B. Science

-Definition: A hurricane is a severe tropical cyclone in which maximum wind speed exceeds 65 knots (74 mph) and has a size on the order of 500 kilometers (300 miles) in diameter.

Origin: hurricanes start as tropical disturbances originated in association with easterly waves in the tropical oceans including North Atlantic Ocean, Caribbean Sea, Gulf of Mexico and in the eastern North Pacific off the west coast of Mexico. Similar storms called Typhoon originate in western Pacific and most of the south Pacific. Conditions favorable for hurricane development include deep oceanic surface layer of temperatures above 27°C (81°F) in the latitudes between 5°-20°, unstable atmosphere throughout the troposphere, and weak tropospheric winds. These conditions provide enough latent heat that is transported upward from the surface and Coriolis force for strong cyclonic rotation. As long as these conditions continue, storm winds can increase, fueled by the release of latent heat. With winds of at least 20 knots (23 mph), the developing tropical cyclone is called a tropical depression. If winds reach 35 knots (40 mph), the cyclone is called a tropical storm and is given a name, i.e., Andrew. See resources for list of hurricane names. When winds reach 65 knots (74 mph), it becomes a hurricane.

The strength of a tropical cyclone is determined by its maximum wind speeds and not necessarily by its size. Therefore, a classification was adopted to measure the hurricane strength in terms of wind velocities, the Saffir-Simpson Scale (see below). Since pressure gradients are responsible for wind speeds, the lower the central surface air pressure, the stronger the winds. Super Typhoon Tip on October 11, 1979, had a record low sea level pressure of 870 millibars and winds gusting to 190 kts. (over 230 mph).

Hurricane is categorized into 5 category based on the Saffir-Simpson Scale.

Decay: At the surface, hurricanes can diminish rather quickly given the right conditions. These conditions include: (1) the storm moving over cooler water that can't supply warm, moist tropical air; (2) the storm moving over land, again cutting of the source of warm, moist air; and finally (3) moving into an area where the large-scale flow aloft is not favorable for continued development or sustainment. For example, during El Nino, a strong subtropical jet crosses over the Caribbean, which shears off the tops of developing tropical cyclones. Most El Nino years, there are few hurricanes forming in the North Atlantic. For La Nina years, the subtropical jet is weak and therefore more hurricanes usually occur in the North Atlantic.

While most hurricanes dissipate quickly once they cross land, there are exceptions. Hurricane Andrew crossed southern Florida in 4 hours, weakening only 1 category, from 4 to 3, thus still potent enough to cause the costliest weather disaster in U.S. history (see mitigation). Hurricane Hazel, in 1954, crossed the Gulf coast and moved north as far as Toronto, Canada, where it deluged the city with rains, killing 81, while still maintaining nearly hurricane strength winds.

Structure of Hurricanes: Hurricane has an eye (clear sky) in the middle of strong counter-clockwise rotating and uprising air (eye walls). The eye size ranges from 20-50km (15 to 30 mi). The eye results from the rapid rotation of inflowing surface air that rises and spins away from the storm’s center. To replace this outward and upward flow, air from above descends in the central area. The subsidence of this drier air causes warming and evaporates clouds, so that the eye region may be free of clouds.

Hurricane season:

Over the Atlantic Ocean, the hurricane season is from June to November with maximum in early to middle September. A Caribbean nursery rhyme goes:

June, too soon

July, winds will fly

August, come they must

September, remember

October, all over.

The Northeast Pacific basin has a broader peak with activity beginning in late May or early June and going until late October or early November with a peak in storminess in late August/early September.

The Northwest Pacific basin has tropical cyclones occurring all year round regularly though there is a distinct minimum in February and the first half of March. The main season goes from July to November with a peak in late August/early September.

The North Indian basin has a double peak of activity in May and November though tropical cyclones are seen from April to December. The severe cyclonic storms (>33 m/s winds [76 mph]) occur almost exclusively from April to June and late September to early December.

The Southwest Indian and Australian/Southeast Indian basins have very similar annual cycles with tropical cyclones beginning in late October/early November, reaching a double peak in activity - one in mid-January and one in mid-February to early March, and then ending in May. The Australian/Southeast Indian basin February lull in activity is a bit more pronounced than the Southwest Indian basin's lull.

The Australian/Southwest Pacific basin begin with tropical cyclone activity in late October/early November, reaches a single peak in late February/early March, and then fades out in early May.

Globally, September is the most active month and May is the least active month. (Neumann 1993)

Long Range Forecasting of Atlantic Hurricane Activity.

Since hurricane development depends on the right combination of atmospheric and oceanic conditions, scientists have studied what factors should be looked at to predict whether conditions are favorable or not. Several recent studies have shown that ocean and atmosphere conditions months before the hurricane season can be used to make some judgement about whether it will be an active season or not. For example, El Nino conditions in the Pacific are followed by less hurricane activity, while La Nina years usually produce more than average hurricanes.

To look at the government’s forecast for this summer’s hurricane season, see: http://www.cpc.noaa.gov/products/outlooks/hurricane.html

For Prof. Gray’s 2003 N. Atlantic hurricane season prediction, see: http://Hurricane.Atmos.ColoState.Edu/Forecasts/2003/april2003/

A recent increase in Atlantic hurricane activity was reported in Science magazine (July 20, 2001). Supplemental material from the article are posted at:

http://www.sciencemag.org/cgi/content/full/293/5529/474/DCI along with an abstract.