The Wettest Places on Earth

Mawsynram and Cherrapunji, India

The two wettest places with reliable long-term records sit within 15 kilometres of each other on the southern edge of the Meghalaya plateau in north-east India. Mawsynram holds the WMO record for the highest average annual rainfall: 11,873mm (467 inches). Cherrapunji -- known locally as Sohra -- holds the records for the wettest single calendar month (9,300mm in July 1861) and the wettest single year in recorded history (26,461mm in 1860-61).

The mechanism is straightforward to describe but extreme in its output. The Bay of Bengal drives warm, saturated air northward during the summer monsoon, from June through September. This air meets the steep southern escarpment of the Khasi Hills, which rise abruptly from the plains of Bangladesh to over 1,500m. The air has nowhere to go but up, and as it rises it cools rapidly, shedding its moisture as rain at rates that can exceed 100mm per hour during the peak of the monsoon. The escarpment acts as a wall that intercepts moisture from an enormous catchment area of warm ocean to the south.

Outside the monsoon season, both towns are relatively dry. This seasonality means that despite the extraordinary annual totals, the hillsides around Cherrapunji are not dense rainforest -- the months-long dry season limits what can grow. The town faces a peculiar problem: it receives among the most rainfall on Earth and yet faces periodic water shortages, because the infrastructure to store and distribute the monsoon deluge has historically been inadequate.

Reunion Island

A small French overseas territory in the Indian Ocean, roughly 800 kilometres east of Madagascar, Reunion holds more world rainfall records than any other location on Earth. The 24-hour record stands at 1,825mm (71.9 inches) at Cilaos on 15-16 March 1952. The 48-hour record is 2,493mm, and the 72-hour record is 3,929mm. All were set during different tropical cyclone events. To put the 24-hour figure in context, London receives approximately 600mm in an entire year.

The physics behind Reunion's dominance of short-duration records come from the intersection of two factors. First, the island sits in one of the world's most active tropical cyclone tracks. Cyclones carry enormous volumes of moisture, and their spiral bands can produce rainfall rates of 50-100mm per hour for extended periods. Second, Reunion is mountainous -- the interior rises to over 3,000m just a few kilometres from the coast. When a cyclone approaches, its moisture-laden bands collide with this topography and are forced steeply upward, concentrating rainfall in a small area with extraordinary intensity. The island's geography essentially acts as a rain amplifier for any cyclone that passes within range.

Lloro, Colombia

Lloro, a small town in the Choco department of Colombia's Pacific coast, is frequently cited as the world's wettest location by annual rainfall, with figures of approximately 13,300mm (524 inches) quoted by multiple sources. If accurate, this would place it substantially above Mawsynram. The reason it is not the universally accepted record holder is measurement consistency: the Choco region's rain gauge network has historically been sparse and the data more variable than the long-running Indian records.

The rainfall mechanism at Lloro is different from India's seasonal monsoon and more continuous throughout the year. Colombia's Pacific coast sits directly in the path of warm, moisture-laden air from the Pacific Ocean. The Western Andes rise steeply just inland, forcing this air upward almost constantly. Unlike Cherrapunji, there is no dry season -- rainfall occurs in every month, and the surrounding Choco bioregion is among the most biodiverse on Earth precisely because of this year-round moisture. Some estimates for annual rainfall in parts of the Colombian and Ecuadorian Choco exceed 12,000mm in multiple years, and the region almost certainly contains locations wetter than anything currently measured.

The Pacific Northwest -- Henderson Lake and Milford Sound

The Pacific Northwest of North America and the south-west coast of New Zealand's South Island both receive very high annual rainfall through a different mechanism from the tropical and monsoon locations above. Henderson Lake on Vancouver Island in British Columbia averages approximately 6,655mm annually, making it one of the wettest locations in North America. Milford Sound in Fiordland, New Zealand, averages around 7,000mm -- on some estimates higher -- and receives rain on roughly 200 days per year.

Both locations intercept frontal weather systems arriving from the open Pacific. The mid-latitude westerlies drive moisture-laden air onshore year-round, and in both cases the air immediately encounters steep mountain ranges -- the Vancouver Island Ranges and the Southern Alps respectively -- that force it rapidly upward. Unlike monsoon locations, the rainfall is spread fairly evenly across the year rather than concentrated in a single season. This produces a different kind of wetness: persistent, drizzly and pervasive rather than seasonal and extreme. The resulting landscapes -- temperate rainforests of cedar and spruce in British Columbia, and beech forest in Fiordland -- reflect sustained wetness rather than peak intensity.

The Central Amazon

The Amazon basin does not rank among the world's wettest places by peak annual rainfall -- the central Amazon receives between 2,000 and 3,500mm annually, which is high but not exceptional by global standards. What makes the Amazon remarkable is the sheer scale of that rainfall across a basin roughly the size of the continental United States, and the mechanism by which the forest actively generates much of it.

Through evapotranspiration, the forest releases enormous volumes of water vapour into the atmosphere. Trees essentially pump groundwater upward through their roots and release it through their leaves, where it rises, condenses and eventually falls again as rain. This recycled moisture can travel hundreds of kilometres before falling, and it has been estimated that a water molecule entering the Amazon basin from the Atlantic may fall as rain five or six times before reaching the Andes. Scientists refer to these streams of airborne moisture as flying rivers -- and they transport quantities of water comparable to those flowing in the Amazon river itself.

The implication is that large-scale deforestation does not simply remove trees -- it disrupts the moisture recycling that keeps the interior of the continent wet. Studies have shown that deforested areas receive less rainfall than nearby forested areas, and that there are tipping points beyond which the forest could shift from a wet, self-sustaining system to a dry savanna state, with consequences for agricultural productivity across South America.

What Makes a Place Extremely Wet?

Orographic lift is the single most reliable producer of extreme rainfall. When warm, moist air meets a mountain range, it is forced upward, cools at a predictable rate, and sheds moisture as rain or snow. The steeper and more abrupt the topography, and the warmer and wetter the incoming air, the more intense the rainfall. This explains why the world's wettest places are almost universally located on windward slopes at medium elevations -- high enough to force the air up, but not so high that the moisture has already been exhausted.

Warm sea surfaces provide the moisture source. Warm ocean water evaporates more rapidly, loading the overlying air with water vapour. The Bay of Bengal feeding Mawsynram, the Indian Ocean feeding Reunion, and the Pacific feeding both the Choco and the Pacific Northwest all share this characteristic. Tropical cyclones are a special case: they carry extraordinary concentrations of moisture and release it in extremely short periods, which is why Reunion dominates all short-duration rainfall records despite not being the wettest place by annual average.

Monsoon systems concentrate rainfall seasonally, delivering annual totals in a matter of months. Year-round westerly flow -- as in the Pacific Northwest -- produces lower peak intensities but continuous exposure. The rarest and most extreme combination is persistent onshore flow, steep orography and a warm, vast ocean source: this is what Mawsynram, Reunion and the Colombian Choco all have in common.