Hurricanes From Allan to Zelda

By Dave Esser

Knowledge of weather is vital to conducting safe flight operations, and the more a pilot knows about the workings of weather, the better. This article concludes the year-long series of articles by Dave Esser explaining weather phenomenon. Woman Pilot has been honored to bring you his interesting and informative explanations. We hope to continue bringing you more of his writing in future issues. Dave is a professor of aeronautical science at Embry-Riddle Aeronautical University. He can be contacted by email at esserd@cts.db.erau.edu, or by phone at 904-226-6987 if you have an aviation-related question or topic you would like addressed.hurican2.gif

Over the past century, hurricane related deaths have decreased drastically as tracking methods have improved. Unfortunately, hurricane related property damage has increased dramatically as communities developed in desirable but potentially dangerous coastal areas. The weather satellite is the greatest tool for predicting hurricanes. Before the 1960s the only way to monitor hurricane movement at sea was to listen to radio reports from unfortunate ships that had sailed into them. How do these fascinating and deadly storms form and how is their movement forecast?

Warm ocean water provides the energy source for a hurricane. This is why hurricanes form in the summer months in the warmth of the Gulf of Mexico and the southeastern Atlantic ocean. The first stage of a hurricane is a tropical wave. The continent of Africa creates thermal low-pressure areas, spawned from high temperatures in the inter-tropical convergence zone around the equator. As this disturbance moves eastward into the south Atlantic, it is fed by evaporating warm water. As the evaporating water adds heat to the lower atmospheric levels, the higher temperatures decrease air density. The lower density air then rises and, in turn, the rising air pattern lowers the surface pressure. In areas north of the equator, the Coriolis effect results in faster counter-clockwise wind flow as surface pressure drops. If the conditions are right, the area deepens (the pressure decreases) until the isobars (lines of equal pressure) form a closed pattern around the disturbance, which is then called a tropical depression. Storm intensity is categorized by the maximum sustained winds in the depression. When the upward-circulating air reaches the tropopause, some of the airflow will vent into the upper levels of the atmosphere, and some will be recirculated downward in the “eye” of the storm. The eye is created by the downward airflow stifling cloud development, and is also the place where the strongest winds are found.

As the hurricane’s eye passes over a particular location, the wind speed will drop and the sky will clear. This lull, however, is temporary. The wind, just as strong but in the opposite direction, will soon return as the other side of the eye wall moves over the location.

hurican1.gifWhen sustained wind speeds in a tropical depression reach 39 mph, the tropical storm threshold is reached and the storm is given a name. By international agreement, Atlantic storms are named alphabetically using names of both genders. Tropical storm names can be reused in later seasons, unless a storm has caused a significant number of casualties. In those cases the storm name is retired. Storms in the Pacific follow the same system, but use different names than those in the Atlantic. A hurricane that forms in the Asian Pacific, across the international  date line, is called a typhoon.

 When sustained wind speeds reach 74 mph, a stage 1 hurricane is born. The highest category in the Saffir-Simpson Hurricane Intensity scale is stage 5, when top wind speeds reach 155 mph. Three things will inhibit a hurricane: strong upper level winds, rough terrain, and cold water. Strong upper level winds will shear or tear off the top of the storm destroying the organized circulation. If the storm passes over land, the friction of the topography can interfere with the wind pattern. As a storm moves north into colder temperatures, the warm water energy source is shut off and the storm will dissipate.

Hurricane observations from satellites are augmented by wind speed and pressure measurements from aircraft flying through the storm. In an average year the Atlantic may have 12 named tropical storms. Nine of these may form hurricanes, six of which may become strong hurricanes. Global warming and particularly wet African rainy seasons can increase the number of storms. A strong El Nino year can reduce the frequency of Atlantic storms; a La Nina year can have the opposite effect. As forecasters sharpen their ability to predict hurricanes, more lives will be saved. Unfortunately, property damage is likely to increase if coastal communities continue to develop at present rates.

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Chicago, Illinois

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