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TROPICAL CLIMATES AND HURRICANE INGREDIENTS

METEOROLOGIST JEFF HABY

NATIONAL HURRICANE CENTER

The weather in the tropics has many differences from mid-latitude weather. This section will summarize these differences.

THE GEOSTROPHIC WIND:

Because the pressure gradient and Coriolis forces are weaker in the tropics, the wind deviates much more from geostrophic than in the mid-latitudes. Streamlines are used to analyze the wind flow in the tropics instead of isobars and height contours. The wind direction is more constant in the tropics. There are no fronts to cause sudden changes in wind speed and direction. Forecast models are unable to use the geostrophic approximation in tropical environments. The Coriolis force is a minimum at the equator. Therefore, tropical storms do not develop within 5 of the equator. There is no earth vorticity to initiate tropical systems close to the equator.

INVERTED TROUGHS:

Troughs in the tropics have an opposite tilt to those in the mid-latitudes. Mid-latitude troughs dig to the south while tropical trough amplify to the north. This is due to tropical troughs being generally south of high pressure. They also propagate in the opposite direction from mid-latitude troughs. Tropical troughs generally follow the prevailing easterlies.

LACK OF FRONTS:

Tropical environments have a lack of fronts. Weather is fairly uniform throughout the year as far as temperature goes. At low elevations, temperature is a primary function of cloud cover. During the rainy season, temperatures will tend to be a little cooler. In places where high pressure dominates year round, temperatures only change slightly. This slight change is due to the earth's tilt. The earth's tilt causes precipitation patterns to change throughout the year. In the Northern Hemisphere summer, the ITCZ moves further north and hurricanes become prevalent.

PREVAILING EASTERLIES:

Weather systems move from east to west in the tropics. Hurricanes and thunderstorm complexes generally drift toward the west. There are exceptions to this rule, especially as tropical systems move into higher latitudes. In these cases they will be picked up by the westerlies.

HURRICANE DEVELOPMENT

There are six widely accepted conditions for hurricane development. The first condition is that ocean waters must be above 26 degrees Celsius (79 degrees Fahrenheit). Below this threshold temperature, hurricanes will not form or will weaken rapidly once they move over water below this threshold. Ocean temperatures in the tropical East Pacific and the tropical Atlantic routinely surpass this threshold.

The second ingredient is distance from the equator. Without the spin of the earth and the resulting Corioles force, hurricanes would not form. Since the Corioles force is at a maximum at the poles and a minimum at the equator, hurricanes can not form within 5 degrees latitude of the equator. The Corioles force generates a counterclockwise spin to low pressure in the Northern Hemisphere and a clockwise spin to low pressure in the Southern Hemisphere.

The third ingredient is that of a saturated lapse rate gradient near the center of rotation of the storm. A saturated lapse rate insures latent heat will be released at a maximum rate. Hurricanes are warm core storms. The heat hurricanes generate is from the condensation of water vapor as it convectively rises around the eye wall. The lapse rate must be unstable around the eyewall to insure rising parcels of air will continue to rise and condense water vapor.

The fourth and one of the most important ingredients is that of a low vertical wind shear, especially in the upper level of the atmosphere. Wind shear is a change in wind speed with height. Strong upper level winds destroy the storms structure by displacing the warm temperatures above the eye and limiting the vertical accent of air parcels. Hurricanes will not form when the upper level winds are too strong.

The fifth ingredient is high relative humidity values from the surface to the mid levels of the atmosphere. Dry air in the mid levels of the atmosphere impedes hurricane development in two ways. First, dry air causes evaporation of liquid water. Since evaporation is a cooling process, it reduces the warm core structure of the hurricane and limits vertical development of convection. Second, dry air in the mid levels can create what is known as a trade wind inversion. This inversion is similar to sinking air in a high pressure system. The trade wind inversion produces a layer of warm temperatures and dryness in the mid levels of the atmosphere due to the sinking and adiabatic warming of the mid level air. This inhibits deep convection and produces a stable lapse rate.

The sixth ingredient is that of a tropical wave. Often hurricanes in the Atlantic begin as a thunderstorm complex that moves off the coast of Africa. It becomes what is known as a midtropospheric wave. If this wave encounters favorable conditions such as stated in the first five ingredients, it will amplify and evolve into a tropical storm or hurricane. Hurricanes in the East Pacific can develop by a midtropospheric wave or by what is known as a monsoonal trough.

SAFFIR-SIMPSON SCALE

CAT Pressure min (mb) Max wind (mph) Storm Surge (m) PD
1 >979 74 to 95 1 to 2 1
2 965 to 979 96 to 110 2 to 2.5 4
3 945 to 964 111 to 130 2.5 to 4 9
4 920 to 944 131 to 155 4 to 5.5 16
5 <920 155+ >5.5 25

**PD= Potential Damage (CAT 5 is at least 25 times more destructive than a CAT 1)