Hurricanes
A large, rotating tropical weather system that has wind speeds of at least 120 km/h is called a hurricane. Hurricanes are the most powerful storms on Earth. Hurricanes have different names in different parts of the world. In the western Pacific Ocean, hurricanes are called typhoons. Hurricanes that form over the Indian Ocean are called cyclones.
Most hurricanes form in the areas between 5° and 20° north latitude and between 5° and 20° south latitude over warm, tropical oceans. At higher latitudes, the water is too cold for hurricanes to form. Hurricanes vary in size from 160 to 1,500 km in diameter and can travel for thousands of kilometers.
How a Hurricane Forms
A hurricane begins as a group of thunderstorms moving over tropical ocean waters. Winds traveling in two different directions meet and cause the storm to spin. Because of the Coriolis effect, the storm turns counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The Coriolis force causes the resulting low-level winds to spiral in a counterclockwise direction around the center of the low in the Northern Hemisphere. (Winds swirl clockwise in the Southern Hemisphere.) The rotation of the storm causes the air being drawn into the central low pressure to curve. The air rises as it rotates. This rising air, which is saturated with water, cools and condenses, forming clouds. Hurricanes do not occur within 300 miles (500 kilometers) of the equator because there is no Coriolis Effect at the equator.
A hurricane gets its energy from the condensation of water vapor. Once formed, the hurricane is fueled through contact with the warm ocean water. Moisture is added to the warm air by evaporation from the ocean. As the warm, moist air rises, the water vapor condenses and releases large amounts of energy. The hurricane continues to grow as long as it is over its source of warm, moist air. When the hurricane moves into colder waters or over land, it begins to die because it has lost its source of energy.
Where do hurricanes get their energy?
Tropical cyclones can develop in the Gulf of Mexico, the Caribbean Sea, the Pacific and Atlantic Oceans. They are born in moist tropical air. About every four to five days, a tropical wave of low pressure moves along with westerly winds. Some of these waves develop into tropical depressions, tropical storms, and hurricanes.
Once formed, hurricanes take energy from the warm ocean water to become stronger. A storm will strengthen if there is a supply of warm, moist air to feed it. Warm, moist air is found above warm, tropical ocean waters. While a hurricane is over warm water it will continue to grow. A hurricane dies when it moves away from the tropics. When a hurricane moves into areas with cooler ocean water, it weakens. It will also weaken if it travels over land.
At the center of the rotating storm is a small area of calm weather and clear skies called the eye. Typically, an "eye" forms when the tropical cyclone reaches hurricane strength, but an eye is not necessary for a tropical cyclone to become a hurricane.
Another way to think of a hurricane is as a large heat engine. The fuel is moisture from warm ocean water. The moisture is converted to heat in the thunderstorms that form. Spiral rain bands that surround the tropical cyclone's core help feed the circulation more heat energy. As air nears the center, it rises rapidly and condenses into clouds and rain. The condensation releases tremendous amounts of heat into the atmosphere. The result is lower surface pressure and strengthening winds.
In this way, the tropical cyclone's engine refuels itself, concentrating its power in a donut-shaped area, called the eye wall, surrounding the center. The eye wall typically contains the strongest surface winds. Sinking air at the center clears the tropical cyclone of clouds and forms the "eye." Falling surface pressure can occur only if air mass is removed from the circulation center. This is accomplished by wind flowing away from the circulation in the upper atmosphere
- Coastal residents should form evacuation plans before a warning is issued to identify a safe shelter and a route to get there.
- Stock up on emergency supplies including food, water, protective clothing, medications, batteries, flashlights, important documents, road maps, and a full tank of gasoline.
- As a storm unfolds, evacuees should listen to local authorities on radio or television. Evacuation routes often close as a storm develops.
- Dedicated professionals and improved technology have made hurricane forecasting more accurate than ever before—but it’s far from precise.
- If forced to weather a storm, get inside the most secure building possible and stay away from windows. Remember that a lull often signifies the storm’s eye—not its end. Anyone riding out a hurricane should wait for authorities to announce that the danger has passed.
TORNADOES
Tornadoes happen in only 1% of all thunderstorms. A tornado is a small, spinning column of air that has high wind speeds and low central pressure and that touches the ground. A tornado starts out as a funnel cloud that pokes through the bottom of a cumulonimbus cloud and hangs in the air. The funnel cloud becomes a tornado when it makes contact with Earth’s surface.
What is a tornado?
A tornado is a violent rotating column of air extending from a thunderstorm to the ground. The most violent tornadoes are capable of tremendous destruction with wind speeds of up to 300 mph. They can destroy large buildings, uproot trees and hurl vehicles hundreds of yards. They can also drive straw into trees. Damage paths can be in excess of one mile wide to 50 miles long. In an average year, 1000 tornadoes are reported nationwide.
How do tornadoes form?
Formation of tornadoes is poorly understood. For some reason especially violent thunderstorms cause the air which rushes inward and upward to spiral downward creating a funnel from a few hundred feet to over a mile in width. The tornado travels along the ground, usually in a northeasterly direction, for distances of a few hundred feet to tens of miles eventually dissipating. Tornadoes - violent wind storms produced by a spiraling column of air that extends downward from a cumulonimbus cloud. Most tornadoes form from thunderstorms. You need warm, moist air from the Gulf of Mexico and cool, dry air from Canada. When these two air masses meet, they create instability in the atmosphere. A change in wind direction and an increase in wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere. Rising air within the updraft tilts the rotating air from horizontal to vertical. An area of rotation, 2-6 miles wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.
Forecasting Tornadoes
Tornadoes can occur at any time of the year and just about anywhere in the world. However, the unique geography of the United States helps to produce some of the most favorable conditions for their development. The months with the greatest number of tornadoes overall are April, May, and June, but tornadoes can and do occur during any month of the year. Tornado seasons vary in different parts of the United States. In the Southeast, the peak season for tornadoes is February through April. In the northern Plains, tornadoes are most likely to develop from June through August. Generally, tornado frequency is high in the South in late winter and early spring; and in the Plains, Midwest, and Ohio Valley from early spring through summer.
The reason for this is that low-level heat, moisture, and instability necessary for tornado formation are usually confined to southern regions early in the year, and only reach northern sections with regularity late in the spring. Also, the jet stream (associated with the upper-level weather systems that can contribute to thunderstorm and tornado formation) migrates northward from winter to summer.
A secondary season of tornado activity has been observed across the South in late autumn as the jet stream, located well to the north in the summer, migrates southward to its typical wintertime position.
Since most tornadoes are formed in conjunction with severe thunderstorms, forecasters first must determine where thunderstorms are likely to form and reach the severity necessary for tornado formation. The difference between whether a thunderstorm will be severe with large hail and strong winds but no tornado, or whether it will spawn a potentially deadly tornado is very subtle. When evaluating areas for tornadic potential, forecasters traditionally examine observations and computer data to locate regions where strong instability and wind shear coexist.
What are some other factors for tornadoes to form?
Several conditions are required for the development of tornadoes and the thunderstorm clouds with which most tornadoes are associated. Abundant low level moisture is necessary to contribute to the development of a thunderstorm, and a "trigger" (perhaps a cold front or other low level zone of converging winds) is needed to lift the moist air aloft.Once the air begins to rise and becomes saturated, it will continue rising to great heights to produce a thunderstorm cloud, if the atmosphere is unstable. An unstable atmosphere is one where the temperature decreases rapidly with height. Atmospheric instability can also occur when dry air overlays moist air near the earth's surface. Finally, tornadoes usually form in areas where winds at all levels of the atmosphere are not only strong, but also turn with height in a clockwise or veering direction.
BEFORE A TORNADO:Have a disaster plan. Make sure everyone knows where to go in case a tornado threatens. Make sure you know which county or parish you live in. Prepare a kit with emergency food for your home. Have enough food and water for at least 3 days.
DURING A TORNADO:Go to a basement. If you do not have a basement, go to an interior room without windows on the lowest floor such as a bathroom or closet. If you can, get under a sturdy piece of furniture, like a table. If you live in a mobile home get out. They offer little protection against tornadoes. Get out of automobiles. Do not try to outrun a tornado in your car, leave it immediately. If you’re outside, go to a ditch or low lying area and lie flat in it. Stay away from fallen power lines and stay out of damaged areas.
IF YOU’RE AT SCHOOL DURING A TORNADO:Every school should have a disaster plan and have frequent drills. Basements offer the best protection. Schools without basements should use interior rooms and hallways on the lowest floor away from windows. Crouch down on your knees and protect your head with your arms.
AFTER A TORNADO:Stay indoors until it is safe to come out. Check for injured or trapped people, without putting yourself in danger. Watch out for downed power lines. Use a flashlight to inspect your home.
THUNDERSTORMS
Thunderstorms can occur along cold fronts. But thunderstorms can develop in other places, too. An air mass generated thunderstorm is typically associated with a warm, moist air mass that is in place over a region and usually occurs during the summertime. There are only two atmospheric conditions required to produce thunderstorms: warm and moist air near Earth’s surface and an unstable atmosphere. The atmosphere is unstable when the surrounding air is colder than the rising air mass. The air mass will continue to rise as long as the surrounding air is colder than the air mass. When the rising warm air reaches its dew point, the water vapor in the air condenses and forms cumulus clouds. If the atmosphere is extremely unstable, the warm air will continue to rise, which causes the cloud to grow into a dark, cumulonimbus cloud. Cumulonimbus clouds can reach heights of more than 15 km.
Like tornadoes, thunderstorms can occur at any time of the year and just about anywhere in the world. Severe thunderstorms are most prevalent during the spring and fall, while air mass thunderstorms are usually associated with certain geographic regions, and others are associated with frontal systems. Thunderstorms are often classified by where they form and what kind of weather they produce. A thunderstorm occurs from a cumulonimbus cloud. The risk of lightning is perhaps the most common and deadly of what can occur with a thunderstorm. In a thunderstorm, you will also find thunder. Thunder is an explosion of noise heard when atmospheric gases are suddenly heated by a discharge of lightning. The lightning present in all thunderstorms is an electrical discharge that balances the difference between positive and negative charges within a cloud, between two clouds, or between a cloud and the ground. They also bring rain, gusty winds, and hail to many areas of the United States, providing much needed moisture to plants and animals, keeping reservoirs replenished and rivers, lakes and streams full.
IF YOU’RE OUTDOORS: Keep an eye at the sky. Look for darkening skies, flashes of lightning, or increasing winds. Lightning often proceeds rain, so don’t wait for the rain to begin. If you hear the sound of thunder, go to a safe place immediately. The best place to go is a sturdy building or a car, but make sure the windows in the car are shut. Avoid sheds, picnic areas, baseball dugouts and bleachers. If there is no shelter around you, stay away from trees. Crouch down in the open area, keeping twice as far away from a tree as far as it is tall. Put your feet together and place your hands over your ears to minimize hearing damage from thunder. If you’re with a group of people stay about 15 feet from each other. Stay out of water, because it’s a great conductor of electricity. Swimming, wading, snorkeling and scuba diving are not safe. Also, don’t stand in puddles and avoid metal. Stay away from clotheslines, fences, and drop your backpacks because they often have metal on them. If you’re playing an outdoor activity, wait at least 30 minutes after the last observed lightning strike or thunder.
IF YOU’RE INDOORS: Avoid water. It’s a great conductor of electricity, so do not take a shower, wash your hands, wash dishes or do laundry. Do not use a corded telephone. Lightning may strike exterior phone lines. Do not use electric equipment like computers and appliances during a storm. Stay away from windows and doors and stay off porches.
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