We often hear meteorologists on television speaking of a tropical depression “becoming better organized” as it develops into a tropical storm and then a hurricane. And probably everyone has seen satellite images of the distinctive spiral shape of these great storms. That spiral shape is only the most obvious indication of the complex structure of a hurricane. Ironically, these storms that cause so much chaos, death, and destruction, are themselves one of the most structured weather phenomena of all.
A hurricane is essentially a gigantic heat engine. It draws its energy from the warm tropical waters it sucks up from the ocean. That heat is converted into the powerful winds and surging waves that give the hurricane its destructive power. Hurricanes are not only powerful; they are also huge storm systems, averaging around 300 miles in diameter. Some have grown to as much as a thousand miles across.
The basic components of a hurricane are the spiral of rain bands, the eyewall, and the eye. Together they make up a dynamic structure that starts with warm, humid air from the surrounding ocean being drawn in to the circular band of cirrocumulus clouds that make up the eyewall. The winds here are the most intense, and draw in huge volumes of moist air, throwing it upward and outward. There it spreads out to form the great, spiraling bands of rain that stretch out for a hundred miles or more as they rotate with the storm. The moisture is cooled in the upper atmosphere and falls as torrential rain. Wind speeds here are extreme. In a category 5 hurricane, they exceed 155 miles per hour. In a hurricane that is moving fairly rapidly (15-25 miles per hour) the wind rises higher still as the movement of the storm system is added to the rotational wind of the bands that are moving in the same direction.
The eyewall also forms a division between the spiral and the eye of the hurricane. This is one of the most striking features of a hurricane, for the eye (from 20 to as much as 60 miles across) is calm. It is an area of very low barometric pressure. Relatively little of the air and moisture drawn up by the eyewall ends up in the eye itself. In the past, this sometimes led to tragedies when people unfamiliar with hurricanes would leave safe shelter thinking the storm had passed, only to be caught in the open when the eye passed and the storm returned in full fury.
An important part of the anatomy of a hurricane is its effect on the ocean surface. The intense winds create massive waves, called the storm surge. When a hurricane strikes land, the waves pile up, especially if the seabed just offshore is a shallow slope, creating a wall of water that can exceed 20 feet in height. Even while it is still at sea, these waves spread out far beyond the storm itself, creating large swells and high surf on beaches. Sometimes these “forerunner waves” extend for hundreds of miles beyond the hurricane.
A hurricane, for all its size and power, is not a stable system. It depends on constant renewal of energy and moisture from the ocean. When a hurricane makes landfall, its source of energy and water is cut off, and it quickly weakens and then disintegrates.