Convectional rain

Topic: Water in the Atmosphere: Humidity, Condensation, Clouds and Precipitation · NCERT: Class 11, Ch 10 "Water in the Atmosphere"

Meaning

Convectional rain is rain that falls when the hot ground heats the air above it, and this warm, light air rises by itself in convection currents (upward streams of warm, light air), cools, and forms tall clouds that give heavy, short showers.

It is the daily rain of the equatorial regions. It is also the type behind India's pre-monsoon thunderstorms, such as the Nor'westers (Kalbaisakhi), which damage crops and property and kill people with lightning.

Explanation

The basic rule: rain needs rising air

• Each rainfall type is named after how the air is lifted and cooled. In convectional rain, heating from below lifts the air.
• The chain that makes all rain:
• Moist air rises.
• Higher up, air pressure is lower, so the rising air expands.
• Air that expands cools. This is adiabatic cooling (the air cools only because it expands, not because heat leaves it).
• The air cools to its dew point (the temperature at which air becomes saturated, or full of water vapour). The vapour condenses into droplets, clouds form, and rain falls.

How convectional rain forms (cause → process → result)

• Cause: the sun heats the ground strongly. The hot ground then heats the air just above it.
• Process:
• The warm air becomes light and rises in convection currents.

• As it rises, it expands and cools.

• Result:

• The vapour condenses into cumulus clouds (heaped clouds shaped like a cauliflower).
• These grow into cumulonimbus clouds (very tall, dark thunder clouds).
• Heavy rain falls, with thunder and lightning.

What the rain is like and when it falls

• Nature: heavy, but it does not last long.
• Season: summer, when the sun heats the ground most.
• Time of day: the hottest part of the day, so it comes as afternoon showers.
• The "four o'clock rain" of the equator follows a daily pattern:
• Hot morning.
• Clouds build up by noon.
• A storm breaks in the late afternoon.

Conditions it needs

• Strong heating of the ground. This happens in the tropics, and in summer over land far from the sea.
• Moist air, so that rising air has enough vapour to condense.
• Free upward movement of air. No mountain or front is needed. The air rises because of its own warmth.

Where it occurs

• Equatorial regions (about 0°–10° N and S):
• Amazon basin (South America).
• Congo basin (Africa).
• Indonesia (South-east Asia).

• It rains almost every day here. This is the "four o'clock rain".

• Continental interiors of the Northern Hemisphere, in summer.

• India: the pre-monsoon thunderstorms are convectional. Thunderstorms during breaks in the monsoon are also convectional.
• Nor'westers (Kalbaisakhi): India's best-known convectional storms.
• Season: pre-monsoon, March to May [2].
• Areas hit: eastern and north-eastern India, southern Nepal, Bhutan and Bangladesh. They usually move from north-west to south-east [2].
• How they form: dry north-westerly winds meet hot, moist south-easterly winds [2].
• Where they start: storm cells form over Jharkhand, Odisha and sub-Himalayan West Bengal. They grow stronger as they move into Bangladesh and North-east India [2].
• Their weather: cumulonimbus clouds, squalls (sudden strong winds) that sometimes go above 100 km/h, heavy showers, lightning, hail, dust storms, downbursts (strong rushes of air down from a storm cloud) and sometimes tornadoes [2].

Don't confuse with

• Orographic (relief) rain: here a mountain barrier forces moist wind up, not heating of the ground. It falls on the windward side (the side facing the wind), as on the Western Ghats or Khasi hills. It lasts as long as the wind blows, while convectional rain is short.
• Cyclonic (frontal) rain at a warm front: warm air slides slowly up over cold air. It forms nimbostratus clouds (thick, grey, layered rain clouds) and gives long, steady rain. The winter rain of north-western Europe is of this type, not convectional.
• Cold-front rain: this also gives short, heavy showers from cumulonimbus clouds, often with thunder. But here cold air pushes under warm air and lifts it. The air does not rise because the ground heated it.
• Rain at the ITCZ (Inter Tropical Convergence Zone, the low-pressure belt near the equator where the trade winds of the two hemispheres meet): here winds flow together and push the air up (convergence). It also gives daily rain near the equator, but the lifting comes from converging winds, not only from ground heating.

Prelims Hooks

• "Four o'clock rain" = convectional rain of equatorial regions. It falls almost daily in the afternoon from cumulonimbus clouds.
• Cloud sequence in convection: cumulus → cumulonimbus. (Warm front = nimbostratus, so it is a wrong option for convectional rain.)
• Nor'westers = Kalbaisakhi. They are convectional, pre-monsoon (March–May) storms of eastern and north-eastern India [2].
• Nor'wester cells start over Jharkhand, Odisha and sub-Himalayan West Bengal. They grow stronger over Bangladesh and North-east India [2].
• Trap: the winter rain of north-western Europe is cyclonic (frontal), not convectional. Convectional rain comes in summer, in the afternoon.
• Classification: in India's rainfall, pre-monsoon thunderstorms and storms in monsoon breaks = convectional. Western Ghats rain = orographic. Monsoon depressions = cyclonic.

Mains Points

• Convectional storms are a disaster risk (GS-III):
• Nor'westers bring squalls above 100 km/h, hail, lightning and sometimes tornadoes in March to May [2].
• They damage the rabi harvest and power lines. They cause lightning deaths in Odisha, West Bengal, Jharkhand and Bihar.

• This makes the case for nowcasting (very short-range warnings) and lightning early-warning systems.

• Each rain type brings its own hazard:

• Convectional storms bring hail and lightning.
• Orographic cloudbursts bring landslides and flash floods.
• Cyclonic rain brings coastal flooding.

• So disaster planning in each region must match the main rain type there.

• India's monsoon mixes all three types, which is why rainfall varies so much:

• Convectional thunderstorms before the monsoon and during breaks.
• Orographic rain on the Western Ghats and the north-eastern hills.
• Cyclonic rain from monsoon depressions and post-monsoon cyclones.
• This variation in place and time shapes cropping patterns and the need for reservoirs (GS-I).

Related concepts

• Orographic rain
• Cyclonic rain

Read more

• Types of rainfall: convectional, orographic and cyclonic