Hurricane Backgrounder

What Is A Hurricane?

A hurricane is a tropical storm with winds that have reached a constant speed of 74 miles per hour or more. Hurricane winds blow in a large spiral around a relative calm center known as the "eye." The "eye" is generally 20 to 30 miles wide, and the storm may extend outward 400 miles. As a hurricane approaches, the skies will begin to darken and winds will grow in strength. As a hurricane nears land, it can bring torrential rains, high winds, and storm surges. A single hurricane can last for more than 2 weeks over open waters and can run a path across the entire length of the eastern seaboard. August and September are peak months during the hurricane season that lasts from June 1 through November 30.

The center, or eye, of a hurricane is relatively calm. The most violent activity takes place in the area immediately around the eye, called the eyewall. At the top of the eyewall (about 50,000 feet), most of the air is propelled outward, increasing the air's upward motion. Some of the air, however, moves inward and sinks into the eye, creating a cloud-free area.

Tropical cyclones are classified as follows:

Tropical Depression - An organized system of clouds and thunderstorms with a defined circulation and maximum sustained winds of 38 mph (33 knots) or less.

Tropical Storm - An organized system of strong thunderstorms with a defined circulation and maximum sustained winds of 39 to 73 mph (34-63 knots).

Hurricane - An intense tropical weather system with a well-defined circulation and maximum sustained winds of 74 mph (64 knots) or higher. Hurricanes are called "typhoons" in the western Pacific, while similar storms in the Indian Ocean are called "cyclones."

Hurricanes form in the Atlantic Ocean, Gulf of Mexico, Indian Ocean, Caribbean Sea and Pacific Ocean. Hurricane winds in the Northern Hemisphere circulate in a counterclockwise motion around the hurricane's center or "eye," while hurricane winds in the Southern Hemisphere circulate clockwise. Natural phenomena, which affect a storm, include temperature of the water, the Gulf Stream, and steering wind currents. Powered by heat from the sea, they are steered by the easterly trade winds and the temperate westerlies as well as by their own ferocious energy. Around their core, winds grow with great velocity, generating violent seas. Moving ashore, they sweep the ocean inward while spawning tornadoes and producing torrential rains and floods.

In the eastern Pacific, hurricanes begin forming by mid-May, while in the Atlantic, Caribbean, and Gulf of Mexico, hurricane development starts in June. For the United States, the peak hurricane threat exists from mid-August to late October although the official hurricane season extends through November. Over other parts of the world, such as the western Pacific, hurricanes can occur year-round. Areas in the United States vulnerable to hurricanes include the Atlantic and Gulf coasts from Texas to Maine, the territories in the Caribbean, and tropical areas of the western Pacific, including Hawaii, Guam, American Samoa, and Saipan.

Hurricane Threats

The 74 to 160 mile per hour winds of a hurricane can extend inland for hundreds of miles. Hurricanes can spawn tornadoes, which add to the destructiveness of the storm. Floods and flash floods generated by torrential rains also cause damage and loss of life. Following a hurricane, inland streams and rivers can flood and trigger landslides. Even more dangerous than the high winds of a hurricane is the storm surge-a dome of ocean water that can be 20 feet at its peak and 50 to 100 miles wide. The surge can devastate coastal communities as it sweeps ashore. Nine out of 10 hurricane fatalities are attributable to the storm surge.

Hurricane Winds
Coastal communities deciding how strong their structures should be need to consider the strength of hurricane winds and the pressure they generate. As winds increase, pressure against objects is added at a disproportionate rate. Pressure against a wall mounts with the square of windspeed so that a threefold increase in windspeed gives a nine-fold increase in pressure. Thus, a 25 mph wind causes about 1.6 pounds of pressure per square foot. A four by eight sheet of plywood will be pushed by a weight of 50 pounds. In 75 mph winds, that force becomes 450 pounds, and in 125 mph winds, it becomes 1,250 pounds. For some structures, this force is enough to cause failure. These winds will weaken after landfall due to loss of warm-water energy source; and the encountering of great friction over land.

Rainfall and Flooding
Heavy rains and ocean waters brought ashore by strong winds can cause flooding in excess of 50 cm (20 in) over a 24 hour period. The runoff systems in many cities are unable to handle such an increase in water because of the gentle topography in many of the coastal areas where hurricanes occur. Hurricanes are capable of producing copious amounts of flash flooding rainfall. During landfall, a hurricane rainfall of 10 to 15 inches or more is common. If the storm is large and moving slowly-less than 10 mph-the rainfall amounts from an well-organized storm are likely to be even more excessive. To get a generic estimate of the rainfall amount (in inches) that can be expected, divide the storm's forward motion by 100, i.e. Forward Speed/100 = estimated inches of rain. Rainfall and Flooding fact: Tropical Storm Claudette (1979) brought 45 inches of rain to an area near Alvin, Texas, contributing to more than $600 million in damage.

The heaviest rain usually occurs along the coastline, but sometimes there is a secondary maximum further inland. This heavy rain usually occurs slightly to the right of the cyclone track and usually occurs between 6 hours before and 6 hours after landfall. The amount of rain depends on the size of the cyclone, the forward speed of the cyclone and whether it interacts with a cold front. Interaction with a cold front will not only produce more tornadoes but more rainfall as well.

Storm Surge
Storm surge is an abnormal increase in the ocean's level, sometimes in excess of several meters high and miles wide. Storm surges can come ashore up to five hours before the storm and destroy low-elevation coastal areas. It is especially damaging when the storm surge occurs during high tide and consequently is often responsible for most hurricane-related deaths. Storm surge is a large dome of water often 50 to 100 miles wide that sweeps across the coastline near where a hurricane makes landfall. Storm surge can range from 4 to 6 feet for a minimal hurricane to greater than 20 feet for the stronger ones. The surge of high water topped by waves is devastating. The stronger the hurricane and the shallower the offshore water, the higher the surge will be. Along the immediate coast, storm surge is the greatest threat to life and property, even more so than the high winds.

• Over 6000 people were killed in the Galveston Hurricane of 1900, most by storm surge.
• Hurricane Camille produced a 25-foot storm surge in Mississippi.
• Hurricane Hugo in 1989 generated a 20-foot storm tide in South Carolina.

Tornadoes
Hurricanes also produce tornadoes, which add to the hurricane's destructive power. Typically, the more intense a hurricane is, the greater the tornado threat. When a hurricane brings its winds inland, the fast-moving air hits terrain and structures, causing a frictional convergence which enhances lifting. Frictional convergence may be at least a contributing factor to tornado formation in hurricanes. The greatest concentration of tornadoes occurs in the right front quadrant of the hurricane. A number of theories exist about their origin, but in the case of Hurricane Andrew, severe damage was inflicted by small spin-up vortices that developed in regions of strong wind-shear found in the hurricane's the eye wall. The strong damaging winds of the hurricane frequently cover the smaller tornado paths, making the separation of their damaging effects very difficult.

Tropical Cyclone Spawned Tornadoes Facts

• 10% of deaths in the United States are associated with hurricanes are a result of tornadoes.
  
• Most tornadoes occur within 24 hours after hurricane landfall. The exception is when there is interaction with a cold front after landfall. Then more tornadoes will occur two or three days after landfall, well inland.
  
• Most tornadoes occur within 150 miles of the coastline.
  
• More tornadoes occur during the morning and afternoon rather than evening or night due to the need for a tornado to have a heat source.
  
• The Gulf of Mexico hurricanes produce more tornadoes than Atlantic storms.
  
• The majority of tornadoes occur within 30 miles of the center of the cyclone, but there is a secondary maximum further away in the outer rain bands (100-150 miles away from the center).
  
• Tornado winds can reach up to 300 mph at a forward speed of 60 mph and are usually 100-300 yards wide.