Almost every afternoon, from September to March, a thunderstorm hits the Tiwi Islands in northern Australia. It happens so regularly that meteorologists refer to it by the name Hector. The cumulonimbus thundercloud is visible around 3 a.m., and is said to be so reliable that anyone can set their clocks by it.
Hector, also known as Hector the Conveyor, earned his name during World War II, when pilots and sailors in the region used the thundercloud’s recurring position as a sort of navigational beacon. Hector can reach an altitude of about 20 km, and is visible all the way from Darwin, a city 100 km away.
A view of Hector from Gun Point in the Northern Territory. visual: jambalawa / Wikimedia
Hector is mainly caused by the collision of several sea breeze boundaries in the Tiwi Islands.
“Southeasterly trade winds extending south from the subtropical ridge will normally suppress typhoon activity over much of the Northern Territory during the late dry season (September) and occasionally during the wet season (October and November), causing Hector will leave as a prominent but isolated feature on the Darwin horizon,” said senior meteorologist Ian Shepherd of the Australian Bureau of Meteorology.
“The size, shape and location of the Tiwi islands make them an ideal location for Hector to grow,” said Ben Domencino, a meteorologist at DTN company WeatherZone. “Sea winds develop from around on the islands and meet in between. These convergent winds, which are carrying moisture from the surrounding ocean, have to go somewhere when they collide … so they go up. The wind This rising column of water becomes cooler with height, which condenses the water vapor into liquid droplets, forming clouds.”
“The tropical environment is almost always accompanied by thunderstorms during the wet season, when convergent sea winds provide the trigger for lifting,” Shepherd said.
Hector is most common during the warm months of the “build up season” of September–December. Hector can also occur during the “rainy season” from January to March. Sometimes Hector stops in April if the temperature is warm enough.
Hector as seen from Stokes Hill Wharf in Darwin. visual: jambalawa / Wikimedia
Hector is not formed only on days when there is an active west monsoon flow.
“The cloudy conditions and strong winds associated with an active monsoon suppress sea breezes over the islands, making Hector less likely to form regularly,” Shepherd said. “An active monsoon also generates regular rainfall and thunderstorms that move rapidly across the islands from west to east in a relatively calm oceanic air current.”
The most suitable conditions for the growth of Hector are when there are clear skies with light winds, which allow not only strong solar heating, but also sea breezes during the day. Hector does not occur in the winter months (May to August in the Southern Hemisphere) because cooler temperatures reduce the strength of local oceanic winds and limit the convergence of winds needed to form the hector.
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