As hurricane season 2010 begins – a look back at past storms

It is now 2010, the 50th Anniversary of the infamous Hurricane Donna which, in the fall of 1960, made landfall in Collier County, literally changing overnight the history of Southwest Florida. As we begin to get ready for hurricane season, it is important to examine not only exactly what a hurricane is, but also to review the impact of several of these historical storms on our area’s geography, and the effects on our local economy and residents.

The English word, “hurricane” comes from Spanish. In 1555, the book Historia General y Natural de las Indias first mentions the word Huracán to describe the strange new storms the Spanish conquistadors were encountering in the Caribbean. The Spanish adopted the word from the ancient Central American God or Wind Deity known as “Hurakan,” who the Mayans believe repeated the word “earth” several times until the land rose up from the depths and blew a great wind over the ocean. Later, when the Gods became angry with humans, it was Hurakan whom the Mayans credited with causing a great flood to destroy the earth. Hurakan is always depicted as having one leg with the other being a snake (similar to a hurricane which only has one “leg” and moves like a snake). By the late 16th century the word was being used in English in various spellings: harrycain, hurlecane, and even Shakespeare used the word “hurricano” in two plays, but apparently in a reference to waterspouts.

All North Atlantic hurricanes heading toward the Caribbean originate in a band roughly between the latitudes of 10 degrees and 15 degrees north, in the region off the upper west coast of Africa, due

west of the Sahara desert. A lethal combination of thunderstorms and warm water at sea level (usually 80 degrees or higher) creates nurseries for these large storms. As water vapor from the Atlantic Ocean rises, it condenses and releases latent heat which makes the surrounding air rise, creating a low pressure system below, which in turn draws in more warm sea air. A vortex can then form with a loosely organized eye wall around the center. The vortex can start to rotate based upon the “Coriolis Effect”—a force that gives hurricanes their circular shape and makes them spin and have a forward motion like old toy tops. In the northern hemisphere the spin will be counterclockwise, while in the southern it will be clockwise.

The Coriolis Effect is best generally understood with an extreme example—if one could stand north of the equator and throw a ball straight toward Iceland, the ball would never hit Iceland as the earth beneath the ball spins to the east, meaning that the ball would fall and land west of Iceland (assuming one could throw that far!) Standing in Iceland the trajectory of the ball would appear to curve to the west or look to be turning counterclockwise. Added to this simple example is the more complex fact that the earth spins more at the equator then it does in Iceland, because a point closer to the equator has further to travel in a 24-hour period (approximately 25,000 miles), then does a point in Iceland (approximately 10,800 miles). The different speed at different latitudes also impacts the rotation of air masses above the earth.  These forces create the counterclockwise rotation in

North Atlantic hurricanes.

As the storms, initially known as tropical disturbances, develop to 23 mph they become Tropical Depressions. At 39 mph they become classified as Tropical Storms and usually follow the track of the North Equatorial Current, which flows due west from the African coast, skirting north of the South American Continent and then entering the Caribbean Sea, where the North Equatorial Current splits into the Caribbean Current and the Antilles Current. As it moves north, the Caribbean Current joins with part of the Antilles Current to become the Florida Current or the Gulf Steam, which flows thru the Straits of Florida, between the Keys and Cuba, and then up the eastern seaboard of the United States. Tropical Storms that stay directly above these warm water currents increase in speed, becoming hurricanes when their sustained winds hit 74 mph; while those storms that veer into the mid-Atlantic Ocean quickly lose their energy source over colder water.

One would logically think that counterclockwise rotating and spinning storms heading due west would, on occasion, spin to the south and hit South America, but this never happens – why? Because there is little to no Coriolis Effect along the equator or roughly within 10 degrees latitude north or south of it.  In fact the Coriolis Force actually pulls hurricanes away from the equator.

Hurricanes have been called the “Greatest Storms on Earth” by NASA and the largest ones have intense rainfall and a large storm surge, and can hit coastal areas with over 155 mph winds.  In Florida, the hurricane season starts on June 1st and runs to November 30th.

The peak months are August and September with September 10th the deadliest day, at least statistically. Remarkably, U.S. hurricane data for North Atlantic Hurricanes has been recorded and kept since 1851! Before 1953, hurricanes were not named and only given the year, i.e. the “1910 Hurricane” or the “Labor Day 1935 Hurricane.” Upon reaching Tropical Storm status (39 mph) they now receive a name; starting in 1953 women’s names were given exclusively, and after 1979 male names were added to the potential name list.

The eye wall of a hurricane consists of numerous thunderstorms with tremendous winds gusting up to and over 155 mph and dumping literally tons of rain. The most destructive portion of a North Atlantic hurricane is the upper right quadrant—an area where the winds blowing in the eye wall are moving in the same direction as the forward movement of the hurricane. Coming off of the eye wall and typically streaming around the hurricane are the curved clouds and areas known as “rain bands,” creating the large intense rain around hurricanes. The eye of the hurricane is calm because it is inside a cylinder of circling air with centrifugal force; the tighter and smaller the eye, the faster it is spinning and the more severe the storm. The most dangerous part of a hurricane is the storm surge. The low pressure under the eye can create a trough and push a wave ahead of the hurricane, while the strong winds can multiply that wave creating a large wall of water devastating almost everything in its path. Storm surges are even worse when

combined with high tides. However, on the west coast of Florida, we are fortunate to have a very large and high continental shelf that creates a very shallow ledge for miles off shore, which tends to break up the height of any landward headed waves.

The Saffir-Simpson Hurricane Scale was invented in 1971 to rate hurricanes on their power, rating intensity from 1-5, with 1 being the weakest and 5 the strongest. Historic hurricanes (pre-1971) were also given ratings to compare their strengths. The scale takes into account each hurricane’s maximum sustained winds (a key factor), central pressure, and storm surge (a factor now less important). The pressure gradient is important as the greater the difference in pressure, the faster the wind will flow from high to low pressure.  Hurricanes rated above 3 are considered major hurricanes.

As destructive as hurricanes can be, there are ecological positives: Hurricanes often redistribute water into drought areas, and they help equalize the Earth’s atmosphere by moving heat and energy away from the topics. With today’s advanced technology, there are weeks of notice and ample warnings to provide time to protect loss of life and property. In contrast, there is little notice paid to nature’s other destructive forces such as earthquakes, volcanic eruptions, tsunamis, tornadoes, etc.

Keep an eye out for future articles on specific storms and their impact to Southwest Florida.