365 Days of Climate Awareness 40 - Hurricanes 1: Formation and Development

A hurricane, known elsewhere around the world as a cyclone or typhoon, is a very large (>300 miles in diameter) rotating storm system with a low-pressure center and circling bands of high wind and rain. The defining feature of a hurricane is sustained wind speeds of 74 mph (33 m/s) or greater around an organized storm center. Wind speeds can reach 160 mph (71 m/s) and their destructive potential is massive. But hurricanes form an important component in the global heat budget, bringing large pulses of heat poleward from the tropics.

Formation of a hurricane.

Atlantic hurricanes typically form off the northwest coast of Africa, with winds from inland bringing very hot, dusty air out over the ocean. This leads to upward convection which, if strong enough, becomes sustained. As the upward convection strengthens, storms and precipitation form around it. The storm system reaches a size where the rotation of the planet begins to affect it.

The basic dynamic is the same as with sea/land breezes and the monsoon: higher-pressure air is moving toward a lower-pressure region. "The atmosphere is trying to fill the hole," as my professors used to put it. But The distance the winds are traveling is great enough that the rotation of the earth diverts them. In the northern hemisphere, the winds are diverted to the right; in the southern hemisphere, to the left.

The result is a coherent storm structure with an eye, filled with placid, low-pressure air, extending from a few tens to a few hundred miles across, surrounded by a cloud wall and then the bands of storm clouds rotating at high speed. In the northern hemisphere, the storm winds rotate counterclockwise around the eye; in the southern hemisphere, clockwise. So long as the storm is over warm water, it can continue drawing on the warm vapor to sustain and strengthen the convection. It is now a mature hurricane.

The National Weather Service established several categories for tropical and subtropical storms. Among those are tropical depressions, storms with sustained winds of 38 mph (17 m/s) or less, and tropical storms, with sustained wind speeds of 39 mph (17-33 m/s). When a storm has reached tropical storm strength, it is named by the National Weather Service. When it passes 74 mph sustained wind speed, it's a hurricane.

Tomorrow: Hurricanes part 2: maturity to dissipation.

Be well!