A tropical cyclone is a general description given to storms that have formed in the tropics with sustained winds of at least 74 miles per hour.
Both hurricanes and typhoons are local names given to these storms in specific parts of the world.
Hurricanes are the tropical storms which form in the south Atlantic, across the Caribbean and the eastern side of the Pacific Ocean. Typhoons are the same thing, but are found on the western side of the Pacific, through the Philippines, China, Vietnam and Japan.
The storms that frequent the Bay of Bengal and cause so much misery and destruction to Bangladesh, for example, are known by the generic name of tropical cyclones. They also affect other areas, from the Indian Ocean to eastern Africa and through Indonesia to northern coasts of Australia.
These storms form over the warm tropical oceans, although not within about 300 miles of the equator. The 'Coriolis' force is too small here to allow any low pressure to develop. It is the force that usually pushes the air to the right of its orginal path in the northern hemisphere, and to the left, south of the equator.
The sea temperature must be at least 26°C (80°F). As the warm humid air rises up from the ocean, it condenses and releases large amounts of latent heat that fuels the developing tropical storm. Even with these prevailing conditions the vast majority of tropical disturbances never develop into full blow storms. This is because conditions in the upper atmosphere also have a bearing on them.
Once they become full tropical cyclones, hurricanes or typhoons, they are given a category depending on the winds blowing around them. The Saffir-Simpson intensity scale is used, which works on a similar principle to the Beaufort scale.
Saffir-Simpson scale:
| Category |
Winds |
Effects |
| 1 |
Strongest gusts less than 77 mph. |
Damage to some crops, trees and caravans. |
| 2 |
Strongest gusts 77-106 mph. |
Minor house damage. Small craft may break moorings. Heavy crop damage. |
| 3 |
Strongest gusts 106-140 mph. |
Some roof and structural damage. Power failure likely. |
| 4 |
Strongest gusts 140-175 mph. |
Significant structural damage. Widespread power failure. Dangerous airborne debris. |
| 5 |
Strongest gusts above 175 mph. |
Widespread damage. |
As well as a category, most of the storms are given names alternating from male to female. If any become major disasters, then the name is only ever used once. These destructive storms are normally expected in the northern hemisphere between June and November and in the Southern Hemisphere between November and June. The numbers and categories of the storms can vary quite a lot from year to year.
During an El Niño event, when the eastern Pacific warms up, hurricanes in the Caribbean and the eastern USA seem to decrease. However, activity increases in the eastern Pacific instead.
One last important distinction to make is between hurricanes and hurricane force winds. Hurricanes are tropical revolving storms. When they reach the UK, they have lost most of their characteristics by moving over the colder Atlantic. Hurricane force winds, as we had in the 1987 storm, are purely and simply winds reaching at least force 12 on the Beaufort scale. They do not necessarily have anything to do with tropical storms.
Related Links:
- Beaufort Scale
- Weatherwise - Hurricanes
- Hurricane Season
- Predicting Hurricanes
- El Niño
- La Niña
