National Geographic : 2005 Aug
SURGING INTENSITY Sunshine illuminates the aftermath of Ivan in Orange Beach, Alabama. Forecasting when hurricanes will gain or lose strength is a challenge, but a new sat ellite imaging technology allows researchers to monitor rainfall inside storms to find "hot towers"-clouds that rise high above the ocean and release heat into the atmosphere, pow ering and intensifying the storm. Frances's hot towers (red, opposite) rose more than ten miles at Category Three on August 30, 2004, as the storm passed northeast of the Leeward and Virgin Islands. A day later it reached Category Four, with sustained winds of 131 mph and higher. circulation probably increased in that period in response." Increased circulation brings mighty storms born as air spirals into a low-pressure zone charged with warm, humid air over warmer sea surfaces. The winds meet and ascend, causing clouds to billow upward, further lowering air pressure and causing winds to barrel even faster toward the center. The Earth's rotation lends spin to the gathering cyclone. When water vapor in the ascending clouds cools and falls as rain, the amount of heat energy released dwarfs the amount of electricity consumed daily by all of humanity. The energy warms the eye, further low ering the pressure and strengthening the storm. The cyclone can continue to strengthen if 80 NATIONAL GEOGRAPHIC * AUGUST 2005 atmospheric currents guide it over warm water, and if it is not destroyed by vertical wind shear the differential between wind speeds at lower and higher altitudes. Strong wind shear can dissipate a storm, but the warm phase of the AMO tends to weaken vertical wind shear in the Atlantic. The combined effect of changes in the AMO and the Atlantic conveyor belt has been dra matic. In the Caribbean, production of cyclones skyrocketed 400 percent. In the entire Atlantic Basin, major hurricanes, with sustained winds of 111 miles an hour or higher, increased 150 percent. The intensifying is most pronounced in powerful storms like Ivan, whose winds at times exceeded 155 miles an hour as it smashed past Jamaica and headed for landfall near Pensacola.