Bermuda is bracing for the arrival of Hurricane Fabian, and Florida will soon be drenched by the less violent Tropical Storm Henri. Both storms were categorized as tropical depressions before developing into more serious meteorological events. Why do some tropical depressions become hurricanes and others become mere tropical storms?
Tropical cyclones—the catch-all term for these disturbances—remain a meteorological mystery, but it's thought that vertical wind shear plays a big role in determining a tropical depression's fate. A TD is basically a gathering of thunderstorms that's started to coalesce into a unified weather system, thanks primarily to lowered atmospheric pressure, which facilitates the uptake of energy from the water. By definition, a TD's winds are between 25 miles per hour and 39 miles per hour. The next step up the storm hierarchy is a tropical storm, which occurs when a TD's winds exceed the 39 mph mark and the mass of thunderstorms begins to rotate in a cohesive manner.
That's where the impact of vertical wind shear comes into play. Wind shear is the magnitude of wind change with height—in laymen's terms, the difference between wind strength near the ocean's surface versus wind strength way up in the air. High vertical wind shear means that winds are quite strong near the storm's top, and that tends to rip apart or cripple the tropical storm before it can develop into a hurricane. Low vertical wind shear, by contrast, means the storm is more likely to develop winds in excess of 74 mph, an "eye," and spiral banding—all telltale signs of a hurricane.
Another factor is water temperature. The warmer the water, the more energy it releases when it evaporates and then condenses in a patch of storm activity. And more energy means higher winds, as well as more evaporation—think of it as powering a gigantic meteorological feedback loop. The general rule of thumb is that water temperatures in constant excess of 80 degrees Fahrenheit are necessary for hurricane formation.
Also worth mentioning is the impact of the Coriolis Force, a physical force related to the Earth's rotation. This force is thought to bolster the spin of loosely aggregated storms by applying a little rotational kick. There is virtually no Coriolis Force present at or near the equator, so hurricanes are unlikely to form there despite the presence of warm water. Virtually all of the world's tropical cyclones form between at 5 degrees and 20 degrees latitude.
Explainer thanks John Molinari of the StateUniversity of New York at Albany and David Zierden at FloridaStateUniversity.