Hurricanes By the Numbers Home > Hurricanes By the Numbers
Our "storm meister" is Senior Contributor Steve Faber, who came to study meteorology out of necessity, learning the skills out of an aversion to death and dismemberment -- his own! For years he was a private pilot and understanding weather was a basic survival skill....
Hurricane season is a numbers game, and though many of those numbers may seem arbitrary, almost all have their origins in either history or the physics of tropical meteorology. For example:
80 Degrees: This is the water temperature (in Fahrenheit) at which the formation of tropical cyclones is possible. Warm water is the basic fuel of these storms, which is why they peter out almost immediately once crossing the shoreline, and rarely occur (in the northern hemisphere) during the months of December through May. Currently, in the southern Gulf of Mexico and southwestern Caribbean Sea -- June's main spawning ground for new tropical disturbances -- temperatures in the range of 82 through 84 degrees have been recorded, making tropical cyclone development possible at any time.
74 M.P.H.: This is the minimum sustained wind speed (one-minute average) at which a storm becomes a hurricane. So who pulled that number out of their hat? Well, it's not as if King Neptune rises from the sea, waves his magic trident, and "poof," declares a storm a hurricane. But the number isn't arbitrary either. 74 m.p.h. is the wind speed at which a tropical cyclone develops a stable eye, a feature that is one of the key characteristics of hurricanes, but not of tropical storms.
39 M.P.H.: Okay, if a tropical cyclone develops an eye feature at 74 m.p.h., what happens at 39 m.p.h. that changes a storm's designation from tropical depression to tropical storm? Answer: nothing. Well, almost nothing. It's not quite arbitrary either. It probably relates back to the Beaufort Scale, developed in the early 19th century, which breaks wind velocity into 12 levels from calm to hurricane. Based on observed sea states and wind effects on land objects, the scale was developed to alert seamen to the danger level of being at sea in an era before the advent of accurate anemometers (wind speed indicators). Later, when such instruments became available, actual wind speeds were attached to the different levels of the Beaufort Scale, based on measuring the wind when the Beaufort observed criteria were present.
The scale was broken down further into three groups of four. (Beaufort 1 through 4, 5 through 8, and 9 through 12). The lowest grouping was considered safe for all sized vessels; the second group, was safe for larger vessels; and the last four levels were considered hazardous for all vessels. Beaufort 8 is designated "Gale Force," and begins at 34 knots (39 m.p.h.), so storms reaching Beaufort 8 would naturally send up a figurative warning flag to mariners of any sized vessel that they were only one level short of seriously hazardous winds and seas.