Shelf cloud of squall looms ominously. Photo by
Richard Seaman.
Hurricanes are powerful Atlantic storms which cause severe damage and change to the entire coastline. Hurricanes are driven by warm summer oceans and atmospheric convection, and can have a lifetime of up to a week. However, they often only last up to a few hours over any single area. They are well-known to the public as their intensity often causes billions of dollars worth of damage to people and the environment. Nor’easters, the often less publicized Atlantic coastal storm type, can have a similar if not stronger impact than hurricanes. This is because they are often larger than hurricanes and last longer over any given area. typical hurricane season is from May to November whereas the typical Nor’easter season is from December to April.
The tropics are a region of the Earth extending from about 23°N to 23°S, encompassing the Equator. This region of the planet receives the most sunlight on an annual basis, and as such, is subject to higher temperatures in both the air and the sea. These warm waters help to develop large and powerful thunderstorms at a rate directly proportional to the water temperature.
Thus, with higher sea surface temperatures during the summer months, with a maximum around 80°F, more thunder-storms form. With certain atmospheric disturbances, these thunderstorms can agglomerate into a single rotating mass called a tropical cyclone. As this cell strengthens due to the warm ocean waters, the cyclone can grow and increase in intensity by lowering its central pressure and increasing wind speeds.
These cells have their distinctive westward, counterclockwise rotating motion due to general atmospheric wind patterns in the tropics and the Coriolis force. Once winds hit 64 mph, the tropical cyclone is classified as a hurricane and is given a predetermined name for that season. Typically hurricanes are characterized by multiple rain bands rotating around an intense band called the eye wall. Inside of this circle is a region of relatively calm winds called the eye.
Hurricanes go by various names such as a typhoon in the North Pacific Ocean and a cyclone in the South Pacific. Often, Atlantic based hurricanes form in an area aptly named the Main Development Region which is located roughly from West Africa to the Eastern Caribbean Sea. Typically, Atlantic Hurricane season spans from June 1st to November 30th.
This is because the waters of the Caribbean reach 80°F relatively early in the summer and as time progresses the warm waters stretch out further into the Atlantic basin. However, hurricanes, and especially tropical cyclones, are not limited to this time frame as evidenced by numerous December tropical storms.
Hurricanes have winds that vary from 74 mph to greater than 140 mph and central pressures that can dip below 900mb (standard sea-level pressure is around 1013mb). These can induce sea waves up to, but not limited to, 24 feet. These, along with the winds, cause massive damage and large inland flooding. However, once over land the storms lose their source of fuel and dissipate quickly.
The Saffir-Simpson Scale
The Saffir-Simpson hurricane intensity scale was developed in the 1970’s by Herbert Saffir and Robert Simpson (Williams, 2005). It breaks hurricanes into 5 categories, grouped by the maximum sustained wind speeds, which are the peak winds over a period of 1 minute at a height of 10 meters above the surface. From these measured speeds, theoretical ranges for minimum central pressures and storm surges are calculated.
However, central pressure ceased to be used as a classification aid in the 1990s, leaving wind speed to be the lone parameter (Landsea, 2009). The estimates of minimum central pressure and storm surge noted in the table to the right are for reference only. Actual values will vary for each storm. Additionally, it should be noted that the scale does not account for other hurricane-related impacts such as rainfall and tornados.
For more information visit the Saffir-Simpson Hurricane Scale webpage at the National Hurricane Center’s website (http://www.nhc.noaa.gov/aboutsshs.shtml)
The Great Miami Hurricane of 1926 The costliest Atlantic based storm on record after accounting for inflation.
The 1900 Galveston Texas Hurricane The deadliest Atlantic hurricane caused a storm surge that completely flooded Galveston Island.
Hurricane Andrew One of the most severe storms in recent history, Andrew caused irreparable damage in Florida.
Hurricane Wilma The most intense Atlantic storm on record with a central pressure of 882 millibars.
Hurricane Katrina One of the worst storms of the century causing the infamous flooding of New Orleans.
Return Intervals
Category 1
Category 1 hurricanes occur quite frequently, every 4 to 5 years along the southern tip of Florida and the Outer Banks of North Carolina. The tracks of hurricanes allow them to directly hit these areas while other areas are sheltered from direct hits due to the shape of the coastline. The coastline of Virginia is directly impacted by a Category 1 hurricane every 15 years. This return interval increases to once every 30-35 years in the Northeast where very few hurricane hit these shores.
Category 2
The southern part of Florida receives a Category 2 hurricane an average of once every 7 or 8 years, with this frequency being every 11 years along the Outer Banks. These areas are hotspots for hurricane activity. Along the Gulf Coast, these storms occur close to every 20 years. Category 2 storms occur less frequently farther North with a return interval of 130 years off of New Hampshire
Category 3
Category 3 storms rarely hit the Northeast once every 370 years off of New Hampshire, but can occur as frequently as every 9 years in southern Florida. Typically along the Gulf Coasts, these storms hit once every 30 some years. Another area of frequent strikes is the Outer Banks with a return interval of 20 years for Category 3 storms.
Category 4
Category 4 hurricanes occur once every 16-30 years in southern Florida and every 47 years off of North Carolina. The Gulf Coast receives Category 4 storms as frequently as every 48 years southern Alabama, or as infrequently as every 120 years in south central Louisiana. The Northeast has a return interval for Category 4 hurricanes of around 500 years.
Category 5
The largest storms on Earth, Category 5 hurricanes affect southern Florida as frequently as once every 35-40 years. The Gulf Coast only gets a Category 5 storm every 100+ years. For most of the Eastern Shoreline from Virginia to the Northeast, Category 5 hurricanes are extremely rare with a return interval of once every 500+ years.
