Hurricane, typhoon and cyclone are one kind of storm with three regional names
A hurricane over the Atlantic, a typhoon near Japan and a cyclone in the Indian Ocean are the same machine: a spinning low fuelled by evaporation from warm seas. Around 80 to 90 are named worldwide each year, and more than half reach hurricane force. None can form right at the equator.
A tropical cyclone is a warm-cored low-pressure system with a closed circulation, spiral bands of thunderstorms and strong winds, typically 100 to 2,000 kilometres across. Unlike mid-latitude storms such as nor'easters, which feed on contrasts between warm and cold air, these draw their power from ocean water that evaporates, rises, cools and condenses, releasing heat. Once sustained surface winds exceed 35 knots, the system is considered self-sustaining. The name hurricane applies in the Atlantic and northeastern Pacific, typhoon in the northwestern Pacific.
Spin comes from Earth's rotation. As air rushes inward it conserves angular momentum and whirls faster, turning clockwise south of the equator and the opposite way north of it. Because that rotational effect vanishes at the equator, cyclones rarely form within 5 degrees of it. Other requirements include seas of about 27 °C, low wind shear, unstable and humid air, and an existing disturbance to organise around. The South Atlantic almost never produces them because of persistent shear and a weak convergence zone.
On average 86 reach tropical storm strength annually; 47 exceed 119 kilometres per hour and 20 reach at least Category 3 on the Saffir-Simpson scale. Worldwide, September is busiest and May quietest, and November is the only month when every basin is in season. The Atlantic season runs from 1 June to 30 November, peaking statistically on 10 September, while the northwestern Pacific sees storms all year. El Niño has the largest influence of any climate pattern, shifting where storms track.
Coasts take the brunt, from wind, waves, storm surges and tornadoes, since storms weaken once cut off from warm water, though flooding reaches inland too. A cyclone gathers moisture from a huge area and dumps it over a smaller one, capable of multi-day downpours that overwhelm rivers and drainage. Scientists find that climate change can lengthen, intensify and multiply these storms by warming oceans and strengthening the water cycle. Ten centres designated by the World Meteorological Organization issue forecasts of position and intensity.
Source: Tropical cyclone
