World distribution of rainfall; cloudbursts and cloud seeding

Detail

1. Key terms

• Precipitation: water that falls from clouds to the ground. It can fall as rain, snow, sleet or hail.
• Annual rainfall: the total rain a place gets in one year. It is measured in cm or mm.
• Windward side: the side of a mountain that faces the wind. Moist air rises here, cools and gives rain.
• Leeward side / rain-shadow area: the side of a mountain away from the wind. Air sinks here and warms, so it is dry.
• Rainfall regime: the pattern of rain at a place. It covers both how much rain falls and in which season it falls.

2. General rules of world rainfall distribution

• Rule 1: rain decreases from the equator towards the poles.
• Near the equator, strong heat makes moist air rise every day. This gives convectional rain (rain from rising warm air) in the ITCZ (Inter Tropical Convergence Zone: the low-pressure belt near the equator where the trade winds of the two hemispheres meet).

• Towards the poles, the air is cold. Cold air holds little water vapour, so it gives little rain.

• Rule 2: coasts get more rain than continental interiors, and oceans get more than land.

• Winds pick up water vapour over the sea. They drop much of it soon after they reach the coast.

• By the time air reaches the middle of a large continent, it has little moisture left. Central Asia and interior Australia are examples.

• Rule 3: between 35° and 40° N and S, east coasts are wetter and rain decreases westward.

• At these latitudes, the trade winds and monsoon-type winds blow onto the eastern side of continents. Examples are south-east USA, south-east China and eastern Australia.

• West coasts at the same latitudes lie under the subtropical high-pressure belt. Here the air sinks and is dry. Cold ocean currents also flow along these coasts. The result is west-coast deserts such as the Atacama, Namib and Sahara (west).

• Rule 4: between 45° and 65° N and S, the westerlies wet the west coasts first, and rain decreases eastward.

• The westerlies (winds blowing from the west in the middle latitudes) bring moist air from the oceans onto western margins. Examples are Norway, the British Isles, British Columbia (Canada) and southern Chile.

• As the air moves east over land, it dries out.

• Rule 5: where mountains run parallel to the coast, the windward coastal plain is wet and the leeward side is dry.

• Moist wind hits the mountain and is pushed up. Rising air cools, its water vapour condenses, and it rains on the windward side.
• The air then comes down the other side, warms up and becomes dry. This is orographic (relief) rain (rain caused by mountains).
• India: the Western Ghats make the Konkan and Malabar coasts very wet. Places to their east, such as interior Maharashtra and Karnataka, are dry rain-shadow areas.
• World: the Andes in southern Chile get rain on their western (windward) side, while Patagonia in Argentina, to the east, stays dry.

3. Precipitation regimes by amount

Annual rainfall	Regions	Main reason
Over 200 cm	Equatorial belt (Amazon, Congo basin, Indonesia); windward west coasts of cool temperate lands; monsoon coasts (Western Ghats, Meghalaya, Myanmar coast)	Daily convection; onshore westerlies; monsoon winds hitting mountains
100–200 cm	Interior continental areas (wetter parts) and coastal areas in general	Moderate supply of moisture
50–100 cm	Central parts of tropical lands; eastern and interior parts of temperate lands	Far from the sea; moisture has already been used up
Under 50 cm	Rain-shadow interiors; high latitudes; subtropical deserts under the high-pressure belts	Sinking dry air; cold air; mountain barriers

4. Seasonal distribution: when the rain falls

• Rain-use efficiency (how much of the rain is actually useful to plants and people) depends on the season of rain as well as its amount.

Pattern	Regions	Cause
Even through the year	Equatorial belt; western parts of cool temperate lands	ITCZ convection all year; westerlies all year
Mostly in summer	Monsoon lands (South and South-East Asia, northern Australia)	Wind direction reverses, bringing sea winds in summer
Mostly in winter	Mediterranean lands (30°–45° on west coasts: Mediterranean basin, California, central Chile, south-west Australia, Cape Town)	Westerlies and their cyclones move towards the equator in winter

• This pattern links directly to the Köppen climate classification. Köppen uses letters such as f (rain all year), w (dry winter) and s (dry summer) to show the season of rain.

5. Extremes

• Wettest: Mawsynram in the Khasi Hills of Meghalaya gets about 11,000–12,000 mm a year (NCERT Class 7). Nearby Sohra (Cherrapunji) is also among the wettest places in the world.

• Cause: moist monsoon winds from the Bay of Bengal are funnelled into the funnel-shaped Khasi Hills and forced to rise steeply. This gives very heavy orographic rain.

• Driest: the Atacama Desert in northern Chile is among the driest places on Earth.

• Cause: it lies in the rain shadow of the Andes, under the subtropical high, and next to the cold Humboldt (Peru) Current.

• Map exercise (NCERT Class 11, Ch 10, exercise 3(i)): shade the four rainfall regimes on a world outline map.

6. Cloudbursts

• Definition: a cloudburst is very heavy rain over a small area in a very short time.
• IMD treats rainfall of about 100 mm (10 cm) or more in one hour at a station as a cloudburst [2].

• The area affected is small, roughly 20–30 sq km.

• How it happens (cause → process → result):

• Cause: moist monsoon air reaches steep mountain slopes and is forced up very fast.
• Process: a tall cumulonimbus cloud forms (a thunderstorm cloud that can reach the top of the troposphere). Strong upward air currents hold up a large amount of water. When these currents weaken, the cloud drops all its water at once on one spot.

• Result: flash floods (sudden floods in narrow valleys) and debris flows (fast-moving mud, rock and water).

• Where: mainly the Himalaya, including Uttarakhand, Himachal Pradesh, Jammu & Kashmir and Ladakh. Uttarakhand had several cloudbursts in the 2017 monsoon [2]. They are also seen in other steep regions of the world.

• Major Indian events:

Event	Date	What happened
Leh, Ladakh	August 2010	Cloudburst over a cold desert town; flash floods and mudflows swept through Leh
Kedarnath, Uttarakhand	June 2013	Very heavy rain together with a glacial lake breach (Chorabari Lake) caused floods in the Mandakini valley; heavy loss of life among pilgrims
Himachal Pradesh and Uttarakhand	Recurring, every monsoon	Flash floods, landslides and road washouts

• Cloudbursts are hard to forecast because they are very small and short-lived. Doppler radar networks help detect them. How such disasters are managed is covered under natural hazards and disasters.

7. Cloud seeding

• Definition: cloud seeding is a type of weather modification. Weather modification means changing local weather on purpose. In cloud seeding, particles are spread into clouds to change how much rain or snow falls, to reduce hail, or to clear clouds [6].
• Two methods:

Type	Clouds targeted	Seeding agent	How it works
Glaciogenic seeding	Cold clouds: colder than 0 °C, with supercooled water (water that stays liquid below 0 °C)	Silver iodide, dry ice (solid CO₂)	Silver iodide has a crystal shape similar to ice, so ice forms on it. The ice crystals grow, become heavy and fall [7]
Hygroscopic seeding	Warm clouds: convective clouds that are mostly warmer than −10 °C	Salt and other hygroscopic (water-attracting) particles	Larger droplets form, collide with other droplets, grow and fall as rain [7]

• Limit: seeding needs suitable clouds that are already there. It cannot make rain from a clear sky. WMO says claims to create rain systems, change wind patterns or remove severe weather have no scientific basis [6].
• Evidence: glaciogenic seeding of winter mountain clouds has shown clear cause-and-effect results. Results from hygroscopic seeding are mixed, and a result from one place cannot be applied directly to another (WMO statement, 14 June 2025) [6].
• Global use: operational seeding programmes run in more than 50 countries [6].

8. Cloud seeding in India

• Early trials: cloud seeding was tested at Delhi during the monsoon months of 1957–59 [8].
• CAIPEEX (Cloud Aerosol Interaction and Precipitation Enhancement Experiment) is run by IITM, Pune (Indian Institute of Tropical Meteorology) under the Ministry of Earth Sciences [5].
• Phase IV (2017–2019) tested seeding in the rain-shadow region of Solapur, Maharashtra [3].
• Rain rose by up to about 46 ± 13% at some places, and by about 18 ± 2.6% on average over a 100 sq km area downwind of the seeding spot [3].
• It added about 867 million litres of water, with a positive cost–benefit ratio [3].

• These gains are modest and appear only when suitable clouds are present.

• State programmes: Karnataka and Maharashtra have run their own seeding operations during droughts.

• Delhi smog: trials have tested seeding to clean Delhi's polluted winter air by washing pollutants out with rain. Results depend on whether suitable moist clouds are present, and winter air over Delhi is often dry.
• Mission Mausam (Ministry of Earth Sciences; Phase I: 2024–26) [4]:
• It has a weather-modification research area, the "Weather_MoD" vertical, at IITM [3].
• Plans include a 16 m tall cloud chamber, hired seeding aircraft, seeding drones built with industry, and work on seeding materials and dispensers [3].

9. Debates

• Does it work? It is hard to prove that rain fell because of seeding and would not have fallen anyway [6].
• Cost: aircraft, radar and chemicals are expensive, and the gains are small.
• Chemical residue: past operations show no significant health or environmental harm from silver iodide [6]. Some people still worry about it building up over time.
• "Rain theft": areas downwind may get less rain. WMO says these downwind effects need more study [6].
• Governance: who should decide, and who is responsible if floods or droughts follow? WMO advises independent expert review before new projects, regular checks on running projects, and reporting to WMO [6]. It neither promotes nor discourages weather modification [6].

Prelims Hooks

• Over 200 cm a year: equatorial belt, windward west coasts of cool temperate lands, and monsoon coasts. Under 50 cm: rain-shadow interiors and high latitudes.
• 35°–40° N/S: east coasts are wetter. 45°–65° N/S: west coasts are wetter because of the westerlies. A common trap is to swap these two.
• Mediterranean lands get winter rain. Monsoon lands get summer rain. The equatorial belt and west coasts of cool temperate lands get rain all year.
• Mawsynram (Khasi Hills, Meghalaya) gets about 11,000–12,000 mm a year. The Atacama (Chile) is among the driest places, lying in the Andes rain shadow beside the cold Humboldt Current.
• Cloudburst (IMD): about 100 mm or more of rain in one hour over a small area of roughly 20–30 sq km [2].
• Glaciogenic seeding = silver iodide or dry ice in cold clouds. Hygroscopic seeding = salt in warm clouds [7].
• Cloud seeding cannot create rain from a clear sky. Suitable clouds must already be present [6].
• CAIPEEX is an IITM, Pune programme. Its Phase IV (2017–19) seeding was in the Solapur rain-shadow area and gave about 18% average rain gain [3].
• Mission Mausam belongs to the Ministry of Earth Sciences, with Phase I in 2024–26 [4].

Mains Points

• World rainfall pattern (GS-I): rainfall is shaped by latitude (ITCZ and the subtropical highs), the main wind systems (trade winds and westerlies), distance from the sea, ocean currents and mountain barriers. Example contrasts: the Western Ghats coast against the Deccan rain shadow; southern Chile against Patagonia. The season of rain decides how useful it is for farming.
• Cloudbursts as a Himalayan hazard (GS-III): steep slopes, monsoon moisture and cumulonimbus clouds combine with fragile slopes and unplanned building on riverbanks. Leh 2010 and Kedarnath 2013 show the risk. Responses include denser Doppler radar cover, nowcasting (very short-range forecasts), river-zone rules and early warning for pilgrims.
• Cloud seeding as a drought or pollution tool (GS-III): CAIPEEX shows modest, cloud-dependent gains, about 18% in Solapur [3]. Seeding is not a substitute for water conservation or for cutting emissions at source. It needs regulation, independent checks and rules for affected neighbouring areas [6].