Mandatory evacuations were ordered for parts of Florida as forecasters said Hurricane Dorian would head just off the U.S. coast, CBS News reported early Monday morning. The impact and strength of the storm will play out throughout the week. In the meantime, brush up on hurricanes.
According to the CBS article, even if Dorian remains just off the U.S. coast, Florida would experience “major problems with high winds, a devastating storm surge and heavy rain.” As always, inclement weather is rarely predictable. Thunderstorms, hail, nor’easters, tornadoes, and hurricanes rear up, die down, deviate from predicted patterns, and can stump even the best meteorologists. Both typhoons and hurricanes have the same weather system, called tropical cyclones, of a rotating, organized system of clouds and thunderstorms with a closed, low-level circulation that originates over tropical or subtropical waters. The weakest tropical cyclones are called tropical depressions, intensifying to tropical storms, then classified as hurricanes once they reach maximum sustained winds of 74 mph. Typically, the six-month period from June 1 to November 30 is hurricane season for the Atlantic region.
The Saffir-Simpson Scale and Hurricane Categories
Hurricanes are often described in ominous tones as “Category 3,” “Category 4,” and so on, but these numbers bear little context for many people. As it turns out, the categories are based on a model called the Saffir-Simpson Scale.
“Herbert Saffir was an engineer and Bob Simpson was the director of the National Hurricane Center,” said Professor Eric R. Snodgrass, Director of Undergraduate Studies for the Department of Atmospheric Sciences at the University of Illinois at Urbana-Champaign. “These two teamed up to develop a scale that used a maximum sustained winds speed to measure the intensity of a hurricane. In the development of their scale, they used one-minute averaged wind speeds as the metric by which hurricane wind intensity could be categorized—average sustained wind speed was used instead of wind gusts because the damage caused by the wind is proportional to the square of the sustained wind.”
Professor Snodgrass broke down the Saffir-Simpson Scale by category as follows, using the hurricane’s minimum production of sustained winds as a metric.
Category 1: 73 miles per hour
Category 3: 110 mph
Category 4: 130 mph
Category 5: 155 mph
The National Hurricane Center clarified that a Category 2 hurricane produces 96 mph winds.
Staying Informed in Case of Irregular Hurricanes
Professor Snodgrass pointed out that, in general, hurricane tracks follow a specific pattern in the Northern Hemisphere. “These weather systems tend to start in the low altitudes, move to the west, and then curve to the north in the Northern Hemisphere,” he said. “Many times, this curved path causes the tropical cyclone to hit land in the Caribbean, Central America, the Gulf Coast, and the East Coast of the United States. However, occasionally these stream winds are very weak, allowing a hurricane to follow its own path.”
Citing Hurricane Kyle in 2002 and Hurricane Ivan in 2004, Professor Snodgrass provided ample evidence on the difficulty—yet importance—of staying up-to-date on developing hurricane formations. Kyle circled back around the Atlantic Ocean near the East Coast three times before hitting the Carolinas. Ivan was even less predictable.
“At the beginning, Ivan looked very typical by tracking west at first and then veering to the north,” he said. “After hitting the U.S. near Mobile Bay, Ivan tracked through the southern Appalachian Mountains and then exited the United States, near Delaware. Then in a very atypical way, Ivan tracked southward—turning west—and hit Florida. Then Ivan moved across the Gulf and hit the United States for a third time near Texas and Louisiana.”
Dorian is the latest in an unending list of hurricanes to threaten the southern United States. In light of its status as a Category 5 hurricane and the worst-case projections made by national weather services, The Great Courses Daily urges those living in Dorian’s possible affected areas to be safe and take appropriate precautions until it has passed.
Professor Eric R. Snodgrass contributed to this article. Professor Snodgrass is the Director of Undergraduate Studies for the Department of Atmospheric Sciences at the University of Illinois at Urbana-Champaign, where he also received his master’s degree. Previously, he earned his bachelor’s degree in Geography from Western Illinois University.