5.3 Nature’s Deadliest Storms: Hurricanes

Explain how hurricanes develop, where they occur, and what makes them dangerous.

The fastest wind ever officially recorded was produced by the release of latent heat in tropical cyclone Olivia. The storm produced a wind gust of 408 km/h (253 mph), measured on April 10, 1996, on Barrow Island, Australia.

Hurricane is the North American name for a tropical cyclone with sustained winds of 119 km/h (74 mph) or greater. Hurricanes are capable of producing meters of rainfall, heavy flooding, and damaging high winds. A single storm can devastate extensive coastal regions and cause hundreds of thousands of fatalities. To make matters worse, tornadoes often form in the severe thunderstorms that precede hurricanes.

hurricane

The North American name for a tropical cyclone with sustained winds of 119 km/h (74 mph) or greater.

What Is a Hurricane?

Question 5.4

What does the word hurricane mean?

Hurricane means “god of storms” in the Taino language.

The word hurricane is derived from Huracán/Juracán, meaning “god of storms” in the Taino language of the Lesser Antilles islands in the Caribbean Sea. Hurricanes are deep low-pressure systems occurring in the tropics. Like all cyclonic storm systems, hurricanes rotate counterclockwise (in the Northern Hemisphere) around a region of low barometric pressure. Meteorologists refer to hurricanes as tropical cyclones. A tropical cyclone is a cyclonic storm with sustained winds of 119 km/h (74 mph) or greater.

tropical cyclone

A tropical cyclonic storm with sustained winds of 119 km/h (74 mph) or greater.

170

Tropical cyclones go by many names depending on the geographic region in which they occur. North America and Central America use hurricane, southeastern Asia uses typhoon, and countries bordering the Indian Ocean use cyclone, except Australia, which uses tropical cyclone.

typhoon

A name used for tropical cyclones in Southeast Asia.

In addition, individual tropical cyclones are given identifying names, such as Hurricane Sandy or Typhoon Jelawat (which means “carp”). Naming systems are different for each of the ocean basins where tropical cyclones occur. Atlantic Ocean hurricanes are named according to an alphabetical list of common first names in English, French, and Spanish. The names of destructive hurricanes, like Hurricane Sandy and Hurricane Katrina, are taken off the list and replaced by other names starting with the same letter. The names of Pacific typhoons west of the 180th meridian describe a range of phenomena, including food, star constellations, plants, and wildlife.

171

Why Are Hurricane Winds So Fast?

Hurricane winds are among the fastest and most sustained on the planet. Air rapidly flows in toward the low-pressure center of a hurricane, and Coriolis force causes that airflow to be deflected, creating a rotating pinwheel structure (Figure 5.16).

Figure 5.16

Tropical cyclone anatomy. This image shows Typhoon Haiyan on November 7, 2013. This storm was notable for being the strongest tropical cyclone ever to strike land. Arrows show the direction of rotation within the storm. Haiyan formed a well-defined eye, eyewall, and rain-band structure. It had maximum 1-minute sustained wind speeds of 315 km/h (195 mph). Haiyan took over 6,200 lives, mostly in the Philippines. The inset map shows Haiyan’s path over the Philippines, then mainland Southeast Asia.
(NASA)

Like that of a thunderstorm, the strength of a hurricane depends on how much water vapor condenses to liquid. Condensation is important because it releases latent heat into the storm (see Section 3.1). As the rotating airflow grows faster, it pulls in more moisture, which releases latent heat and further strengthens the storm, creating a positive feedback (Figure 5.17).

Figure 5.17

GEO-GRAPHIC: Hurricane latent heat positive feedback. Hurricanes derive their high wind speeds from latent heat positive feedback.

Animation

Hurricane positive feedback

http://qrs.ly/vs434g3

The latent heat positive feedback in hurricanes results in high winds and, sometimes, extraordinary amounts of precipitation. Rainfall amounts, however, are more often a function of the hurricane’s forward speed than its strength. Cyclone Gamede moved slowly over the island of Réunion in the Indian Ocean in 2007. The storm brought 3.93 m (12.9 ft) of rain in a three-day period. After six days of continuous rainfall, precipitation totals came to 5.51 m (18.1 ft), setting world records for rainfall totals during a single storm.

172

Stages of Hurricane Development

We learned in Section 4.3 that the trade winds of the tropics flow from east to west. As these winds flow across northern Africa, they develop ripples called tropical waves. Some tropical waves are generated by the Ethiopian Highlands in eastern Africa and the Atlas Mountains of northwestern Africa. Land-sea temperature contrasts between the Sahara and the Atlantic Ocean can also form tropical waves from which a hurricane may later develop. The ITCZ provides the instability that allows a hurricane to develop and persist. Figure 5.18A shows the stages of hurricane development from tropical disturbance to tropical storm. Tropical storms are cyclonic systems that have sustained winds between 63 and 118 km/h (39 to 73 mph).

tropical storm

A tropical cyclonic storm with sustained winds between 63 and 118 km/h (39 to 73 mph).

Figure 5.18

Hurricane development. (A) A tropical disturbance may develop into a hurricane if conditions are right. This image shows the progression of a single tropical disturbance into a hurricane as it crosses the Atlantic Ocean. Not all Atlantic hurricanes form off the west coast of northern Africa, however; hurricanes also form within the Caribbean Sea and the Gulf of Mexico. (B) The Saffir-Simpson scale of hurricane intensity.

Once a hurricane has formed, it may strengthen if conditions are favorable. Meteorologists use the Saffir-Simpson scale, a hurricane ranking system based on measured wind speeds, to describe five categories of hurricane intensity (Figure 5.18B).

Saffir-Simpson scale

A hurricane ranking system based on measured wind speeds.

173

For a hurricane to persist and strengthen, it must have an ample supply of warm seawater that readily evaporates. Warm seawater is the main fuel for hurricanes. Generally, water temperature must be about 26°C (80°F). Cooler water reduces the moisture supply to hurricanes because it does not evaporate as quickly. The high waves stirred up by the storm will mix deeper, cooler seawater with the surface water, so the warm water must extend to about 60 m (200 ft) in depth. There must also be little to no wind shear, which tears hurricanes apart.

As hurricanes approach populated areas, meteorologists monitor them closely to forecast where they will make landfall and how strong they will be when they do (Figure 5.19).

Figure 5.19

SCIENTIFIC INQUIRY: How do meteorologists monitor hurricanes? Knowing where and when a hurricane will strike, as well as its likely strength, is essential to efforts to save lives. Decisions to evacuate coastal areas are based on close monitoring of these storms by NASA, NOAA, and the National Weather Service.
(NOAA PMEL Carbon Group and the University of Georgia; http://www.pmel.noaa.gov/co2)

Video

Hurricane monitoring

http://qrs.ly/t8434gb

174

Hurricane Geography

Subtropical highs guide hurricanes in all ocean basins. Where hurricanes make landfall (move onshore) in the eastern United States (assuming they do make landfall) depends on the steering influence of the Bermuda high, which is part of the subtropical high-pressure belt (see Section 4.3). Figure 5.20 summarizes the factors that control the paths of hurricanes and the resulting geographic extent of hurricanes worldwide.

Figure 5.20

GEO-GRAPHIC: The geography of hurricanes. This map shows the paths of all hurricanes, tropical storms, and tropical depressions in recorded history. TD = tropical depression. TS = tropical storm.
(Robert Rohde/globalwarmingart.com)

Why Are Hurricanes Dangerous?

Hurricanes present two basic hazards: flooding and sustained high winds. Hurricanes bring two types of flooding, inland flooding from rivers over-flowing their banks and coastal flooding from storm surges.

Coastal flooding is the most dangerous aspect of hurricanes. The strong winds and low atmospheric pressure of a hurricane cause a rise in sea level called a storm surge (Figure 5.21). Low-lying coastal areas are inundated by seawater as a storm surge comes ashore. The storm’s high winds also create large waves that break on top of the storm surge. The storm surge is typically about 80 to 160 km (50 to 100 mi) wide.

storm surge

A rise in sea level caused by the strong winds and low atmospheric pressure of a hurricane.

Figure 5.21

Storm surge. A dome of seawater causes sea level to rise as a hurricane comes ashore. About 95% of the raised sea level is the result of strong winds that pile up the seawater. Decreased atmospheric pressure, which allows the sea to expand upward, contributes to about 5% of the storm surge.

High winds do not cause as many human fatalities as flooding, but if people are caught outside in high winds, they are at risk of being struck by flying debris. High winds also cause much property damage.

175

Table : TABLE 5.3 The 10 Deadliest Hurricanes Worldwide

RANK

APPROXIMATE DEATHS

LOCATION

DATE

1

500,000

West Bengal/Bangladesh

1970

2

300,000

Hooghly River,Bengal, India

1737

3

300,000

Haiphong, Vietnam

1881

4

300,000

Coringa, southern India

1839

5

200,000

Bengal

1584 or 1582

6

200,000

China

1975

7

200,000

Calcutta, Bengal, India

1876

8

140,000

Burma

2008

9

140,000

Bangladesh

1991

10

100,000

Arabian Sea, western India

1882

Inland flooding from hurricanes does sometimes cause considerable loss of life. For instance, Hurricane Mitch struck Honduras and Nicaragua in 1998 and caused rivers there to overflow their banks and cut into steep cliff faces. This created a dangerous mix of mudslides and flooding. In all, some 11,000 people died in that storm as a result of inland flooding, and 1.5 million people were left homeless.

Worldwide, hurricanes are the main meteorological killer. The losses of life they inflict can be staggering (Table 5.3). With satellite technology, hurricanes can no longer catch us by surprise, but this fact does not always prevent the loss of human lives. Some particularly deadly hurricanes have occurred in recent times. The most recent devastating hurricane was in 2008 in Burma.

All but two of the disasters listed in Table 5.3 occurred in countries bordering the Indian Ocean. Burma, Bangladesh, and India have suffered the greatest losses of life. Presumably, such large death tolls could have been avoided by getting people out of harm’s way. Yet large populations live at sea level near the coast where there is no high ground. Dire poverty also makes it impossible to evacuate everyone.

As Table 5.4 shows, the United States also experiences deadly hurricanes, but loss of life is far less there because of better warning systems and better ability to evacuate people in coastal areas to higher ground.

Hurricane Katrina occurred in recent times, yet caused the third most fatalities. New Orleans lies in a topographic bowl that is below sea level and easily flooded. When the hurricane struck the city, some of the protective walls surrounding it failed, and much of the bowl filled with water. Poverty and inability or unwillingness to evacuate were also important factors in that disaster. The Geographic Perspectives at the end of this chapter explores the threat posed to New Orleans and other U.S. cities by hurricanes.

Video

Storm surge protectors

http://qrs.ly/vm434gf

Table : TABLE 5.4 The 10 Deadliest Hurricanes in the United States

RANK

APPROXIMATE DEATHS

LOCATION

DATE

1

8,000

Galveston, Texas

1900

2

2,500

Lake Okeechobee, Florida

1928

3

1,836

Hurricane Katrina, New Orleans

2005

4

1,800–2,000

Louisiana and Mississippi

1893

5

1,000–2,500

South Carolina and Georgia

1893

6

700

Georgia and South Carolina

1881

7

638

New England

1938

8

600

Florida

1919

9

500

Georgia and South Carolina

1804

10

450

Corpus Christi, Texas

1919

176

177