Meteorology is fascinating, and since weather is something that has thwarted human kind’s attempts to control and harness it since the beginning of time it’s something worth watching [closely].

Hurricanes [also known as tropical cyclones or typhoons] are one of the most devastating of storms Mother Nature throws at us; and on the Eastern Coast of the United States, the Atlantic Hurrican Season has started for this year and I thought I’d gather some information about hurricanes and put it here on my BLOG.



Named for Huracan, the Carib god of evil, the hurricane is an amazing yet destructive natural phenomenon that occurs about 40 to 50 times worldwide each year. Hurricane season takes place in the Atlantic, Caribbean, Gulf of Mexico, and Central Pacific from June 1 to November 30 while in the Eastern Pacific the season is from May 15 to November 30.

Hurricane Formation

Due to the Coriolis effect, the regions between 5° and 20° north and south of the equator are the belts where hurricanes can form (there is not enough rotary motion between 5° north and south. The term cyclone is used in the Bay of Bengal and Arabian Sea and the term typhoon is used in the Pacific Ocean north of the equator and west of the International Dateline.

The birth of a hurricane starts as a low pressure zone and builds into a tropical wave of low pressure. In addition to a disturbance in the tropical ocean water, the storms that become hurricanes also require warm ocean waters (above 80°F or 27°C down to 150 feet or 50 meters below sea level) and light upper level winds.
Growth and Development of Tropical Storms and Hurricanes

A tropical wave grows in intensity and then may grow to become an organized area of showers and thunderstorms known as a tropical disturbance. This disturbance becomes an organized area of tropical low pressure that is called a tropical depression based on cyclonic winds (counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere). A tropical depression’s wind speed must be at or below 38 miles per hour (mph) or 62 km/hr when averaged out over one minute. These winds are measured at 33 feet (10 meters) above the surface.

Once average winds reach 39 mph or 63 km/hr then the cyclonic system becomes a tropical storm and receives a name while tropical depressions are numbered (i.e. Tropical Depression 4 became Tropical Storm Chantal in the 2001 season.) Tropical storm names are preselected and issued alphabetically for each storm.

There are approximately 80-100 tropical storms annually and about half of these storms become full-fledged hurricanes. It is at 74 mph or 119 km/hr that a tropical storm becomes a hurricane. Hurricanes can be from 60 to almost 1000 miles wide. They vary widely in intensity; their strength is measured on the Saffir-Simpson scale from a weak category 1 storm to a catastrophic category 5 storm. There were only two category 5 hurricanes with winds over 156 mph and a pressure of less than 920 mb (the world’s lowest pressures ever recorded were caused by hurricanes) that struck the United States in the 20th century. The two were a 1935 hurricane that struck the Florida Keys and Hurricane Camille in 1969. Only 14 category 4 storms hit the U.S. and these included the nation’s deadliest hurricane – the 1900 Galveston, Texas hurricane and Hurricane Andrew which hit Florida and Louisiana in 1992.

Hurricane damage results from three primary causes:

  • Storm Surge. Approximately 90% of all hurricane deaths can be attributed to the storm surge, the dome of water created by the low pressure center of a hurricane. This storm surge quickly floods low-lying coastal areas with anywhere from 3 feet (one meter) for a category one storm to over 19 feet (6 meters) of storm surge for a category five storm. Hundreds of thousands of deaths in countries such as Bangladesh have been caused by the storm surge of cyclones.
  • Wind Damage. The strong, at least 74 mph or 119 km/hr, winds of a hurricane can cause widespread destruction far inland of coastal areas, destroying homes, buildings, and infrastructure.
  • Freshwater Flooding. Hurricanes are huge tropical storms and dump many inches of rain over a widespread area in a short period of time. This water can engorge rivers and streams, causing hurricane-induced flooding.

Unfortunately, polls find that about half of Americans living in coastal areas are unprepared for a hurricane disaster. Anyone living along the Atlantic Coast, the Gulf Coast and the Caribbean should be prepared for hurricanes during hurricane season.

Fortunately, hurricanes ultimately diminish, reverting to tropical storm strength and then into a tropical depression when they move over cooler ocean water, move over land, or reach a position where the upper level winds are too strong and are thus unfavorable.

 Hurricane Strength (Classification)

The Saffir-Simpson Hurricane Scale is utilized to determine the relative strengths of hurricanes that may impact the United States coast. Since the 1990s, only wind speed has been used to categorize hurricanes.

  • Category One Hurricane
    Maximum Sustained Wind Speed: 74-95 miles per hour
    Damage Category: Minimal
    Approximate Pressure: Above 980 mb
    Approximate Storm Surge: 3-5 feet
    Examples: Hurricane Lili (2002) in Louisiana; Hurricane Gaston (2004) in South Carolina
  • Category Two Hurricane
    Maximum Sustained Wind Speed: 96-110 miles per hour
    Damage Category: Moderate
    Approximate Pressure: 979-965 mb
    Approximate Storm Surge: 6-8 feet
    Example: Hurricane Isabel (2003) in North Carolina
  • Category Three Hurricane
    Maximum Sustained Wind Speed: 111-130 miles per hour
    Damage Category: Extensive
    Approximate Pressure: 964-945 mb
    Approximate Storm Surge: 9-12 feet
    Examples: Hurricane Katrina (2005) in Louisiana; Hurricane Jeanne (2004) in Florida; Hurricane Ivan (2004) in Alabama
  • Category Four Hurricane
    Maximum Sustained Wind Speed: 131-155 miles per hour
    Damage Category: Extreme
    Approximate Pressure: 944-920 mb
    Approximate Storm Surge: 13-18 feet
    Example: Hurricane Charley (2004) in Florida; Hurricane Iniki (1992) in Hawaii; the Galveston Hurricane (1900) in Texas
  • Category Five Hurricane
    Maximum Sustained Wind Speed: Above 155 miles per hour
    Damage Category: Catastrophic
    Approximate Pressure: Below 920 mb
    Approximate Storm Surge: More than 18 feet
    Examples: Only three Category 5 hurricanes have struck the United States since records began: The Labor Day Hurricane (1935) in the Florida Keyes, Hurricane Camille (1969) near the mouth of the Mississippi River, and Hurricane Andrew (1992) in Florida

There is no Category Six for hurricanes. While some have suggested such a category, since only three Category Five storms have struck the United States, Category Five would appear to be more than adequate.

Originally posted 2009-08-10 01:00:36.