Hurricane Michael rammed into the Florida Panhandle Wednesday, October 10th, causing significant damage to property, public safety, and life. With winds reaching 155 mph, Michael was just 2 mph shy of a Category 5 hurricane upon landfall and is the strongest storm to impact the continental U.S. since 1992.  Hurricane Michael also had the third lowest recorded pressure of all hurricanes making landfall on U.S. soil. Michael was the first Category 4 storm to ever make landfall on Florida’s panhandle as well as being the strongest storm to ever impact the United States during the month of October, making it one for the record books.

NOAA Geostationary Operational Environmental Satellite16(GOES) East CONUS GeoColor image of Hurricane Michael.

As another major hurricane (Michael) impacts the United States, we wanted to take a moment to dissect some of the confusion behind tropical cyclone terminology, hurricane formation, and (some light) science behind the cyclones.

Often, people ask: “What’s the difference between a cyclone and a hurricane?” Well, technically, all hurricanes start out as “tropical cyclones.” In fact, depending on where it forms and ultimately travels, a tropical cyclone may change its title several times.

Hurricane Formation Stages (source).

Step-By-Step Hurricane Formation 

Initially, storms that form over warm ocean water are coined “tropical disturbances,” which are unorganized weather systems that consist of thunderstorms and low pressure. Often, the hurricanes that impact the United States begin their lives as tropical disturbances that move off the African Coast.

If the tropical disturbance is located in a favorable area with warm water, high moisture, low wind shear (change of wind speed or direction with height in the atmosphere), and a decent distance from the equator, then it may mature and organize into a “tropical depression.” The word “depression” is used to describe this stage because the tropical depression has very low (depressed) air pressure at its center. The area of low pressure allows warm, moist air to rise and, ultimately, cool and condense, creating thick layers of clouds and precipitation. These rainbands make up the largest, most identifiable part of the storm. Throughout this process, the system rotates counterclockwise in the northern hemisphere (a result of the Coriolis Force/ earth’s rotation).

Once the tropical depression has maximum sustained winds of 39 mph, it becomes a “tropical storm.” When a system reaches tropical storm stage, it is given a name! As the storm strengthens further and rotates faster an eye will form at the center. In this center portion of the storm, air sinks, keeping the eye cloud-free and free of precipitation.

The (now named) tropical storm will continue to strengthen and spin (given a favorable environment/ conditions) until it has maximum sustained winds of 74 mph, where it will be classified as a “hurricane” (North Atlantic, central North Pacific, eastern North Pacific), “typhoon” (Northwest Pacific) or “cyclone” (South Pacific and Indian Ocean).

Saffir- Simpson Hurricane Wind Scale (source).

Hurricane Categories: An Imperfect System

Narrowing our focus to the United States, once a storm reaches hurricane stage, it will be classified based upon its intensity, using the Saffir-Simpson Hurricane Wind Scale.

Upon landfall, hurricane damage can depend on factors other than wind speed, like the rate at which the hurricane travels, overall size/ extent of the hurricane’s rainbands, or even where the hurricane makes landfall. Hurricanes bring (what would seem to be the obvious) hazards like strong winds (often, severe wind gusts are confined to a relatively small area on the northeastern part of the storm near the coast), storm surge, and intense flooding, but can also act as the catalyst for other dangerous weather phenomena. It is not uncommon for a hurricane to produce lightning and even damaging tornadoes.

Unfortunately, some of the most costly and deadly hurricanes are well short of being Cat 4 or 5 monsters. With hurricane season in full swing (spanning from June 1- November 30), Hurricane Michael may not be the final storm of the season for the United States.

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