Size, intensity and frequency
Tropical cyclone size
Since winds spiral inward toward the central low pressure area in the eye of a hurricane, hurricane-wind velocity increases toward the eye. The distance outward from the eye to which hurricane strength winds occur determines the size of the hurricane. Winds in the eye wall itself have the highest velocity and this zone can extend outward from the center to distances of 16 to 40 km. Hurricane force winds (winds with velocities greater than 119 km/hr) can extend out to 120 km from the center of the storm. The largest tropical cyclone recorded, Typhoon Tip, had gale force winds (54 km/hr) which extended out for 1100 km in radius in the Northwest Pacific in 1979.
The smallest, Cyclone Tracy, had gale force winds that only extended 50 km in radius when it struck Darwin, Australia, in 1974. There is very little association between hurricane intensity (either measured by maximum sustained winds or by central pressure) and size. Hurricane Andrew is a good example of a very intense tropical cyclone of small size. It had 922 mb central pressure and 230 km/hr sustained winds at landfall in Florida, but had gale force winds extending out to only about 150 km from the center.
Hurricane intensity and frequency
Once a hurricane develops, the Saffir-Simpson Scale is used to classify a hurricane's intensity and damage potential. There are five possible categories. Category 1 storms are more common than category 5 storms. In a typical year, there may be many category 1 storms, but category 5 storms occur very infrequently.
| Saffir-Simpson Hurricane Damage-Potential Scale | ||||
|---|---|---|---|---|
| Scale Number Category |
Central Pressure mb (inches of mercury) |
Wind Speeds mi/hr (km/hr) |
Storm Surge feet (meters) |
Observed Damage |
| 1 | >980 (>28.94) |
74-95 (119-153) |
4-5 (1.2-1.5) |
some damage to trees, shrubbery, and unanchored mobile homes |
| 2 | 965-979 (28.50-28.91) |
96-110 (154-177) |
6-8 (1.8-2.4) |
major damage to mobile homes; damage buildings' roofs, and blow trees down |
| 3 | 945-964 (27.91-28.47) |
111-130 (178-209) |
9-12 (2.5-3.6) |
destroy mobile homes; blow down large trees; damage small buildings |
| 4 | 920-944 (27.17-27.88) |
131-155 (210-249) |
13-18 (3.9-5.5) |
completely destroy mobile homes; lower floors of structures near shore are susceptible to flooding |
| 8 | <920 (<27.17) |
>155 (>250) |
>18 (>5.5) |
extensive damage to homes and industrial buildings; blow away small buildings; lower floors of structures within 500 meters of shore and less than 4.5 m (15 ft) above sea level are damaged |
- Ships at sea transmit weather reports that help meteorologists locate centers of low pressure that may develop into tropical disturbances.
- Images from weather satellites, which are collected every 30 minutes, are then scanned to look for any development or growth of the disturbance. In particular, the images are examined to detect any rotational development of the storm, an indication that it may be approaching tropical storm strength.
- If a tropical storm or hurricane is detected and appears to pose a threat to land areas, observation airplanes are sent to examine the storm. Such planes fly into the storm at an altitude of about 3,000 meters or lower. The plane collects data on wind speed, air pressure, and moisture content by dropping devices called dropsondes into the storm. These dropsondes transmit the meteorological data continuously has they fall to the ocean surface, and thus provide information on the vertical structure of the storm. In addition, radar devices carried on the plane collect data about the intensity of the rainfall and wind velocities. The planes fly completely through the storm, passing though the eye, sometimes making several passes. The data collected give meteorologists a 3 dimensional picture of the structure of the storm.
- Satellite images and radar from land based stations allow scientists to track the position of the storm and report it to all agencies that may be affected if the storm makes landfall
