Simulations

Hurricanes are intense tropical cyclones of large mesoscale proportions that develop over warm ocean water in many parts of the world. A deep layer of warm ocean water provides the storm with its energy source in the form of large fluxes of sensible and (especially) latent heat. Also required are a preexisting, large-scale wind flow system that imposes a low-level convergent and/or upper-level divergent flow on the region, along with an air mass that is warm, humid, and free of strong inversions or wind shear. Due to an absence of Coriolis force on and near the equator, hurricanes do not form in this region.

To qualify as a hurricane, a tropical cyclone must have sustained winds in excess of 64 knots. Hurricane-like storms with lesser winds are designated tropical depressions (maximum winds between 20 and 34 knots) or tropical storms (maximum winds between 35 and 64 knots). Hurricanes are known by different names in different parts of the world: typhoons (in the Western Pacific) and cyclones (in the Indian Ocean) are two examples.

Hurricanes differ from frontal cyclones in many ways. They are smaller than frontal storms, are roughly circular, consist of a single (mT) air mass, and have no fronts. Being a warm-core cyclone, the hurricane's circulation is strongest near sea level and weakens with height (Why? Refer back to Figure 7.25.). As air spirals toward the storm's center, it gains speed, sometimes reaching well over 100 knots. Tornadoes and smaller vortices spawned in the hurricane's eyewall create even stronger, short-duration winds and gusts. One of the hurricane's most distinctive features is its eye, a region of relative calm at the very center of the storm.

The Simulation associated for this chapter, as well as the Exploration, offer you many views of hurricane structure, circulation and motion.

Hurricanes generally move westward, steered by the trade winds on the equatorward side of the subtropical high pressure belt. Reaching the west side of the subtropical high pressure cell, many storms recurve poleward. If a hurricane makes landfall, it can cause widespread devastation due to a combination of strong winds, storm surges of ocean water, and heavy rainfall. Moving over land or colder ocean waters, hurricanes lose energy and dissipate, often merging with mid-latitude weather systems.

As with other circulations from tornadoes to frontal cyclones, we have much to learn before we can predict accurately just when and where hurricane formation will occur. Once a hurricane has formed, however, forecasts generally are much more accurate. With guidance from satellite imagery and aircraft reconnaissance to provide an accurate record of the storm's past, charts of past hurricane tracks, and computer modeling, forecasts have shown considerable improvement in skill in recent years. The dramatic reduction in hurricane fatalities in recent years is important evidence of this improvement.

Simulation 1 (10119.0K)