4.4 - Tropical Cyclones

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Tropical cyclones are intense, low pressure disturbances, that develop in the tropics (8° - 23.4°N and S) and often move poleward into the midlatitudes. Tropical cyclones are known by different names in different parts of the world: hurricanes in North and Central America, typhoons in the western North Pacific, and cyclones in the Indian Ocean and Australia.

Hurricanes are much smaller than extratropical cyclones, less frequent and effect many less people.  Most have a diameter of between 100 and 600 miles (compare that to the 1000 ft diameter of extratropical cyclones). But what hurricanes lack in size they make up for in strength. They have much higher wind speeds and the potential for significant damage is great.

Hurricanes can only form over warm tropical oceans. The water temperature has to be at least 80°F down to a depth of almost 200 feet. They can only form at least 8° to 10°  north or south of the equator because the Coriolis effect is necessary for their formation and the Coriolis Effect is absent at the Equator. More than 80% form on the poleward side of the intertropical convergence zone. 

Meteorologists do not completely understand how and why hurricanes form. They always start with some sort of existing disturbance in the atmosphere - thunderstorms over north western Africa, for example. Hurricanes can only form when there is no significant wind shear (a significant change in wide direction or wind speed at higher altitudes within the atmosphere). 

Hurricanes occur in a number of low-latitude regions around the Globe.  They are most common in the North Pacific basin originating east of the Philippines and west of southern Mexico and Central America. The third region in terms of frequency is the west central portion of the North Atlantic, the Caribbean and the Gulf of Mexico.  Other basins include the western portion of the South Pacific, the South and North Indian Oceans. They are very rare in the South Atlantic and in the southeastern portion of the Pacific, apparently because the water is too cold, high pressure dominates and wind shear is common. The strongest and largest hurricanes are typically those in the China Sea. These are the "super" typhoons. Hurricanes usually occur in late summer and fall, when ocean temperatures are at their highest.

Classification and Anatomy of Tropical Cyclones

Tropical disturbances with sustained winds of 38 mph or less, are called tropical depressions. If the wind speeds in a tropical disturbance reach sustained winds of 39 to 73 mph, they become tropical storms and they are named by the National Weather Service. In order to be designated a hurricane, a storm must have sustained wind speeds of 74 mph or greater. For several hundred years, hurricanes in the Atlantic Basin were named after the particular Saint's day on which the hurricane occurred. Informally, meteorologists began giving women's names to storms in the 1940's and this was an especially common practice among military meteorologists during WW II. In 1953, the use of women's name became the official practice for identifying storms. In 1979, men's names were included. If a storm is particularly deadly, that storm's name is retired and is not used again.

Hurricanes are classified by the Saffir Simpson Scale. The Saffir Simpson Hurricane Wind Scale is a 1 to 5 rating based on a hurricane's sustained wind speed. This scale estimates potential property damage. Hurricanes reaching Category 3 and higher are considered major hurricanes because of their potential for significant loss of life and damage. Category 1 and 2 storms are still dangerous, however, and require preventative measures. In the western North Pacific, the term "super typhoon" is used for tropical cyclones with sustained winds exceeding 150 mph.

 

At the center of the hurricane is the eye, a cloud-free area of sinking air and light winds that is usually from 6 to 40 miles in diameter. As air rises in the thunderstorms surrounding the eye, some of it is forced towards the center, where it converges and sinks. As this air sinks, it compresses and warms to create an environment (mostly) free of clouds and precipitation. The eye is the calmest part of the storm because the strong surface winds converging towards the center never actually reaching the exact center of the storm, but instead form a cylinder of relatively calm air. Bordering the eye of a mature hurricane is the eye wall, a ring of tall thunderstorms that produce heavy rains and very strong winds. The most destructive section of the storm is in the eye wall.

Surrounding the eye wall are curved bands of clouds that trail away in a spiral fashion, suitably called spiraling rain bands. The rain bands are capable of producing heavy bursts of rain and wind, perhaps one-half or two-thirds the strength of those associated with the eye wall. Tornadoes may develop within a hurricane. 

 

Damage and Destruction

Certainly the high winds associated with hurricanes cause significant damage as do the torrential rains that occur and the tornadoes that often accompany hurricanes. However, the overwhelming cause of damage and loss of life is the flooding caused by the storm surge. Storm surge is produced by water being pushed toward the shore by the force of the winds moving cyclonically around the storm.

How high the storm surge is at any given location depends on the orientation of the coast line with the storm track; the intensity, size, and speed of the storm; and the slope of the sea floor. The shallower and flatter the sea floor the higher the surge. The highest storm surge (43 feet) recorded occurred in Cyclone Mahina in 1899 in Australia. The highest in the United States was 28 feet in Hurricane Katrina in 2005.

Watch this animation of storm surge to help you understand it.  Storm Surge. Links to an external site.

The greatest loss of life has occurred in the Bay of Bengal where the Ganges and Brahmaputra Rivers form a delta. The area is very flat with high population densities, very few roads or vehicles and extreme poverty. In 1991, a storm killed nearly 200,000 people.  The single most deadly natural disaster in U.S. history, by comparison,  was the Great Galveston Storm of 1900.  12,000 people were killed and the city was destroyed. In August 2005, 1,833 people died when Hurricane Katrina hit Louisiana, Mississippi, Florida and Alabama. The historic flooding topped levees, submerging New Orleans and displacing hundreds of thousands of people. Many of the dead were left to decompose on rooftops and streets, drawing sharp criticism of the government's response. Tens of thousands of people were forced to seek refuge in the Superdome, which descended into chaos and became a symbol of the city’s lack of preparedness.  To understand what is being done to improve Hurricane preparedness in New Orleans, please watch this video: Hurricane Katrina. Links to an external site.

In the U.S., hurricane season is usually June through November. By the beginning of September in an average year we would expect to  have 4 named storms, two of which would be hurricanes and one of which would be a Category 3 or greater. Hurricane activity peaks in the Atlantic Basin in late summer, usually around mid September.  The 2020 Atlantic hurricane season featured tropical cyclone formation at a record-breaking rate. There were a total of 30 named storms, 13 hurricanes, and 6 major hurricanes. With 30  named storms, it is the most active Atlantic hurricane season on record, beating the prior 2005 Atlantic hurricane season record of 28 named storms. It is also only the second tropical cyclone season to feature the Greek letter storm naming system (because they have run through all the names chosen for the season - one for each letter of the alphabet), with the other season being 2005.

Recent research suggests that there has been an increase in the intensity of hurricane activity in the North Atlantic since the 1970s. Climate change is a likely contributor to this increase in intensity due to increasing ocean temperatures.  While there aren’t necessarily more Atlantic hurricanes than before, those that form appear to be getting stronger, with more Category 4 and 5 events.