Hurricane info

An overview of terminology related to tropical cyclones.

A tropical hurricane is the same as a typhoon and the same as a tropical cyclone. All these three systems carry sustained winds of 65 knots (75 mph) or more. The different names are used in different areas of the world.

A tropical storm carries winds between 34 and 65 knots (37 and 75 mph). A tropical depression has winds lower than 34 knots (37 mph). They are precursors to a hurricane, but may sometimes never reach that stage, if atmospheric or oceanic conditions forbid.

Tropical systems differ from the ones at mid-latitude (such as the central and northern US and Europe) in that they are powered by high water temperatures, an atmosphere with windspeeds at the same level and direction at all levels and by the force of the earth's rotation.

At mid latitude, the depressions form as a result of differences in temperature between air near the poles and air near the tropics. The boundaries are called fronts, and the greater the difference, the more vigorous the depression.

A tropical cyclone is a safety mechanism in the earth's atmosphere, which allows heat, radiated onto the tropics to be quickly disseminated to higher latitudes. Warm air rises in a circular motion, giving rise to thunderstorms which rise 10 miles into the atmosphere. Cooler air is being drafted in at sealevel. Water offers very little resistance to wind, so the cyclone winds itself up to phenomenal windspeeds. Hurricane Felix this summer carried sustained winds of 140 knots, that's 165 mph. Its gusts went to 170 knots, a mind boggling 190 mph. Hurricanes are devastating, but they are necessary on a global scale.

I feel privileged to be able to admire their savage beauty through a satellite image from a safe distance. I went through two Atlantic winter storms of hurricane strength, which was frightening enough.