THE TERRIBLE TWISTERS
When a hurricane roars by us, to drift and die and decay in the northern hinterland, we sigh in relief — and forget what a close call it was. But soon another comes in its wake. Let us brace ourselves for it because late summer is middle of the hurricane season: it is a time of devastation and trepidation along the southern and eastern coasts of the USA (and more so among the poor inhabitants of the low-lying Caribbean). The tropical storms called hurricanes in the Atlantic, typhoons in the North Pacific, and cyclones in South Pacific & Indian Oceans, are among the most ferocious tantrums of nature. They are not to be confused with tornadoes or twisters, which are most prevalent in spring and sweep across the Midwest mainland of the USA rather than the coastal areas.
A tropical storm is rated according to the speed of its winds; those are the factors that cause most damage to structures on land. When the wind exceeds 74 MPH (119 KPH) the storm is classified as a hurricane. A hurricane of that minimum speed is rated as Category-1; at the top end, Category-5 hurricanes are characterized by winds with a wallop of 156 MPH or higher. Their names are chosen whimsically but alphabetically, male and female names alternating: Irma soaked Florida recently in the wake of Harvey; Jose is now decaying to a mere “tropical storm” far west of our North Atlantic coast; Maria (this season 7th) is lashing the Caribbean and sidling up to us.
In addition to its linear travel (say from the Caribbean up to Florida), a hurricane spins to form a static vortex called the “eye.” They spin for the same reason a spacecraft launched directly upwards at Cape Canaveral appears to ascend with a slightly counter-clockwise trajectory. It is all due to the west-to-east rotation of the earth, making the sun rise in the east and set in the west. As a rocket ascends directly upward, the spot from which it was fired will seem to be drifting steadily east of the rocket with the spinning earth; conversely, to an observer in the rocket it seems the launching pad is drifting westward. In sum, it is as though a force (the “Coriolis Force”) drags a spacecraft from the upright ascent into a curved path.
The spin of the earth is fastest at the equator and zero at the poles, so the net result of the Coriolis force on a large mass of air is to pull it into a spiral, the signature spiral of a hurricane. That spin or spiral is counter-clockwise in the Northern Hemisphere (USA), and clockwise in the Southern Hemisphere (South Africa). The spin of a tornado is analogous: clockwise in southern hemisphere and counterclockwise in northern.