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740 mm of rain in a day
Northern Australia runs on a wet season and a dry season. Darwin, the largest city on the northern coast, averages more than 1,800 mm of rain a year, nearly triple London's total, but the vast majority of it falls in the four months between December and March, when monsoons and tropical cyclones are active.
Away from the immediate coastline, the interior between the northern coast and the Tropic of Capricorn gets some of the most erratic rainfall anywhere on the planet, largely because of the unpredictable pattern of tropical cyclones.
In 1924, with no cyclones reaching the interior, the same town received 4 mm of rain across the entire year. That is even less than the yearly average of the Sahara desert.
How does one cyclone drop a year of rain?
A tropical cyclone is a heat engine fed by warm ocean water. Moist air spirals inward and rises, and as the vapor condenses into droplets it releases heat that pulls in still more moist air. Bands of rain wrapped around the eye keep that cycle running for hours over the same area. Meteorologists generally say the ocean surface must be about 26.5 degrees Celsius or warmer for a cyclone to form.
Why does Darwin's rain fall in four months?
In the Australian summer, the northern landmass heats up and the zone where trade winds meet, known as the intertropical convergence zone, shifts south. That draws in moist monsoon winds from the northwest, which bring storms and heavy rain. In winter the pattern reverses and dry southeast winds blow from the continent's interior. Darwin's dry season runs from about May to September, when almost no rain falls.
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