Randy Mann: The birth of an Atlantic hurricane

Over the weekend, Hurricane Irene moved up the East Coast, bringing strong winds, heavy rainfall, property damage and loss of life from the Carolinas into New England. Although the storm weakened to a Category 1 by the time it reached the mid-Atlantic region, Irene caused widespread damage that will likely be in the billions of dollars.

My physical geography class students and others have asked me how hurricanes are born in the Atlantic and Caribbean waters.

Like most Atlantic hurricanes, Irene was born just off the coast of West Africa as a tropical wave of thunderstorms. In this part of the world, hot, dry air usually flows westward from North Africa’s Sahara Desert toward the Atlantic Ocean. This atmospheric imbalance creates a kink in the wind patterns near the Canary Islands. Sparked by the increased friction of these widely opposing air masses, strong thunderstorms are ignited, eventually developing into a series of tropical waves. If ocean temperatures are warm enough, combined with the effects of the Earth’s rotation, these systems become tropical storms and later sometimes hurricanes.

As seen from satellite images, a hurricane appears to be a huge spiraling mass of clouds converging around a small area called an “eye” that is usually entirely free of clouds. Many people have been lured from protective shelters when the eye passed overhead, thinking that the worst was over, only to be caught by violent winds often exceeding 120 mph on the back side of the hurricane. That part of the hurricane that is near the eye is usually the most intense part of the storm.

Once a hurricane hits land or encounters cooler waters in the northern latitudes, it usually loses much of its intensity. The storm often weakens fairly rapidly when its supply of moisture from the Gulf of Mexico or Atlantic Ocean is cut off.

Our active 2011 tropical storm and hurricane season should produce another eight to 12 named storms before it ends on Nov. 30.

Our local weather has been dry and hot. This current prolonged dry spell may continue until late September or early October, especially if the strong stationary high pressure ridge centered over the Inland Northwest doesn’t break apart anytime soon. We’ll likely see some showers over the next four to six weeks, and perhaps a thunderstorm or two, but the overall weather pattern will be drier and warmer than normal into the early fall season. The dry conditions also mean high fire danger levels.