Air

Cyclones and hurricanes

Around low pressure areas, air circulation generates a depression vortex called a cyclone. The circumstances that lead to the onset of a cyclonic circulation are always the same: the low pressure centre attracts air from the surrounding areas with an anticlockwise movement in the Northern Hemisphere and a clockwise movement in the Southern Hemisphere. But cyclones are not all the same. Why are some violent and unexpected, real catastrophes? Why do some last for months? Why are others relatively ‘calm’ and do not cause damage? In theory, the greater the difference in pressure, the greater the violence and speed of the winds and therefore the more destructive the cyclone will be. However, generally, the differences in pressure are relatively small and similar in each vortex (normally, a couple of tens of mb). It all depends on the size of the low pressure area. Considering that the difference in pressure is constant, if the low pressure area is extended, the pressure gradient will be low and the winds relatively ‘slow’; if, on the other hand, the area is restricted, the high pressure gradient will provoke violent, very fast-moving winds which in some cases can exceed 250 km/hr, reaching up to 400 km/hr like in tornadoes or whirlwinds. Therefore there will be considerable differences between the power of a so-called extratropical cyclone, like those that occur in our latitudes and that are extended over areas with a diameter of thousands of km, and that of a tropical cyclone, or of a hurrican, that rarely has an extension that exceeds hundred kilometres. Even more destructive, but on a strictly local scale, are whirlwinds and tornadoes (from the Spanish word for ‘vortex’), or their marine equivalent, the waterspouts, that generally have dimensions of a couple of hundreds of metres: these are the most violent meteorological phenomena even though they are not the most destructive because fortunately they are small and they last a very short time.