, snow, sleet and hail, and it occurs when cloud droplets or ice crystals grow too heavy to remain suspended in the air.

What are the types of rainfall?

Rainfall is of three types according to the way the moist air rises: convectional rainfall, caused by heating and rising of air; orographic or relief rainfall, caused by air forced over mountains; and cyclonic or frontal rainfall, caused by air rising at fronts or low pressure centres.

What is convectional rainfall?

Convectional rainfall occurs when the surface is strongly heated, so that moist air rises, expands, cools and condenses into cumulonimbus clouds. It is short and heavy, often with thunder, and is typical of equatorial regions and of hot afternoons in continental interiors.

What is a rain shadow?

A rain shadow is a dry area on the leeward side of a mountain range. Moist air loses its moisture as rain on the windward slope, and the descending air on the leeward side warms and dries. The Deccan plateau lies in the rain shadow of the Western Ghats.

Which is the wettest place on Earth?

Mawsynram in Meghalaya, India, has the highest annual average rainfall, about 11,870 mm. Nearby Sohra, or Cherrapunji, holds the record for the most rain in twelve months, about 26,460 mm between August 1860 and July 1861. Both are on the windward Khasi Hills.

PYQ Practice — Statement Analysis

1 Orographic rainfall occurs on the windward side of mountains.

True

Air is forced to rise and cool.

2 Convectional rainfall is typically light and continuous for days.

False

It is short and heavy.

3 Mawsynram is in Meghalaya.