UD climate expert talks about hurricanes

This satellite image of Hurricane Katrina was taken at 11:45 a.m. EDT, Aug. 28, as the powerful storm churned in the Gulf of Mexico as a Category Five storm with sustained winds near 175 mph, a day before the storm made landfall on the U.S. Gulf Coast. Photo courtesty of the National Oceanic and Atmospheric Administration

5:01 p.m., Sept. 23, 2005--With New Orleans still in ruins and hundreds of thousands of residents displaced in the aftermath of Hurricane Katrina just four weeks ago, the approach of Hurricane Rita has caused the evacuation of nearly 2 million persons along the Texas and Louisiana Gulf Coast.

David Legates, state climatologist and associate professor of geography at UD, said two powerful hurricanes rarely occur so close together. However, “the Gulf of Mexico is exceptionally warm now, and since hurricanes are latent heat engines--deriving their energy from condensing moisture--the extremely warm waters are fuel for any storm that enters the Gulf,” he said.

Legates, who is co-coordinator of the Delaware Geographic Alliance, is among experts being hosted by the National Geographic's Education Network in a monthlong series of interviews on hurricanes and geography. He answered some of the most common questions about hurricanes for UDaily.

What causes hurricanes, and is there any connection to global warming?

Hurricanes are naturally produced as tropical disturbances that grow as they gain fuel from warm ocean waters and from their poleward movement. Looking at long-term records, we average about 90 storms a year globally, and there has been no long-term trend in that figure. Right now, the North Atlantic Basin is on a 30-year cycle of enhanced activity while the activity in the western North Pacific has decreased. El Nino/Southern Oscillation (ENSO) events also affect hurricane activity--with El Nino years leading to decreased Atlantic activity and increased Pacific activity. There are many things that affect hurricane frequencies and intensities; global warming doesn't appear to be one of them, however.

What are the worst hurricanes, and what made them so?

We have had a number of Category 4 and 5 storms--sustained winds above 130 mph--over the years. In the Atlantic, Hurricane Camille (1969), Hurricane Andrew (1992) and the Labor Day Hurricane of 1935 are the only three Category 5 storms to hit land before this year. Warm Gulf and/or Caribbean waters, an avoidance of land during their development and a lack of upper-level wind shear are three ingredients necessary for building the strongest storms.

What areas are most hurricane-prone and why?

Coastal areas, particularly on the southeastern side of a continent, and their associated offshore islands. Hurricanes move from east to west in the tropics following the tropical easterly winds and then swing back to the east as they move into mid-latitudes due to westerly winds in mid-latitudes. This produces the traditional C-shaped signature that we see. Thus, the southeastern part of a continent is the most likely to be hit.

Is there any risk of hurricanes ever striking Delaware, Maryland or New Jersey?

Due to the geography of the Atlantic Coast, it is difficult to get a storm that misses Cape Hatteras, but then turns west to hit this region. Difficult, but not impossible. Often, the remnants of a hurricane can be devastating. Hurricane Danny in 1997 hit Mobile, Ala., doing minimal damage. But, its remnants, with winds of only 15 mph but still with visible circulation, stalled over Charlotte, N.C., producing up to 15 inches of precipitation. Most hurricane deaths come from flooding, not from the storm surge or the high winds.

Delaware has had a number of hurricane remnants that have had significant impact. Recently, Glenville was flooded by heavy rain from Hurricane Henri's remnant, while old-timers will remember the effects of Hurricanes Agnes and Hazel. Although not hurricanes when they arrived, damage still occurred.

More significant to this region is the nor'easter. It is a winter storm that is formed in mid-latitudes, not the tropics, but can have wind speeds that exceed hurricane force. They usually occur from November through early spring.

What causes the most damage during a hurricane, wind, flooding or flying debris?

Usually, the storm surge damage is limited to structures close to the coast. Wind damage, particularly accentuated by flying debris, is most responsible over much of the affected area. But today, most of the deaths come from flooding due to heavy rains, usually inland and well away from the landfall region.

Article by Martin Mbugua
Photo by Kathy F. Atkinson

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