49 Fun Facts About Monsoons

Traditionally, how is a monsoon defined?

A seasonal reversing wind, with matching changes in rainfall, is the traditional definition; in everyday use the word now also means the rainy season those winds bring.

The English word "monsoon" ultimately derives from an Arabic word meaning what?

Season: monsoon goes back to Arabic mawsim, 'season', the word Arab sailors used for the fixed times of year when the Indian Ocean winds reversed.

Which language did "monsoon" pass through on its way from Arabic into English?

Portuguese sailors were the first Europeans to work the Indian Ocean trade winds regularly.

The migration of which atmospheric feature is now considered the driver of monsoons?

The Intertropical Convergence Zone, the tropical rain belt where the trade winds meet, swings north and south with the seasons; its migration over land is now viewed as the engine of monsoon rains.

Why do continents warm up more quickly than the sea in summer, setting up monsoon winds?

Land has a lower heat capacity than water, so the same sunshine warms it faster; the sea also mixes heat tens of metres down by convection, while rock and sand only conduct it slowly.

In summer, why does the air over heated land develop low pressure?

Warm air expands and rises over the heated land, so cooler, higher-pressure air over the ocean flows inland as a moist sea breeze on a continental scale.

Summer monsoon winds over India, West Africa and northern Australia share which dominant directional component?

Westerly: India's and West Africa's summer monsoons blow from the southwest and northern Australia's from the northwest, so a westerly component is common to all; winter monsoons are mostly easterly and dry.

The strengthening of the Asian monsoon has been linked to the uplift of which highland region?

The Tibetan Plateau, raised after India collided with Asia about 50 million years ago, heats the air above it each summer and is widely linked to a stronger Asian monsoon.

Which low-level wind, racing up the East African coast, feeds moisture into India's summer monsoon?

The Somali Jet, a low-level southwesterly jet hugging the Somali coast, crosses the equator and carries moisture from the southern Indian Ocean across the Arabian Sea into India each summer.

During which months does India's southwest monsoon occur?

It normally arrives at the end of May or start of June and fades in late September or early October.

Which arid region's summer heating creates the low pressure that draws the monsoon into India?

The Thar Desert of northwest India and Pakistan bakes in early summer, forming a deep low-pressure cell; moisture-laden winds from the Indian Ocean rush inland to fill it.

Which mountain range forces the monsoon winds to rise and drop their rain rather than pass into Central Asia?

The Himalayas act like a high wall, forcing the winds to rise and drop their rain; some parts of the subcontinent receive up to 10,000 mm a year as a result.

Which Indian state is the first to receive rain from the southwest monsoon?

The Arabian Sea branch hits the Western Ghats there first.

What is the normal date for the monsoon's onset over India, by the weather service's calendar?

June 1 is the India Meteorological Department's normal date for monsoon onset over Kerala, the first state to get the rains; the actual onset is declared each year when its criteria are met.

Into which two arms does the southwest monsoon split at the southern tip of India?

Arabian Sea and Bay of Bengal: the Arabian Sea branch runs up the Western Ghats, while the Bay of Bengal branch swings around the bay towards the eastern Himalayas and the Ganges plain.

Which town on the southern slopes of the Khasi Hills in Meghalaya is one of the wettest places on Earth?

Mawsynram sits on the rain-facing southern slopes of the Khasi Hills, where moist Bay of Bengal monsoon air is forced up the escarpment, giving it one of the highest average annual rainfalls ever measured.

What is the average annual rainfall of the Khasi Hills town reputed to be the wettest place on Earth?

11,872 mm is Mawsynram's long-run annual average — nearly 12 metres of rain — though two Colombian towns, Lloró and López de Micay, dispute its claim to be the wettest place on Earth.

Which town, also called Sohra, holds the records for rain in a single month and year?

Cherrapunji, officially renamed Sohra, logged 26,461 mm in the twelve months from August 1860, the largest rainfall ever measured in a single year anywhere on Earth.

How much rain fell in the Khasi Hills in July 1861, the record for a calendar month?

9,300 mm fell on Cherrapunji in July 1861 — more than 30 feet of water in 31 days — the greatest rainfall ever recorded in one calendar month.

Roughly what share of India's annual rainfall comes from the monsoon?

Nearly 80% of India's yearly rain falls during the June–September southwest monsoon, so a late or weak season cuts the sowing of rice, cotton and pulses across the country.

What term describes the sudden switch to wet, humid weather when the monsoon arrives in India?

Bursting: the 'burst' of the monsoon is the abrupt switch from hot, dry weather to rain and humidity when the moist winds arrive, which the jet-stream theory links to the westerly jet jumping north of the Himalayas.

By what other name is the northeast phase, when cold air sweeps towards the sea, known?

The retreating monsoon: as the southwest monsoon withdraws, cold dry air sweeps off the Himalayas towards the sea, picking up moisture over the Bay of Bengal on the way.

Which Indian city is watered mainly by the northeast monsoon rather than the southwest one?

Chennai lies in the rain shadow of the Western Ghats during the southwest monsoon, so most of its rain comes from the northeast monsoon off the Bay of Bengal later in the year.

In which months does the northeast monsoon bring rain to southern India?

October to December: once the high-pressure system over the cooling northern landmass builds, northeasterly winds cross the Bay of Bengal and rain out over Tamil Nadu and Sri Lanka.

What is the northeast monsoon called in the Philippines?

Amihan is the Filipino name for the cool, dry northeast monsoon that blows across the islands in the northern winter; the wet southwest monsoon is the Habagat.

What is the East Asian summer rain belt called in China?

Meiyu (梅雨) is the Chinese name for the stationary front that soaks the Yangtze valley in June; Korea calls the same belt Jangma and Japan Bai-u or Tsuyu.

The East Asian monsoon's spring-summer rains are referred to in several languages as what kind of rain?

Plum rain: Chinese Meiyu and Japanese Bai-u both mean 'plum rain', as the front arrives when plums ripen; it moves north from Indochina in May to northern China and Korea by July.

Which dry, dusty trade wind is interrupted by the West African monsoon each summer?

The harmattan, the dry dusty northeasterly off the Sahara, gives way each summer to moist southwesterlies as the monsoon front pushes north over West Africa around late June, retreating south by October.

Which semi-arid African belt depends on the West African monsoon for most of its precipitation?

The Sahel, the semi-arid belt along the Sahara's southern edge, gets nearly all its rain from the West African monsoon, driven by the contrast between the hot Sahara and the cooler equatorial Atlantic.

During which months does the North American monsoon occur?

Late June to September: the North American monsoon builds over Mexico's Sierra Madre Occidental in June and spreads into Arizona and New Mexico by early July, fading in September.

Which body of water is the main low-level moisture source for Arizona's monsoon?

The Gulf of California feeds the Arizona monsoon with low-level moisture surges up its length; once the Sierra Madre Occidental forests green up, plant transpiration adds still more.

By which other name is the North American wet phase commonly known?

The Arizona monsoon, also called the Southwest or Mexican monsoon, is the usual name; because so much of the affected area is the Mojave and Sonoran deserts, it is sometimes called the Desert monsoon too.

Roughly how much of its annual precipitation does northwest Mexico get from the monsoon?

Over half: in Sonora and Sinaloa the monsoon delivers most of the year's rain, up to about 70%, while Arizona and New Mexico get nearer 40–50% of theirs from it.

Which Californian mountains mark the western limit of the North American monsoon's push?

The Peninsular and Transverse Ranges of Southern California are about as far west as the monsoon pushes; it rarely reaches the coast, so a wall of desert thunderstorms is often visible from sunny beaches.

What is the Australian monsoon commonly called?

The Wet is northern Australia's name for its monsoon season, the humid, stormy months of the southern summer; the rest of the year is the Dry.

What share of northern Australia's annual rainfall falls during its monsoon?

Over three-quarters: the monsoon's wet season, roughly November to April, delivers the great bulk of northern Australia's rain — in Darwin well over 90% — while the Dry brings months with almost none.

By what name is the so-called European monsoon better known?

The return of the westerlies: Atlantic westerly winds ease in spring and strengthen again in June, bringing a run of wet, cool, windy weather to western Europe.

How does an El Niño year usually affect India's southwest monsoon?

La Niña years bring particularly strong monsoons instead.

Which 1999-discovered climate pattern explains why the 1997 El Niño caused no drought in India?

The Indian Ocean Dipole, identified in 1999, was in a strong positive phase in 1997 and offset El Niño's drying; adding the related EQUINOO effect also explains failed forecasts like the 2002 drought.

Attempts to predict India's monsoon rainfall began after which disaster?

The Great Famine of 1876–1878, which killed millions after the monsoon failed, prompted Henry Blanford of the India Meteorological Department to issue the first monsoon forecasts in 1886, based on Himalayan snow cover.

Which Greek navigator is credited with discovering the monsoon route across the Arabian Sea to India?

Hippalus: the first-century Periplus of the Erythraean Sea credits the pilot Hippalus with discovering the open-sea route from Arabia to India on the southwest monsoon, which Greek sailors then named after him.

Roughly how much rain fell at Mawsynram in the 24 hours to 17 June 2022?

About 1,000 mm — just over a metre of rain in one day — fell at Mawsynram in the 24 hours to 17 June 2022, the town's heaviest June day on record, beating a 1966 mark.

How much rain did Guinness credit the reputedly wettest town in Meghalaya with in 1985?

26,000 mm — 26 metres, roughly the height of an eight-storey building — is the rainfall the Guinness Book of World Records credited to Mawsynram for 1985.

Which Indian megacity was brought to a standstill by monsoon floods in July 2005?

Mumbai: on 26 July 2005 the city recorded 944 mm of rain in 24 hours, flooding roads, rail lines and the airport and killing hundreds; Assam, West Bengal and Bangladesh flood almost every monsoon.

Arabian Sea sediment records classically date the Asian monsoon's strengthening to roughly how many years ago?

Around 8 million years ago: Arabian Sea upwelling records long dated the monsoon's strengthening to about 8 million years; newer plant fossils and South China Sea sediments push its start back to 15–20 million years.

Which director general of observatories in India identified and named the Southern Oscillation?

Sir Gilbert Walker, head of the India Meteorological Department from 1904, found in the 1920s that surface pressure seesawed between the eastern and western Pacific, a pattern he named the Southern Oscillation.

The Southern Oscillation Index measures the pressure difference between Darwin and which island?

Tahiti and Darwin anchor the index; the oscillation turned out to be the atmospheric side of El Niño and La Niña, so together they are called ENSO.

By roughly when does the southwest monsoon normally cover all of India?

Mid-July is when the monsoon, having burst onto the west coast near Thiruvananthapuram at the start of June, normally covers the whole country; withdrawal begins in September.

Which low-pressure band runs from northwest India to the Bay of Bengal during the monsoon?

The monsoon trough is that east–west band of low pressure from Rajasthan to the head of the Bay of Bengal; rain belts follow it, and when it shifts north to the Himalayan foothills a 'break' sets in.