ENVIRONMENTAL NECESSITIES

In stark contrast to their ferocity, hurricanes are remarkably fragile systems. Just the right combination of atmospheric and oceanic ingredients are needed to promote the development of a hurricane. Even after a hurricane forms, a little too much dry air, a decrease in water temperature or an increase in wind shear, and even the most powerful storms begin to show signs of decay. It is ironic that one of Mother Nature's most destructive forces demands careful nurturing in order to maintain its existence. The journey from a depression to a hurricane is a perilous one and the majority of tropical systems don't make it.

The development of a hurricane is predicated upon the existence of several favorable environmental conditions, including sea surface temperature, vertical wind shear, the Coriolis force and a moist environment. Each of the required conditions will be discussed in the context of Hurricane Wilma's development.

Sea Surface Temperature
A reasonably deep layer of warm water (150 feet) with a temperature of at least 26.5° C (80° F) is required for hurricane development.  The depth of the warm layer is very important, particularly with slow moving hurricanes. As a hurricanes passes over water, its extraordinary winds are capable of mixing the ocean to considerable depths. If the layer of warm water is relatively thin, cold water is quickly brought to the surface through a process called upwelling and the hurricane's development is hindered.

Warm, moist water is the fuel that sustains a hurricane, and therefore it must be present in ample amounts. The influx of warm, moist air into a hurricane adds energy to the system as water vapor condenses to form liquid droplets. As this condensation takes place, latent heat is released into the surrounding atmosphere. This process increases the overall temperature profile of the air column, ultimately contributing to the lower barometric pressure at the surface. This warming of an air column by latent heat is an integral facet of hurricane development.

The chart below of mean sea surface temperature for the period October 10 to 15, 2005 (Figure 1) shows that conditions were very favorable for the development of the tropical depression that would become Wilma. Temperatures were 28°C or greater in a wide swath of Atlantic Ocean extending from the coast of Africa into the Gulf of Mexico.

In fact, the chart of surface temperature anomalies for October 10 to 15, 2005 (Figure 2), indicates that the temperature was 1°C warmer than the long-term mean for the period. Granted, this is not a significant difference, but it clearly demonstrates that Wilma had a greater than average source of fuel.

Vertical Wind Shear
Developing tropical systems, and even mature hurricanes, are particularly susceptible to vertical wind shear. Wind shear is the difference in wind direction and speed as you ascend in the atmosphere. By convention, meteorologists calculate wind shear by comparing the values for speed and direction at two levels --850mb and 200mb. These pressure values are roughly found at 1,500 meters (4,921 feet) and 12,000 meters (39,369 feet) above the earth's surface and are two of the mandatory pressure levels where atmospheric conditions are regularly measured.

Low values of vertical wind shear are necessary for hurricane development, but the value need not be zero. In fact, hurricanes are able to persevere as long as vertical wind shear is less than 10 meters per second (22.3 mph). Vertical wind shear of a magnitude greater than 10 meters per second suppresses hurricane development by disrupting the storm's convective forces and altering its overall structure.

The chart below (Figure 3), produced at 8am on October 17, 2005, displays those regions (in light blue) where vertical wind shear was light, and therefore supportive of Tropical Storm Wilma's development into a hurricane.

Meteorologists also have resources that allow them to analyze past trends in vertical wind shear and to predict future conditions. There are a number of sophisticated computer models that use existing atmospheric conditions as input to render a prediction of future weather conditions. In many instances, the data from these programs are used to create graphics, commonly called progs, that visually display the state of the atmosphere at a future time. A variety of the organizations make these resources available on the Internet. For example, the prog below (Figure 4) was produced from the Canadian Meterological Centre (CMC) model and was obtained from the Penn State Tropical Ewall.

his particular panel, one of four available, was produced at 8pm on October 15, 2005 and was "valid" twenty-four hours later at 8pm on the 16th. It provides a prediction of the nature of vertical wind shear by using shading and an indication of its direction by the use of streamlines.

The white shading in the region of Tropical Depression Wilma represents minimal vertical wind shear -- a conducive environment for Wilma's further development. This region is in stark contrast to the substantial westerly shear, on the order of 22 meters per second (49.3 mph), that was predicted along the southern Gulf Coast and well into the Gulf of Mexico. A tropical depression wouldn't stand a chance of reaching hurricane status in such an environment.

A note of caution--no single prog should be taken as gospel. The atmosphere is remarkably complex, and no single computer model derives a correct forecast each time it is initialized. It is not uncommon for the same computer model to forecast dramatically conditions just a few hours later. Meteorologists regularly review output from an assortment of models in an effort to arrive at a consensus forecast that is consistent with their human assessment of current conditions.

Coriolis Force
Although warm water is found near the Equator, it is very rare for tropical cyclones to develop between five degrees south and five degrees north latitude, or within approximately 300 nautical miles (nm) of the Equator. The reason is the virtual absence of the Coriolis force at such low latitudes. Only two tropical cyclones have formed within this region since observations began; Typhoon Sarah in 1956 that formed near 3.3 degrees north latitude and Typhoon Vamei which formed at approximately 1.5 degrees north latitude in December 2001.

The Coriolis force is an apparent force that causes the deflection of moving objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The graphic to the right demonstrates the relationship between moving air parcels and an area of low pressure in the Northern Hemisphere. The Coriolis force is essentially zero at the Equator and increases as you travel north and south toward the poles. The deflection resulting from the Coriolis force also increases as the strength of the wind increases. As it relates to tropical systems, the Coriolis force is responsible for promoting cyclonic (counter-clockwise) rotation.