Figuring Out Hurricanes

Hurricanes, also known as typhoons in the western Pacific and tropical cyclones in the Indian Ocean, were giant, deadly, and unpredictable weather systems until the middle of the 20th century. In large part this was because they were difficult to study. Forming over equatorial waters out of sight of land, unless a ship happened to be unlucky enough to travel underneath one or the hurricane ran over an island in its path, they could be born, develop, and die without anyone ever knowing of their existence. Without satellites watching from space, they could literally move ashore with high winds, accompanying tornadoes, and the wall of water known as the storm surge with absolutely no warning. Thousands of people living in coastal areas died when these monsters landed. Outrage sparked by deaths caused by large hurricanes prompted changes in hurricane forecasting and encouraged additional research into their structure and behavior.

By the mid-193 0s, meteorologists agreed that well-developed hurricanes extended vertically through the entire troposphere and were maintained by the release of latent heat caused by convection near the storm center. By the early 1940s, they had developed a fairly accurate description of the life cycle of a hurricane. In the first stage of a tropical system, the storm became organized (exactly what triggered this event was unclear). If conditions favored development, the system would intensify as the barometric pressure plummeted and the winds increased in the second stage. In the third stage, the winds did not increase appreciably, but the storm itself expanded to cover a larger geographic area. In the last stage, the system would dissipate or fall apart.

Early ideas on hurricane genesis had centered on a tropical comparison to Jacob Bjerknes's polar front theory, but data obtained from additional observations eliminated this possibility. By early in this decade, researchers had shown that the most likely trigger for these storms was the presence of a "wave" or perturbation in the easterly flow of the atmosphere moving off the African coast.

Efforts to determine hurricane structure were aided by the availability and use of upper-air observations. The number of pilot balloon stations and the frequency of observations had increased rapidly in the Caribbean during the 1930s, primarily through the efforts of Pan American Airways. Radiosonde stations had also been added. As a result, the flow patterns of upper-level winds were routinely plotted and found to be more useful than surface reports for determining hurricane movement.

Although the upper-air observations had provided new insight into hurricanes during the late 1930s and very early 1940s, a significant advance was made in July 1943, when the U.S. Army Air Corps' Colonel Joseph P. Duckworth (1902-64) became the first person to fly into a hurricane's eye intentionally. Regular missions to hunt down and penetrate hurricanes began in 1944, with army and navy pilots taking part. Their purpose, of course, was not so much to accurately forecast where hurricanes would go ashore, as to keep ships out of their way. Early in the war, the navy had suffered massive damage to its seagoing assets during

Pacific typhoons. Weather reconnaissance flights allowed navy personnel to locate and determine the intensity of these storms. It is impossible to forecast a hurricane's movement without knowing its position and strength. Aircraft provided the fastest method of finding out. Although expensive and dangerous, the process was less expensive than losing ships at sea in high winds and waves.

Radar also provided the first look at complicated hurricane structure. Meteorologists using radar combined with aircraft observations discovered that the upward vertical motion in hurricanes resided in the eyewall—the thick, doughnut-shaped circle of convective clouds that surrounds the clear eye in the storm's center. Additional data also over-

The introduction of sophisticated instruments on hurricane-penetrating aircraft allowed scientists to determine how air moves within tropical storms.

^ Eyewall 1 Eyewall ^

~ A Y A Subsiding

^ II Latent heat is released i< 1 Eye as water vapor rises 1 ^ B and cools J

pr p

^^^ Vapor-filled air

Warm seawater

© infobase Publishing Approximately 300-1,200 miles (482-2,028 km) wide

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