Hurricanes have become a nearly-constant presence in the news over the last month, with 5 developing across the Gulf of Mexico, Caribbean, and Atlantic Ocean since the middle of August. Unfortunately, this recent spike in tropical activity has also brought devastation to Texas, Florida, Puerto Rico, and many other islands nations of the Caribbean.
After a few months of relative quiet in the tropics, why have things suddenly taken a more active turn?
What is a hurricane?
While we may hear plenty about them when they threaten land in or near the United States, hurricanes are not something we often need to concern ourselves with here in Michigan. Being located so far north, and so far inland from any ocean, it is very difficult for these systems to bring little more than leftover rain to our state.
Like all tropical cyclones, hurricanes develop exclusively in the tropical and subtropical regions of our planet, generally between 23.5°N and 23.5°S latitude on either side of the equator. Though these storms are most dangerous and destructive as hurricanes, this isn't the first step in how we classify them.
When it comes to tropical systems that threaten the United States, it all starts with a small area of low pressure over the tropical waters of the Atlantic Ocean, Caribbean Sea, or even the Gulf of Mexico. As the low becomes better organized, thunderstorm activity increases and the system's rotation intensifies. The increasing rotation around the center of low pressure stirs up gradually stronger and stronger winds.
When the system becomes sufficiently organized, it is classified as a tropical depression as long as sustained winds around the center remain at or below 38 mph. As further intensification occurs, the system becomes a tropical storm when sustained winds range between 39-73 mph. It is at this stage that the system is given a name to distinguish it from other potential tropical cyclones.
Once sustained winds around the center of a tropical storm reach 74 mph, the system officially becomes a hurricane and is then categorized according to the Saffir-Simpson Scale based on its maximum sustained winds. See the included graphic for a full breakdown of the categories.
What does a hurricane need to thrive?
In simple terms, hurricanes are basically enormous heat engines. Their primary purpose is to transport heat from the tropical regions of our planet to the cooler regions closer to the poles. They need access to large sources of heat and moisture in order to develop and intensify, and the warm waters of the tropics are an excellent source of just that.
In order to provide enough fuel to support or strengthen a hurricane, ocean waters must be at or above 80 degrees. Water this warm tends to evaporate more easily, pumping more moisture into the air immediately above the water's surface.
In a manner of speaking, hurricanes feed on this evaporated moisture as it is pulled into the storm's circulation and fuels thunderstorms. The warmer the water is, the more fuel is available to support and intensify the hurricane. By contrast, the storm will weaken if the center moves over a land mass and is cut off from it moisture source
Warm water alone isn't necessarily enough to sustain a hurricane, however. These storms are remarkably timid, and don't like a whole lot of resistance in their path. Hurricanes tend to do best in environments where there is little wind shear both at the surface and in the upper levels of the atmosphere.
In other words, hurricanes don't do well with winds that either oppose or cut across their environment. A symmetrical hurricane is a healthy hurricane.
External winds cutting into or across the storm can interfere with its circulation, making it difficult for its central core of thunderstorms to remain wrapped around the eye. As a result, the circulation slows down and the storm weakens. Wind shear can also inject dry air into the storm, and even expose the center of circulation entirely. Both factors can spell the demise of the system if it is unable to escape the shear and re-establish an enclosed eye.
Why the increase in hurricane activity lately?
Hurricane activity in the Atlantic basin typically peaks in mid-September, so the recent surge in tropical systems isn't unusual in and of itself. By this time of year, tropical waters have had several months of strong Summer sunlight to warm them. Wind shear also tends to be less of a factor in late-Summer before the atmosphere begins to cool and the jet stream dips south.
What is unusual about the recent tropical activity is the sheer strength of the developing hurricanes. It is quite rare to have even a single Category 5 hurricane develop in a given season, with two to our credit in just the past 3 weeks. Also atypical is the fact that these immensely powerful storms are tracking over similar areas, and in such a short time.
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