Rain shadow area

Also called: Rain shadow, Leeward side, Rain-shadow region · Topic: Climate of India and the Monsoon · NCERT: Class 7, Ch 3 "Climates of India"; Class 11, Ch 4 "Climate"; Class 11, Ch 6 "Natural Hazards and Disasters: Causes, Consequences and Management"; Class 11, Ch 10 "Water in the Atmosphere"

Meaning

A rain shadow area is a dry region on the leeward side of a mountain (the side facing away from the wind). It forms because wind that has crossed the mountain sinks and gets warmer, so it gives very little rain.

It explains why places only a short distance apart can get very different amounts of rain. In India, it is why the Deccan plateau east of the Western Ghats, around Pune and Bengaluru, is much drier than the west coast.

Explanation

How it forms: cause, process, result

• Step 1: Moist wind meets a mountain
• In the southwest monsoon season (June–September), moist winds blow from the Arabian Sea onto the west coast of India.

• The Western Ghats (Sahyadri) run north–south, almost at a right angle to this south-west wind, so they block it.

• Step 2: Rain on the windward side

• The wind is pushed up the slopes to 900–1,200 m.
• Rising air cools. Its water vapour condenses (turns from gas into tiny water drops) and falls as heavy rain on the windward side (the side facing the wind).

• Result: 250–400 cm of rain on the Sahyadri and the west coast.

• Step 3: Dry air on the leeward side

• The wind crosses the crest and goes down the eastern slopes.
• Sinking air is squeezed by the air above it, so it gets warmer.
• Warm air can hold more water vapour, so it does not condense and little rain falls.
• Result: a dry belt on the eastern slopes and the Deccan plateau.

Conditions needed

• Moist, steady wind from one direction. The south-west monsoon wind picks up its moisture over the warm Arabian Sea.
• A mountain barrier at a right angle to the wind. It must be high enough to force the air to rise.
• A wind that loses most of its moisture on the windward slope. That leaves dry, sinking air for the far side.
• If a range runs parallel to the wind, nothing forces the air to rise. There is little rain on either side, so no proper windward–leeward contrast forms. The Aravallis are an example.

Key features

• A rain shadow area gets less rain. It is not always rainless.
• It is linked to one wind direction. A place can be in the rain shadow of one wind and still get rain from another wind.

• Example: the Coromandel coast of Tamil Nadu is in the rain shadow of the Arabian Sea branch, but it gets its main rain from the northeast monsoon (October–December).

• It always comes as a pair: a very wet windward side and a dry leeward side of the same mountain.

Where it occurs

• Deccan plateau, east of the Western Ghats: the main Indian example. It lies in the rain shadow of the Arabian Sea branch of the southwest monsoon.
• Pune (Maharashtra) and Bengaluru (Karnataka) are the standard rain shadow cities.

• Interior Maharashtra and Karnataka form a drought-prone leeward belt.

• Windward side for comparison: Kozhikode (Kerala) and Mangalore (Karnataka), and the coastal belt of Konkan, Goa, coastal Karnataka and Kerala, get far more rain.

• Coromandel coast (Tamil Nadu): it is dry in June–September for two reasons:
• It lies in the rain shadow of the Arabian Sea branch, behind the Western Ghats.
• Its coastline runs parallel to the Bay of Bengal branch, so that wind is not forced to rise.

• Tamil Nadu gets 48% of its yearly rain in the northeast monsoon and only 36% in the southwest monsoon (1971–2020) [2].

• Around the world: the same process works wherever a mountain range stands across a steady, moist wind. One side becomes wet and the other side dry.

Don't confuse with

• Windward side: this is the side facing the wind, where rising air cools and gives heavy rain (250–400 cm on the Western Ghats). The rain shadow is on the opposite (leeward) side.
• Dryness from a range running parallel to the wind (the Aravallis and the Thar): the Aravallis run south-west to north-east, parallel to the Arabian Sea arm. The air is never forced to rise, so it gives scanty rain. This is not a rain shadow, because no mountain blocks the wind.
• Moisture used up inland (Chotanagpur): the Narmada–Tapi arm gives only 15 cm at the Chotanagpur plateau. The reason is that it has already dropped most of its moisture along the way, not that it sinks down a leeward slope.
• Break in the monsoon: a break is a dry spell of a week or more in time, in the middle of the rainy season. A rain shadow is a dry belt in space that stays dry every season.

Prelims Hooks

• Windward–leeward pairs: Mangalore and Kozhikode (windward, very wet) vs Pune and Bengaluru (rain shadow, dry).
• The Western Ghats rain shadow is created by the Arabian Sea branch, not the Bay of Bengal branch.
• The Western Ghats force the wind up to 900–1,200 m and give 250–400 cm on the windward side. The Deccan to the east stays dry.
• Process–result pair: rising air cools, so it rains (windward). Sinking air warms, so it stays dry (leeward, rain shadow).
• Trap: the Coromandel coast is dry in June–September because it is parallel to the Bay branch and in the rain shadow of the Arabian Sea branch. Tamil Nadu gets most of its rain (48%) from the northeast monsoon [2].
• Trap: the Thar is dry mainly because the Aravallis run parallel to the monsoon wind. "Rain shadow of the Aravallis" is the wrong answer.

Mains Points

• Relief decides where rain falls:
• Mountains at a right angle to the wind, like the Western Ghats, give very heavy rain on the windward side and a dry rain shadow on the leeward side.

• This explains the sharp rain contrast between coastal Karnataka and Kerala on one side and interior Maharashtra and Karnataka on the other (GS-I).

• Rain shadow belts are drought-prone and need careful water management:

• Interior Maharashtra and Karnataka depend on uncertain monsoon rain, so rain-fed kharif crops are at risk.

• Useful responses are irrigation, watershed management and contingency crop plans (GS-III).

• The same place can depend on two different monsoons:

• Tamil Nadu is in the rain shadow of the Arabian Sea branch, so its main rain comes from the northeast monsoon, 48% against 36% from the southwest monsoon [2].
• Water and crop planning in the state must follow the October–December rains, not the June–September rains.

Related concepts

• Southwest monsoon season
• Arabian Sea branch of the monsoon
• Bay of Bengal branch of the monsoon
• Mawsynram
• Cherrapunji
• Dry Tamil Nadu coast in the southwest monsoon
• Break in the monsoon
• Monsoon depression

Read more

• The southwest monsoon season: the two branches, rain shadow and breaks