MACI - Physical Geography: Tropical Cyclones Lesson

Physical Geography: Tropical Cyclones

Tropical Cyclones (Hurricanes)

Above the oceans just north and south of the equator, a weather phenomenon called a tropical cyclone can develop that can drastically alter the physical and cultural landscape if it reaches land. In the North Atlantic Ocean and the Caribbean Sea, this weather pattern is called a hurricane. These storms are considered tropical because they almost always develop between the Tropics of Cancer and Capricorn.

Map of Cyclone Locations around the world

Hurricanes develop over water that is warmer than 80ºF. As the air heats, it rises rapidly, drawing incoming air to replace the rising air and creating strong wind currents and storm conditions. The rapidly rising humid air then cools and condenses, resulting in heavy rains and a downdraft of cooler air. The rotation of the earth causes the storm to rotate in a cyclonic pattern. North of the equator, tropical storms rotate in a counterclockwise direction. South of the equator, tropical storms rotate in a clockwise direction.

Hurricanes start out as tropical depressions: storms with wind speeds between twenty-five and thirty-eight miles per hour. Cyclonic motion and warm temperatures feed the system. If a storm reaches sustained winds of thirty-nine to seventy-three miles per hour, it is upgraded to a tropical storm. Tropical depressions are numbered; tropical storms are named. When winds reach a sustained speed of seventy-four miles per hour, a storm is classified as a hurricane.

When a hurricane makes landfall (comes ashore), the storm surge causes extensive flooding. More people are killed by flooding because of the storm surge than by any other hurricane effect. At the center of the cyclonic system is the hurricane's eye, where there is a downdraft of sinking air but the wind is calm and there are no clouds. The eye can extend from one to one hundred miles or more. Many people who have been in the eye of the hurricane believe the storm has passed, but in reality they are in the center of it. 

Bordering the eye of a storm is the eye wall, where the strongest winds and heaviest rainfall are found. This is the most violent part of the hurricane. Beyond the eye wall are feeder bands, with thunderstorms and rain showers that spiral inward toward the eye wall. Feeder bands can extend out for many miles and increase as the heat engine feeds the storm. Hurricanes lose their energy when they move over land because of the lack of heat generation. Once on land, the storm system breaks down. Rainfall and winds can continue, but with decreased intensity.

Saffir Simpson Hurrican Scale Category 1: Wind Speeds 74-95 mph; Storm Surge 4-5 ft Category 2: Wind Speeds 96-110 mph; Storm Surge 6-8 ft Category 3: Wind Speeds 111-130 mph; Storm Surge 9-12 ft Category 4: Wind Speeds 131-155 mph; Storm Surge 13-18 ft Category 5: Wind Speeds >155 mph; Storm Surge >18 ftHurricanes are rated according to sustained wind speed using the Saffir-Simpson Scale. This scale rates a hurricane according to five categories. Category 1 hurricanes have sustained wind speeds of at least seventy-four miles per hour and can inflict heavy damage to buildings, roofs, windows, and the environment. Category 5 hurricanes have sustained winds of more than 155 miles per hour and destroy everything in their paths. Hurricanes can also spawn tornadoes, which increase their potential for destruction.

Annually, more than one hundred tropical disturbances develop in the North Atlantic, but only about ten make it to a tropical storm status and five to six become hurricanes.

Hurricane Katrina in 2005 was one of the most costly storms to impact the United States. Katrina started out as a tropical depression while in the Bahamas. The storm reached a category 5 hurricane as it passed through the Gulf of Mexico but diminished in strength when making landfall in Louisiana, with sustained winds of 125 miles per hour (a strong category 3 hurricane). Katrina caused widespread devastation along the central Gulf Coast and devastated the city of New Orleans. At least 1,836 people lost their lives, and the cleanup cost an estimated $100 billion.

Map of Path of Hurricane Katrina

Hurricane Mitch hit the coast of Central America in 1998 and dumped over seventy-five inches of rain across the countries of Honduras, El Salvador, Nicaragua, and Guatemala. Devastating winds and heavy rain caused the deaths of up to twenty thousand people. Destructive category 5 hurricanes Edith and Felix made landfall in Nicaragua in 1971 and 2007, respectively. The Yucatán Peninsula and the coast of Mexico have also witnessed a number of devastating category 5 hurricanes.

The Caribbean Basin is located in the path of many hurricanes developing out of the Cape Verde region of the North Atlantic. For example, 2008 was a particularly devastating hurricane season, with sixteen tropical storms and eight full-scale hurricanes, five of which caused massive devastation. Three category 4 hurricanes (Ike, Gustav, and Paloma) cut through the northern Caribbean to hit the Greater Antilles. The most devastating was Ike, which ripped through the Caribbean, across the entire length of Cuba, and then on to the Gulf Coast of Louisiana and Texas. Ike's immense size contributed to the fact that it was the third most costly hurricane on record. Ike caused an estimated $7.3 billion in damage to Cuba and more than $29 billion in damage in the United States. Hurricane Gustav made landfall in Hispaniola and Jamaica before increasing in strength and causing about $3.1 billion in damage to Cuba. In November of 2008, Hurricane Paloma made landfall in Cuba and caused an additional $300 million in damage to the island. Many of the other Caribbean islands were also devastated by the hurricanes that hit the region in 2008.

Graphical Tropical Weather Outlook, National Hurricane Center, Miami, Florida Atlantic Tropical Clone Activity This photo indicates hurricane Gustav and tropical storm Hanna, as well as an existing tropical depression (which became Hurricane Ike) and a tropical disturbance. Tropical Storm Hanna later developed into a full-scale hurricane. Hurricane Extremes: Hurricane Faith in 1966 started out by Cape Verde Islands near Africa and traveled west past the Bahamas as a category 2 hurricane. It swept north by Newfoundland, Canada, then northeast to Norway as an extra-tropical storm with wind speeds of 60 mph. It diminished 300 miles from the North Pole.

Review

  • Tropical cyclones occur in the tropical regions over warm ocean water.
  • Hurricanes start as tropical depressions with wind speeds of at least twenty-five miles per hour. As wind speeds increase to thirty-nine miles per hour, the disturbances are called tropical storms and are named. When wind speeds reach seventy-four miles per hour, they become hurricanes.
  • The most dangerous feature of the storm is the immense flooding it can cause when reaching land.
  • Hurricane season is between June 1 and November 30. Cruise ships do not usually operate in the Caribbean during this time.

 

Map Activity IconFor this module's map activity, use the atlas of your choice to identify the following areas:

            • Cities: Mexico City, Monterrey
            • Continent: North America
            • Countries: Bahamas, Belize, Cuba, El Salvador, Guatemala, Honduras, Jamaica, Mexico, Nicaragua, Costa Rica, and Panama, Puerto Rico
            • Island Groups: Lesser Antilles, Greater Antilles
            • Isthmus: Isthmus of Panama
            • Latitude: Tropic of Cancer
            • Mountain Ranges: , Sierra ,Sierra Madre Occidental, Sierra Madre Oriental
            • Peninsulas: Baja, Yucatan
            • Rivers: Rio Grande
            • Salt Waters (Oceans, Bays, Seas, Gulfs): Pacific Ocean, Atlantic Ocean, Caribbean Sea, Gulf of Mexico

 

RESOURCES IN THIS MODULE ARE OPEN EDUCATIONAL RESOURCES (OER) OR CREATED BY GAVS UNLESS OTHERWISE NOTED. SOME IMAGES USED UNDER SUBSCRIPTION.