Dennis, as I write, is menacing the Carolina coast. This fourth member of the family of Caribbean hurricanes of 1999 is described at present as being Category 2, and is expected to intensify shortly to Category 3.
It is moving very slowly, which makes its future progress difficult to predict with confidence.
Hurricanes Arlene, Bret, and Cindy have already fretted and strutted their hour upon the stage. Dennis, in the nature of things, will be followed in due course by hurricanes called Emily, Floyd and Gert, and there may well be more in the guise of Harvey, Irene, Jose, Katrina, Lenny, Maria, Nate, Ophelia, Phillippe, Rita, Stan, Tammy, Vince and Wilma.
All hurricanes begin at sea, the first sign being a diffuse area of low pressure. In such a region, a tropical depression may develop, producing winds up to 30 m.p.h., and two or more closed circular isobars on the weather map.
Such low pressure areas are quite frequent in low latitudes. Many of them live out their lives as minor disturbances, but a few develop into major storms. When this happens, the result is a tropical storm, with winds around the centre between 30 and 70 m.p.h.
A tropical storm is considered important enough to be given an individual name; it is subsequently given hurricane status if the winds around it exceed 74 m.p.h.
The potential of hurricanes to cause damage to life and property is rated on the Saffir-Simpson Hurricane Intensity Scale of one to five. The scale was designed in the early 1970s by one Herbert Saffir, a consulting engineer who specialised in wind damage to buildings, and Robert Simpson, a former director of the US National Hurricane Centre in Miami, Florida. Any individual hurricane may pass through one of all of the stages, or categories, on the scale.
A category 1 hurricane on the scale is comparatively harmless. It may lop a branch or two from trees and bring down some power-lines, but its winds are no stronger than 90 m.p.h. At category 2, a hurricane will have winds of more than 100 m.p.h. and a few trees begin to fall.
At category 3 the damage becomes relatively widespread, and affects buildings and other relatively solid structures, while Category 4 brings 150 m.p.h. winds with extreme damage and the doors and windows of buildings in the path of the storm being almost totally destroyed.
Category 5 is catastrophic: buildings in the storm's vicinity are devastated, winds exceed 160 or 170 m.p.h., and storm surges approaching 20ft can be expected on the coast.