Hurricanes in Brief

Hurricanes and typhoons are large and sometimes intensely violent storm systems. In meteorological terms, they are tropical cyclones that have maximum sustained winds of at least 120 km/h (75 mph). Atlantic and eastern Pacific storms are called hurricanes, from the West Indian huracan ("big wind"), whereas western Pacific storms are called typhoons, from the Chinese taifun, "great wind."

The primary energy source for a tropical cyclone is the latent heat released when water vapor condenses. Only extremely moist air can supply the energy necessary to spawn and maintain tropical storms, and only very warm air contains enough moisture. Tropical cyclones, therefore, form only over oceans with water temperatures of at least 27 degrees C (80 degrees F). After they have formed, such storms tend to intensify when passing over warmer water and weaken over colder water.

The rate of condensation heating that results from the intense rainfall associated with tropical cyclones is about 10 to the eleventh power kW. In one day, therefore, a storm produces 24 x 10 to the eleventh power kW h, an amount of energy that lies within the range of the yearly consumption of power by many industrialized nations.

Structure of the Storm

The mature tropical cyclone is characterized by a circular pattern of stormclouds and torrential rains, whipped by winds that may reach velocities of 160 to 300 km/h (100 to 180 miles per hour) within a radius of 10 to 100 km (6 to 60 mi) from the storm center. The winds diminish rapidly with increasing distance. At a radius of 500 km (300 mi), wind speed is usually less than 30 km/h (18 mph). (The winds rotate in a counterclockwise direction in the Northern Hemisphere and in a clockwise direction in the Southern Hemisphere.) The heaviest precipitation occurs in this region of intense convection. Thunderstorms may produce rainfall rates of 250 mm (10 in) a day. The release of latent heat associated with this rain maintains low pressure and strong winds. The total cloud system of a large tropical cyclone may have a diameter of up to about 3,200 km (2,000 mi).

At the center of the storm, within a "wall" of powerful winds, there is an "eye", which is a cloud-free circular region of relatively light winds that has a diameter of 10 to 100 km (6 to 60 mi). Surface pressure reaches its minimum in the eye. Typical values are 950 millibars, but values of less than 900 have been recorded. The sinking motion in the eye, which causes the clearing, also produces adiabatic warming and drying. Temperatures at 5 km (3 mi) above sea level are typically 10 degrees C (18 degrees F) warmer than the tropical storm's environment.

The very high-velocity winds surrounding the eye are maintained in strength by the large differences in horizontal pressure between the eye and the outer region of the storm. Although the winds themselves are responsible for much of the storm damage, the waves and tides generated by the wind often cause most of the damage to coastal areas. Because much of human activity near the coast is concentrated within a few meters above mean sea level, storm surges can result in considerable loss of life and property.

Speed of Rotation

The rapidly whirling tangential circulation of winds in a tropical cyclone can be explained by the conservation of angular momentum. Just as ice skaters spin faster as they bring their arms down, closer to the axis of rotation, so the air rotates faster as it is pulled in toward the center of the storm by the low pressure. Without friction, the wind would increase as the inverse of the distance from the center. Thus, a wind rotating at 5 km/h (3 mph) at a radius of 500 km (300 mi) would have a velocity of 250 km/h (160 mph) if it reached a radius of only 10 km (6 mi). Friction reduces the predicted speed , but the basic principle explains the high rotational velocities near the center.

The air that spirals toward the center and rises in the intense convection in the wall of the eye turns outward in the upper troposphere (about 15 km/10 mi above sea level). As the air moves away from the center, its counterclockwise