Thunderstorms http://www.windows.ucar.edu/tour/link=/earth/Atmosphere/tstorm.html It is late afternoon. The white puffy clouds that have been growing all day are replaced by a greenish sky. A distant rumble is heard...then another. It starts to rain. A flash of light streaks the sky, followed by a huge BOOM. Welcome to a thunderstorm. Thunderstorms are one of the most thrilling and dangerous of weather phenomena. Over 40,000 thunderstorms occur throughout the world each day. Thunderstorms have several distinguishing characteristics that can cause large amounts of damage to humans and their property. Straight-line winds and tornadoes can uproot trees and demolish buildings. Hail can damage cars and crops. Heavy rains can create flash floods. Lightning can spark a forest fire or hurt you. Safety during a thunderstorm is really important. Cumulus Stage: the sun heats the earth’s surface during the day. The warm air starts to rise (known as an updraft). If the air is moist, the air forms a cumulus cloud. The cloud grows as long as warm air continues to rise. Mature Stage: when the cloud gets very large, the water gets large and heavy. Raindrops fall when rising air can no longer hold them up. Cool dry air starts to enter the cloud. The cool air sinks (known as the downdraft), pulling the water with it, making rain. The cloud is now a cumulonimbus. Lightning and thunder and heavy rain occur. Dissipating stage: when the downdraft is stronger than the updraft, warm moist air can no longer rise so no cloud droplets can form. The storm dies out with light rain. The cloud disappears from the bottom up. This whole process takes about an hour for an ordinary thunderstorm. Lightning and Thunder Lightning is the most spectacular element of a thunderstorm. In fact it is how thunderstorms got their name. Lightning is a giant spark. A single stroke of lightning can heat the air around it to 30,000 degrees Celsius (54,000 degrees Farhenheit)! This extreme heating causes the air to expand at an explosive rate. The expansion creates a shock wave that turns into a booming sound wave, better known as thunder. Thus the name thunderstorm. The sky is filled with electric charge. In a calm sky, the + and charges are evenly interspersed thoughout the atmosphere, and therefore neutral. Inside a thunderstorm, electric charge is spread out differently. A thunderstorm consists of ice crystals and hailstones. The ice crystals, (+ charged) are pushed to the top of the thunderstorm cloud by an updraft. Meanwhile, the hailstones (- charge) are pushed to the bottom of the thunderstorm by its downdraft. Thus, the thunderstorm's + and - charges are separated into two levels: the + charge at the top and the - charge at the bottom. During a thunderstorm, the Earth's surface has a + charge. Because opposites attract, the - charge at the bottom of the thunder cloud wants to link up with the + charge of the Earth's surface. Once the amount of charge gets large enough to overcome air resistance, positives and negatives will attract causing lightning. Hail Hail is frozen raindrops that are tossed around the cloud by wind. The top of a cumulonimbus cloud is cold enough to freeze water. A hailstone grows larger when supercooled water droplets freeze on contact with a hailstone. The longer a hailstone stays within a cloud the larger it becomes. When it’s too heavy it will fall to the ground. Most hailstones are about the size of peas. The heaviest one in the United States was the size of a cantaloupe and fell on Coffeyville, Kansas in September of in 1970. Large hailstones can break windows, dent cars, damage roof of homes, and cause damage to crops. One hailstorm can destroy a farmer’s entire crop in minutes. In the United States, hail causes hundreds of millions of dollars of damage each year. Severe Thunderstorms They are capable of producing baseball-sized hail, strong winds, intense rain, flash floods, and tornadoes. Severe thunderstorms can last several hours and can grow 18 kilometers high. Several phenomena are associated with severe thunderstorms. These include the gust front, microburst, supercell thunderstorm, and the squall line. Thunderstorm Safety GO INSIDE! If you hear distant thunder or see a flash of light, get indoors immediately. Seek shelter from sturdy buildings, not lean-tos or outhouses. You can stay in a car if that is your only option, but do not touch any of the metal on the car. If you cannot find shelter, stay away from tall, isolated objects such as trees, poles, or posts. Make sure that you are not the tallest object by crouching down. Crouch down, bend forward, and grab your ankles. Keep your head down. Do not lie flat on the ground and try to keep out of puddles or other standing water. If you can get into a house or school, go down to either the basement or a room that is in the center of the building. Stay away from windows. That way, if a tornado touches down, you'll be safe. Do not use a phone or a computer during a thunderstorm. Do not take a shower or wash dishes. Lightning can strike the plumbing or electrical wires that connect to your house and give you an electrical shock if you use these items. tornadoes....Nature's Most Violent Storms http://www.nssl.noaa.gov/edu/safety/tornadoguide.html Greg Stumpf Although tornadoes occur in many parts of the world, these destructive forces of nature are found most frequently in the United States east of the Rocky Mountains during the spring and summer months. In an average year, 800 tornadoes are reported nationwide, resulting in 80 deaths and over 1,500 injuries. A tornado is defined as a violently rotating column of air extending from a thunderstorm to the ground. The most violent tornadoes are capable of tremendous destruction with wind speeds of 250 mph or more. Damage paths can be in excess of one mile wide and 50 miles long. Once a tornado in Broken Bow, Oklahoma, carried a motel sign 30 miles and dropped it in Arkansas! What causes tornadoes? Thunderstorms develop in warm, moist air in advance of eastward-moving cold fronts. These thunderstorms often produce large hail, strong winds, and tornadoes. Tornadoes in the winter and early spring are often associated with strong, frontal systems that form in the Central States and move east. Occasionally, large outbreaks of tornadoes occur with this type of weather pattern. Several states may be affected by numerous severe thunderstorms and tornadoes. During the spring in the Central Plains, thunderstorms frequently develop along a "dryline," which separates very warm, moist air to the east from hot, dry air to the west. Tornado-producing thunderstorms may form as the dryline moves east during the afternoon hours. Along the front range of the Rocky Mountains, in the Texas panhandle, and in the southern High Plains, thunderstorms frequently form as air near the ground flows "upslope" toward higher terrain. If other favorable conditions exist, these thunderstorms can produce tornadoes. Jim Ladue Tornadoes occasionally accompany tropical storms and hurricanes that move over land. Tornadoes are most common to the right and ahead of the path of the storm center as it comes onshore. Colorado Tornado (David Blanchard) Dr. Joseph Golden (NOAA) Tornado Variations Some tornadoes may form during the early stages of rapidly developing thunderstorms. This type of tornado is most common along the front range of the Rocky Mountains, the Plains, and the Western States. Tornadoes may appear nearly transparent until dust and debris are picked up. Occasionally, two or more tornadoes may occur at the same time. Waterspout Waterspouts are weak tornadoes that form over warm water. Waterspouts are most common along the Gulf Coast and southeastern states. In the western United States, they occur with cold late fall or late winter storms, during a time when you least expect tornado development. Waterspouts occasionally move inland becoming tornadoes causing damage and injuries. How Do Tornadoes Form? Before thunderstorms develop, 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 thunderstorm 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. Tornadoes Take Many Shapes and Sizes Weak Tornadoes Strong Tornadoes Violent Tornadoes 69% of all tornadoes Less than 5% of tornado deaths Lifetime 1-10+ minutes Winds less than 110 mph 29% of all tornadoes Nearly 30% of all tornado deaths May last 20 minutes or longer Winds 110-205 mph Only 2% of all tornadoes 70% of all tornado deaths Lifetime can exceed 1 hour Lifetime can exceed 1 hour Weather Radar Watches the Sky Meteorologists rely on weather radar to provide information on developing storms. The National Weather Service is strategically locating Doppler radars across the country which can detect air movement toward or away from the radar. Early detection of increasing rotation aloft within a thunderstorm can allow life-saving warnings to be issued before the tornado forms. Doppler Radial Velocity TORNADO WATCH: Tornadoes are possible in your area. Remain alert for approaching storms. TORNADO WARNING: A tornado has been sighted or indicated by weather radar. If a tornado warning is issued for your area and the sky becomes threatening, move to your pre-designated place of safety. SEVERE THUNDERSTORM WATCH: Severe thunderstorms are possible in your area. SEVERE THUNDERSTORM WARNING: Severe thunderstorms are occurring. Look out for: Dark, often greenish sky Wall cloud Large hail Loud roar; similar to a freight train Tornado Safety If a Warning is issued or if threatening weather approaches: In a home or building, move to a pre-designated shelter, such as a basement. If an underground shelter is not available, move to an interior room or hallway on the lowest floor and get under a sturdy piece of furniture. Stay away from windows. Get out of automobiles. Do not try to outrun a tornado in your car; instead, leave it immediately. Mobile homes, even if tied down, offer little protection from tornadoes and should be abandoned. In Schools Develop a severe weather action plan and have frequent drills, Make sure someone knows how to turn off electricity and gas in the event the school is damaged. Keep children at school beyond regular hours if threatening weather is expected. Children are safer at school than in a bus or car. Students should not be sent home early if severe weather is approaching. Gymnasiums, cafeterias, and auditoriums offer no protection from tornado-strength winds. Move students quickly into interior rooms or hallways on the lowest floor. Have them assume the tornado protection position (shown at right). Hurricanes (also known as Tropical Cyclones) As a strong hurricane heads towards the coast, people prepare - boarding up houses, packing the car, and evacuating. These storms can spell disaster for people in hurricane prone areas, so they are taken very seriously. They are the most powerful of all weather systems and they are huge - an average of 340 miles across. Hurricanes form in the tropics over warm ocean water and die down when they move over land or out of the tropics. These storms are called hurricanes in the Atlantic and typhoons or tropical cyclones in other areas of the world. In the Northern Hemisphere the storms rotate counterclockwise and in the Southern Hemisphere they rotate clockwise because of the Coriolis Effect. At the center of the rotating storm is a small area of calm weather and clear skies called the eye. Hurricane damage in coastal areas is often due to storm surge, which floods coastal areas. Strong waves and wind also batter coastal areas. Hurricanes also cause a tremendous amount of rain. Not all storms are the same. Large and strong storms cause much more damage than small storms. In the Atlantic and Eastern Pacific, the Saffir-Simpson Hurricane Scale is used to describe the size of a hurricane. Hurricanes usually happen at a particular time of year called hurricane season. The timing of hurricane season is different in different regions of the world. In the North Atlantic, hurricane season is from June 1st to November 30th each year. How Hurricanes Form A tropical thunderstorm can grow into a massive hurricane under certain conditions. Sometimes several thunderstorms start rotating around a central area of low pressure. This is called a tropical depression. If the depression strengthens so that winds reach at least 39 mph, it is called a tropical storm. And if wind speeds increase to more than 74 mph, it is called a tropical cyclone or hurricane. 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. The rotation of the storm is due to the Coriolis Effect, a product of the Earth's rotation. This 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. The Eye of a Hurricane At the center of a fierce tropical storm, there is a small area where the weather is calm, the sky is clear, and the winds are just light breezes. This area is called the eye of the storm. As a hurricane strengthens and wind speeds increase, an eye begins to form at the center of the storm. Usually this happens once winds reach about 80 mph. The eye is usually circular when viewed from above, and about 20 to 40 miles is diameter. It is roughly cylinder shape in cross section, extending through the center of the storm like a chimney. Air from above the storm sinks down in the center of the eye. Surrounding the eye is the eyewall, which is the most violent part of a hurricane. The eyewall is a ring of dense cumulonimbus clouds – thunderstorm clouds. There, the strongest winds in the hurricane are found and warm, moist air is pulled into the storm, rising along the eyewall where it cools and forms more cumulonimbus clouds. Storm Surge One of the most dangerous parts of a hurricane isn’t the rain or the wind. It’s the flooding caused by storm surge. As a hurricane or other tropical storm moves towards a coast, it can cause sea level to rise as much as 20 or 30 feet higher than normal. The sea level rise only lasts a short time, usually just a few hours, but it can cause a huge amount of damage. The rising water may totally submerge low-lying areas and towns along the coast. Huge ocean waves cause damage too, demolishing docks, houses, roads, and eroding beaches. Most storm surge is caused when a storm’s winds push ocean water towards the land. When the water is pushed into the shallow parts of the ocean, it piles up, flooding the coast. Some storm surge is caused by the low pressure of the storm too. When storm surge happens at high tide, there is even more flooding. Safety BEFORE A HURRICANE: Have a disaster plan and a pet plan ready. Before a storm threatens, contact your veterinarian or local humane society for information on preparing your pets for an emergency. Board up windows and bring in outdoor objects that could blow away. Make sure you know which county or parish you live in and know where all the evacuation routes are. Prepare a disaster supplies kit for your home and car. Have enough food and water for at least 3 days. Include a first aid kit, canned food and a can opener, bottled water, battery-operated radio, flashlight, protective clothing and written instructions on how to turn off electricity, gas, and water. Have a NOAA weather radio handy with plenty of batteries, so you can listen to storm advisories. Have some cash handy as well, because following a hurricane, banks and ATMs may be temporarily closed. Make sure your car is filled with gasoline. DURING A HURRICANE: Stay away from low-lying and flood prone areas. Always stay indoors during a hurricane, because strong winds will blow things around. Leave mobile homes and to go to a shelter. If your home isn’t on higher ground, go to a shelter. If emergency managers say to evacuate, then do so immediately. AFTER A HURRICANE: Stay indoors until it is safe to come out. Check for injured or trapped people, without putting yourself in danger. Watch out for flooding which can happen after a hurricane. Do not attempt to drive in flooding water. Stay away from standing water. It may be electrically charged from underground or downed power lines. Don’t drink tap water until officials say its safe to do so.