What are Hurricanes?
Image of Hurricane Katrina on Sunday, August 28, 2005 captured by NASA’s TERRA satellite. By this time, Hurricane Katrina was set to become one of the most powerful storms to strike the United States, with winds of 257 kilometers per hour (160 miles per hour) and stronger gusts. The air pressure, another indicator of hurricane strength, at the center of this Category 5 storm measured 902 millibars, the fourth lowest air pressure on record for an Atlantic storm. The lower the air pressure, the more powerful the storm.
The massive storm covers much of the Gulf of Mexico, spanning from the U.S. coast to the Yucatan Peninsula. Image credit: NASA MODIS Rapid Response Team at Goddard Space Flight Center
Most people associate twisters with tornadoes, but in fact tropical twisters come from hurricanes. Hurricanes are what scientists call "strong Tropical Cyclones". They are formed when large areas of the ocean become heated, and the air pressure over that area drops. This causes thunderstorms and strong surface winds. Cyclones develop over tropical or sub-tropical waters (for example, in the Atlantic off the coast of Africa, or in the Pacific). As they travel long distances gathering energy from the ocean, they are likely to be classified as strong Tropical Cyclones. When the winds of a tropical storm reach 74 mph, then the storm is classified as a hurricane.
One of NASA's most important missions is to develop an understanding of the total Earth system and study the effects of natural and human- induced changes on the global environment. To achieve these goals, NASA has developed satellites and ground programs which study and monitor hurricanes and other climatic events.
How are Hurricanes Created?
The birth of a hurricane requires at least three conditions. First, the ocean waters must be warm enough at the surface to put enough heat and moisture into the overlying atmosphere to provide the potential fuel for the thermodynamic engine that a hurricane becomes. Second, atmospheric moisture from seawater evaporation must combine with that heat and energy to form the powerful engine needed to propel a hurricane. Third, a wind pattern must be near the ocean surface to spirals air inward. Bands of thunderstorms form, allowing the air to warm further and rise higher into the atmosphere.
Hurricane Ivan Rainfall Structure seen by NASA’s TRMM satellite on September 16, 2004. NASA's TRMM spacecraft is used by meteorologists to understand Hurricane Ivan. TRMM view's Ivan on September 16, 2004, just as the storm's most intense rains and winds makes landfall. The VIRS instrument onboard uses visible and infrared data to image the cloud cover. The Tropical Microwave Imager (TMI) lets scientists “see” through the clouds to reveal the rain structure. This data is used to determine the intensity of the rain throughout the hurricane. Blue represents areas with at least 0.25 inches of rain per hour. Green shows at least 0.5 inches of rain per hour. Yellow is at least 1.0 inches of rain and red is at least 2.0 inches of rain per hour.
If the winds at these higher levels are relatively light, this structure can remain intact and grow stronger: the beginnings of a hurricane! Often, the feature that triggers the development of a hurricane is some pre-existing weather disturbance in the tropical circulation. For example, some of the largest and most destructive hurricanes originate from weather disturbances that form as squall lines over Western Africa and subsequently move westward off the coast and over warm water, where they gradually intensify into hurricanes.
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.
The eye of a hurricane is relatively calm. It is generally 20 to 30 miles wide (the hurricane itself may extend outward 400 miles). The most violent activity takes place in the area immediately around the eye, called the "eyewall". At the top of the eyewall (up to 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 Rainfall Measuring Mission
Hurricanes are huge heat engines, converting the warmth of the tropical oceans and atmosphere into wind and waves. The heat dissipates as the system moves toward the poles, sometimes causing a great deal of hardship for people living along the vulnerable coastlines.
NASA scientists are using the TRMM satellite to understand which parts of a hurricane produce rainfall and why. In addition, TRMM may answer the question of how much latent heat or "fuel" hurricanes release into the atmosphere and whether they affect global weather patterns.
Most importantly to people endangered by hurricanes, TRMM will add to the knowledge needed to improve computer-based weather modeling. With this data, meteorologists may be more able to precisely predict the path and intensity of these storms. For more information about TRMM visit the ESE Kids Only TRMM site and the official NASA TRMM web site.
NASA’s TRMM satellite provides unique measurements of rainfall including how much rain has fallen over open seas. This data is valuable to forecasting floods in the tropics. Appropriate hurricane, tropical storm and depression symbols are shown at 0000 Zulu Time (8:00 p.m. the previous day) and 1200 Zulu Time (8:00 a.m.) locations. Labels were only placed above the 0000 UTC positions of Hurricane Ike.
Why do Hurricanes Move?
Hurricanes are "steered" by the prevailing wind currents that surround the storm from the surface to 50,000 feet or more. The storms move in the direction of these currents and with their average speed. The movement of a hurricane affects the speed of the winds that circulate about the center. On one side of the storm, where the circulating winds and the entire storm are moving in the same direction, the wind speed is increased by the forward movement of the storm. On the opposite side of the storm, the circulating wind speed is decreased by the forward motion. In the Northern Hemisphere, the right side of a hurricane, looking in the direction in which it is moving, has the higher wind speeds and thus is the more dangerous part of the storm. The average tropical cyclone moves from east to west in the tropical trade winds that blow near the equator. When a storm starts to move northward, it exchanges easterly winds for the westerly winds that dominate the temperate region. When the steering winds are strong, it is easier to predict where a hurricane will go. When the steering winds are weak, a storm seems to take on a mind of its own, following an erratic path that makes forecasting very difficult.
The major steering wind influence of most U.S. hurricanes is an area of high pressure known as the Bermuda High. This high-pressure dome is over the eastern Atlantic Ocean in the winter, but shifts westward during the summer months. The clockwise rotation of air associated with high pressure zones is the driving force that causes many hurricanes to deviate from their east-to-west movement and start northward. Sometimes this is favorable: huricanes never reaches the shore, and blow out into the Atlantic Ocean. Other times, hurricanes south of the U.S. are steered northward directly towards the coastline.
Because Hurricane movement can be very erratic, scientists have increasingly been called to track them. NASA has been on the forefront on the design, development and deployment of Earth remote sensing spacecraft design to do just this.
GOES - I/M Missions
The GOES I-M series of satellites is owned and operated by the National Oceanic and Atmospheric Administration (NOAA). NASA manages the design, development, and launch of the spacecraft. Once the satellite is launched and checked out, NOAA assumes responsibility for it.
Over the past 30 years scientists have stated a need for continuous, dependable, and high-quality observations of the Earth and its environment. The new generation Geostationary Operational Environmental Satellites (GOES I through M) provide half-hourly observations to fill that need. The instruments on board the satellites measure atmospheric temperature, winds, moisture, and cloud cover.
Each satellite in the series carries two major instruments: an Imager and a Sounder. These instruments acquire high resolution visible and infrared data, as well as temperature and moisture readings from the atmosphere. They continuously transmit this information to ground terminals where it is processed for rebroadcast to primary weather services, both in the US and around the world.
How Dangerous are They?
One of the most damaging, and deadly events that occur is the hurricane. When hurricanes move ashore, they bring with them a storm surge of ocean water along the coastline, high winds, tornadoes, and both torrential rains and flooding.
During a hurricane, homes, businesses, roads and bridges may be damaged or destroyed by high winds and/or high waves. Debris from the high winds can damage property. Roads and bridges can be washed away by flash flooding, or can be blocked by debris. In particularly large storms (such as Hurricane Andrew), the force of the wind alone can cause tremendous devastation. Trees and power lines topple and weak homes and buildings crumble. These losses are not just limited to the coastline -- often damage extends hundreds of miles inland.
Hurricanes and their potential for destruction are rated using a scale from 1 to 5 called the Saffir-Simpson scale. A Category 1 hurricane is the least destructive and a Category 5 hurricane is the most destructive. There are three types of damage caused by hurricanes:
Hurricane damage can be catastrophic.
Hurricane-force winds, 74 mph or more, can destroy poorly constructed buildings and mobile homes. Debris, such as signs, roofing material, siding, and small items left outside, become flying missiles in hurricanes. Winds often stay above hurricane strength well inland.
Storm Surge Damage
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. 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.
Widespread torrential rains often in excess of 6 inches can produce deadly and destructive floods. This is the major threat to areas well inland.
Areas in the vulnerable to hurricanes include the Atlantic and Gulf coasts of the United States from Texas to Maine, the Caribbean, and tropical areas of the western Pacific, including Hawaii, Guam, American Samoa, and Saipan.
Even Category 1 hurricanes can cause death, property damage and flooding and should be taken very seriously. Coastal areas are often evacuated by the police when a hurricane is approaching.
The Saffir-Simpson Hurricane Scale is a 1-5 rating based on the hurricane's present intensity. This is used to give an estimate of the potential property damage and flooding expected along the coast from a hurricane landfall. Wind speed is the determining factor in the scale, as storm surge values are highly dependent on the slope of the continental shelf in the landfall region.
No real damage to building structures. Damage primarily to unanchored mobile homes, shrubbery, and trees. Some damage to poorly constructed signs. Also, some coastal road flooding and minor pier damage. Hurricanes Allison of 1995 and Danny of 1997 were Category One hurricanes at peak intensity.
Some roofing material, door, and window damage of buildings. Considerable damage to shrubbery and trees with some trees blown down. Considerable damage to mobile homes, poorly constructed signs, and piers. Coastal and low-lying escape routes flood 2-4 hours before arrival of the hurricane center. Small craft in unprotected anchorages break moorings. Hurricane Bertha of 1996 was a Category Two hurricane when it hit the North Carolina coast, while Hurricane Marilyn of 1995 was a Category Two Hurricane when it passed through the Virgin Islands.
Some structural damage to small residences and utility buildings with a minor amount of curtainwall failures. Damage to shrubbery and trees with foliage blown off trees and large tress blown down. Mobile homes and poorly constructed signs are destroyed. Low-lying escape routes are cut by rising water 3-5 hours before arrival of the hurricane center. Flooding near the coast destroys smaller structures with larger structures damaged by battering of floating debris. Terrain continuously lower than 5 ft above mean sea level may be flooded inland 8 miles (13 km) or more. Evacuation of low-lying residences with several blocks of the shoreline may be required. Hurricanes Roxanne of 1995 and Fran of 1996 were Category Three hurricanes at landfall on the Yucatan Peninsula of Mexco and in North Carolina, respectively.
More extensive curtainwall failures with some complete roof structure failures on small residences. Shrubs, trees, and all signs are blown down. Complete destruction of mobile homes. Extensive damage to doors and windows. Low-lying escape routes may be cut by rising water 3-5 hours before arrival of the hurricane center. Major damage to lower floors of structures near the shore. Terrain lower than 10 ft above sea level may be flooded requiring massive evacuation of residential areas as far inland as 6 miles (10 km). Hurricane Luis of 1995 was a Category Four hurricane while moving over the Leeward Islands. Hurricanes Felix and Opal of 1995 also reached Catgeory Four status at peak intensity.
Complete roof failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or away. All shrubs, trees, and signs blown down. Complete destructon of mobile homes. Severe and extensive window and door damage. Low-lying escape routes are cut by rising water 3-5 hours before arrival of the hurricane center. Major damage to lower floors of all structures located less than 15 ft above sea level and within 500 yards of the shoreline. Massive evacuation of residential areas on low ground within 5-10 miles (8-16 km) of the shoreline may be required. There were no Category Five hurricanes in 1995, 1996, or 1997. Hurricane Gilbert of 1988 was a Category Five hurricane at peak intensity and is the strongest Atlantic tropical cyclone of record.
How are Hurricanes Named?
Hurricanes names are chosen from a list selected by the World Meteorological Organization. The Atlantic is assigned six lists of names, with one list used each year. Every sixth year, the first list begins again. Each name on the list starts with a different letter, for example, the name of the very first hurricane of the season starts with the letter A, the next starts with the letter B, and so on. The letters "Q", "U", "X", "Y" and "Z", however, are not used. Often when an unusually destructive hurricane hits, that hurricane's name is retired and never used again. Since 1954, forty names have been retired. In 1996 Hurricane Luis was retired. Is your name among the currently used or retired hurricane names?
National Aeronautics and Space Administration, Science Mission Directorate. (2009). Tropical Twisters. Retrieved , from Mission:Science website:
Science Mission Directorate. "Tropical Twisters" Mission:Science. 2009. National Aeronautics and Space Administration.