Tropical cyclonesLarge, rotating storm systems form over warm ocean waters in tropical regions. They do not occur in colder regions; the water surface must be at least 26 degrees for a cyclone to form. Therefore, countries outside the tropics are less likely to be affected. Once a storm system has formed, it can reach a radius of many hundreds of kilometers and travels across the open ocean until it hits land.
Depending on the region, they are called hurricanes, typhoons or cyclones.
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How a tropical storm formsTropical cyclones require warm ocean water with surface temperatures of at least 26°C to form and strengthen. When warm and moist air rises above the ocean, it is called convection. In layers of air up to 5 km high, it cools again and condenses, releasing heat energy that drives the storm. The released heat energy remains in the troposphere and the air pressure there increases. The higher air pressure spreads out and creates a suction effect that pulls in more moist air from below. The Earth's rotational motion causes the storm to spin and develop into a tropical cyclone.
ClassificationTropical cyclones are classified into different categories from 1 to 5 using the Saffir-Simpson scale. The decisive criterion here is the wind speed. However, since weather phenomena are observed before they develop into full-blown storms, there are additional subdivisions:
|Tropical depression||< 61 km/h||< 38 mph|
|Tropical storm||62 - 118 km/h||39 - 73 mph|
|Category 1||119 - 153 km/h||74 - 95 mph|
|Category 2||154 - 177 km/h||96 - 110 mph|
|Category 3||178 - 208km/h||111 - 129 mph|
|Category 4||209 - 251 km/h||130 - 156 mph|
|Category 5||> 251 km/h||> 156 mph|
Typhoons in the Pacific Ocean that reach a speed of 150 km/h (92 mph) are officially designated as "severe typhoons" by the Hong Kong Observatory. Above 190 km/h (= 118 mph), one speaks of a super typhoon. The Hawaiian Joint Typhoon Warning Center, on the other hand, has a different definition: Here, the minimum speed for a super typhoon is 240 km/h (150 mph). In practice, however, these different limits often do not play a role, since the Hong Kong Observatory measures the speed in a 10-minute average, while in Hawaii a 1-minute average is used.
The most frequently affected countriesFrom the past 36 months, we have identified the most frequently affected countries. The table below shows the number of tropical storms per category that made direct landfall. The category indicated corresponds to the Saffir-Simpson scale at the time of impact.
|Country||Cat. 1||Cat. 2||Cat. 3||Cat. 4||Cat. 5|
|United States of America||13||5||5||4||1|
Hurricane, cyclone or typhoon?Which of the words is used depends on where a storm has formed. The word "cyclone" is often confusing because it is also used as a generic term for all rotating storms that originate on the ocean. At the same time, it is equally the term for these storms in the Indian Ocean and on both sides of Australia. Basically, this classification applies:
- Hurricanes: Atlantic and Northeast Pacific
- Typhoons: Northwest Pacific
- Cyclones: Indian Ocean and South Pacific Ocean
When is hurricane season?The peak season for tropical storms depends on the region and the ocean currents with which warm and cold water masses move in the oceans:
North Atlantic and CaribbeanSeason: June 1 - November 30
Strongest in the first half of September
Northeast Pacific and Western MexicoSeason: May 15 - November 30
Strongest end of August to beginning of September
Northwest PacificSeason: July 1 - November 30
Strongest at the beginning of September
Northern Indian OceanSeason: April 1 - December 15
Strongest in May and November
Southwest Indian OceanSeason: November 1 - May 15
Strongest from mid-January to early March
Southeast Indian Ocean to AustraliaSeason: November 1 - May 31
Strongest from January to early March
South Pacific East of AustraliaSeason: November 1 - May 31
Strongest from February to early March