A tropical monsoon climate, coded Am in the Köppen system, is a hot climate with a very wet season and a short dry one. Every month averages above 18 °C, and although the driest month gets less than 60 mm of rain, the annual total is so large that the land never dries out enough to count as savanna. It forms where seasonal winds, steered by the yearly north–south swing of the Intertropical Convergence Zone (ITCZ), carry ocean moisture onshore for part of the year and dry air offshore for the rest. Textbook examples include Kochi in India (about 3,015 mm a year), Conakry in Guinea (about 3,730 mm), Yangon in Myanmar, Cairns in Australia and Miami in Florida.
From Halley’s 1686 chart to the migrating ITCZ
English took “monsoon” from the Portuguese monção, which goes back to the Arabic mawsim, “season”, the name Indian Ocean traders gave to winds that reversed with the calendar. One of the first scientific attempts to explain those winds came from the English astronomer Edmond Halley, who in 1686 published a paper and chart on the trade winds and monsoons from his expedition to St Helena. He identified solar heating as the engine that moves the atmosphere.
In 1735 George Hadley, an English lawyer and amateur meteorologist, added the piece Halley had missed: the Earth’s rotation deflects air moving toward the equator, which is why trade winds blow from the north-east and south-east rather than straight toward the equator. The overturning loop he described, rising air near the equator and sinking air near 30° latitude, is now called the Hadley circulation, and its rising limb is the ITCZ.
For two centuries the monsoon was explained as a giant sea breeze. Water has a heat capacity of roughly 3.9 to 4.2 J per gram per kelvin, against 0.19 to 0.35 for dry soil, sand and rock, and it mixes heat downward by convection, so in summer the land heats faster, its air rises, pressure falls and moist ocean air is drawn in. That contrast is real, but meteorologists no longer treat it as the whole cause. The modern view is that the monsoon is a planetary-scale event: the ITCZ, the belt of converging winds and towering cloud, migrates poleward with the high sun each summer, and where a large, hot continent sits beside a warm ocean it is dragged far from the equator. In South Asia, the heating of the Tibetan Plateau helps generate a tropical easterly jet high over India, which deepens the low over the northern plains and pulls the rains in.
Henry Blanford, first head of the India Meteorological Department, linked the strength of the summer rains to spring snow cover in the Himalayas. When his successor John Eliot’s forecasts for 1899 to 1901 failed badly during drought and famine, the department recruited the Cambridge mathematician Gilbert Walker, who became director general of observatories in 1904. Searching vast weather records for predictors of the monsoon, Walker described the see-saw of air pressure between the Indian and Pacific oceans now known as the Southern Oscillation, half of El Niño–Southern Oscillation, still one of the main signals monsoon forecasters watch.
Köppen’s 60 mm line: why a place is Am rather than Af or Aw
The climate label came from the German-Russian climatologist Wladimir Köppen, who first published his classification in 1884 and revised it in 1918 and 1936; Rudolf Geiger made further changes in 1954 and 1961. Köppen split the always-hot A climates by their driest month. If every month averages at least 60 mm, the place is Af, tropical rainforest. If the driest month falls below 60 mm, the deciding test compares it with a sliding threshold: 100 minus one twenty-fifth of the annual total. A driest month above that threshold makes the climate Am; below it, Aw, tropical savanna.
The formula rewards big totals. A town with 1,500 mm a year has a threshold of 40 mm, so its driest month needs 40 to 59 mm to qualify as monsoon. A town with 2,500 mm or more has a threshold of zero, so any dry season at all, even a rainless one, still counts as Am. That is how two very different places share the code. Some Am places have a damp, muted dry season with regular showers, not quite enough for rainforest. Others have a dry season as harsh as any savanna’s, followed by months of extraordinary rain, sometimes more than 1,000 mm a month for two or more months in a row. The driest month nearly always comes at or soon after the winter solstice for that hemisphere.
Am climates are found in West and Central Africa, South and Southeast Asia, northern South America, Central America, parts of the Caribbean, South Florida and northern Australia. Not every very wet place qualifies. Sohra (Cherrapunji) in Meghalaya received 26,461 mm between 1 August 1860 and 31 July 1861, still the record for a single year, but up in the Khasi Hills its average temperatures are too mild for any A climate; Köppen files it as Cwb, subtropical highland.
Kochi and the Arabian Sea branch: South Asia’s June burst
The South Asian monsoon is the system the word was coined for, and Kerala’s coast shows its Am form most cleanly. Through April and May the Thar Desert and the northern plains bake, creating a deep low. Some time between the end of May and the beginning of June, moisture-laden south-westerlies from the Indian Ocean reach the tip of the peninsula and split into two branches. The Arabian Sea branch strikes the Western Ghats of Kerala first, which is why Kerala is the first Indian state to receive the south-west monsoon each year, and then works north along the Konkan coast and Goa.
Kochi sits on the windward side of those mountains. Forced up the Ghats, the air cools and drops its water, and the city averages about 3,015 mm a year across 124 rainy days, with temperatures held between roughly 23 and 31 °C by the sea and the latitude. From October to December Kochi gets a second, lighter rainy spell from the north-east monsoon, which reaches it from the leeward side. The dry months from December to March, each averaging well under 60 mm, are what keep the city out of the rainforest class.
East of the Ghats, the same winds arrive drained of moisture, so the interior of the peninsula is far drier. Am here is as much a product of relief as of wind: a narrow strip pinned between the Arabian Sea and the mountains.
Conakry and the West African monsoon: 1,100 mm a month after the harmattan
West Africa runs on the same physics with a different geography. There is no Himalaya; instead the contrast is between the hot Sahara and the cooler equatorial Atlantic. The ITCZ begins moving north from the equatorial Atlantic in February, reaches West Africa on or near 22 June and retreats south by October. While it is away, the dry north-easterly trade winds and their dusty extreme, the harmattan, blow off the desert.
Conakry, the capital of Guinea, built on the narrow Kaloum Peninsula, shows the extreme form of Am. Its 1991–2020 normals give 2.5 mm in January and 1.4 mm in February, then 433.6 mm in June, 1,082.6 mm in July and 1,142.1 mm in August, for a yearly total of about 3,730 mm. Because the total is so high, Köppen’s threshold is below zero and the nearly rainless dry season does not push Conakry into the savanna class.
Inland, the same ITCZ swing delivers far less water. The semi-arid Sahel and Sudan depend on the northern edge of this monsoon for most of their rain, which is why a late or weak season there means failed harvests while Conakry is still flooding.
Yangon and Cairns: the Bay of Bengal and Australian versions
Yangon, Myanmar’s largest city, sits on the Bay of Bengal side of the South Asian system. Its wet season runs from May through October and its dry season from November through April; January and February average only 4.5 mm and 3.0 mm, while July brings about 625 mm and the year about 2,841 mm. The Bay of Bengal also breeds cyclones: in 2008 Cyclone Nargis made landfall as a Category 4 storm and became the worst cyclone on record in Myanmar.
In the Southern Hemisphere the calendar flips. When the ITCZ moves south in the austral summer, the Australian monsoon draws moist north-westerlies across the Top End and north Queensland. Cairns has a wet season from November to May and a relatively dry one from June to October, with an average of just under 2,000 mm a year. Wet-season months from December to April can top 1,000 mm; January 1981 delivered more than 1,417 mm, while the whole of 2002 brought only 721 mm. Heavy monsoon rain periodically floods the Barron and Mulgrave rivers and cuts road and rail links to the city.
Miami: the trade-wind coast variety
Not every Am city lies on a classic monsoon wind reversal. Miami is classed Am because its winters, though rainier than Conakry’s, are dry enough, and its summers wet enough, to pass Köppen’s test with about 1,572 mm a year. There the wet season, May to October, is defined locally by humidity: it begins when the average daily dew point climbs above 21 °C (70 °F), which between 1956 and 1997 happened on a median date of 21 May, and ends when it falls back, a median of 17 October. Afternoon thunderstorms in moist, unstable air deliver most of the year’s rain.
This is the pattern found on many trade-wind coasts in Africa and the Americas: rain under the ITCZ or summer convection in the high-sun season, and dry, settled air under the subtropical high in the low-sun season. Meteorologists still debate whether weaker reversals such as the North American monsoon, which brings summer storms, from late June or early July into September, to Mexico’s Sierra Madre Occidental and Arizona, deserve the full name.
Kochi and Yangon owe their rain to the South Asian monsoon, Conakry to the West African swing of the ITCZ, Cairns to the Australian summer monsoon and Miami to trade-wind convection; all share heat year-round and a dry season too short or too wet to make them savanna. For the wind systems behind them in more depth, see how monsoons work; for the drier neighbour on Köppen’s scale, compare this with the tropical savanna climate, and for every letter in the code, our guide to the Köppen climate classification.