As it rises, the air cools and thunderstorms form. The ocean surface under the spiral acts as a low-pressure area. Cool, dry air is cycled downward, where it is warmed by the sea and rises to continue the swirling cycle. A hurricane's strength depends largely on the degree to which air is allowed to flow unhindered into and out of the central column, or eye. Hurricanes gain their classification when winds roar above 74 mph.
Tornadoes are funnel clouds.
Scientists cannot agree on exactly what causes such a phenomenon. In any case, a spinning funnel cloud has been known to lift cars and rip apart buildings as its vacuum action sucks in air from the ground and carries it upward. The weakest type of tornado blows out from a thunderstorm, usually lasting less than five minutes with winds rarely exceeding 150 mph. Large tornadoes can stir up enough wind to travel as fast as 300 mph. In rare instances, twisters have lasted for hours, measured a half-mile wide and traveled more than 200 miles.