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Natural Disasters Thunderstorm and Tornado Presentation

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  • Thunderstorms
    A thunderstorm is rain or hail accompanied by thunder and lightning and gusty winds. Thunderstorms usually develop when there is sufficient heating of air near the Earth's surface which rises in a very unstable atmosphere. Thunderstorms are a violent example of atmospheric convection, with uplift and cooling of air, and subsequent cloud formation. As the cloud forms, water vapor changes to liquid and/or frozen cloud particles. This results in a large release of heat that takes over as the principal source of energy for the developing cloud. Once the cloud starts to form by other forces, this release of heat helps keep it growing. The cloud particles grow by colliding and combining with each other, forming rain, snow, and hail. When the droplets become heavy enough to fall against the updraft, precipitation begins, which may be short-lived but very heavy.
    Having reached its final stage of growth, the towering cumulonimbus cloud may be several miles wide and often 10,000 meters or more in height. High level winds shear the cloud top into the familiar anvil shape. When the Sun illuminates these cloud towers, they appear as huge white mountains. When moving several abreast they may form a squall line.
    Once precipitation begins the updraft which initiated the cloud's growth weakens and is joined by a downdraft generated by the precipitation. This updraft-downdraft couplet constitutes a single storm "cell". On the ground the updrafts and downdrafts of air are felt as rapid gusts of wind . Most storms are composed of several cells that form, survive for about half an hour, and then die. New cells may replace old ones, and it is possible for some storms to continue for several hours.
    Lightning always accompanies the thunderstorm. Lightning arises from a discharge of electrical energy which has built up within the cumulonimbus cloud as a result of repeated separation and splitting of water and ice particles in the turbulent conditions that prevail. Although air is a fairly good insulator, eventually the separation of electric charge becomes so great that the insulation breaks down and a lightning strike results. Lightning discharge may occur entirely within the cumulonimbus cloud or between the cloud and the ground. The lightning strike causes a rapid heating of the surrounding air, resulting in a sudden expansion and contraction of air that is heard as thunder. Close to the lightning strike the thunder may be heard as a short loud crack. Further away, the thunder rumbles or echoes, because sound from different parts of the lightning strike are not all heard at the same time. One can work out how far away the lightning strike was by counting the time taken for the thunder to arrive. A 5 seconds difference is roughly equal to a distance of 1 mile.
    In warmer regions of the world thunderstorms can be particularly violent because they contain so much energy, made available from the strong surface heating by the Sun. Such thunderstorms may also be accompanied by tornadoes, rapidly spinning columns or spouts of air. Tornadoes are particularly common in the central United States.
    It is estimated at any given moment nearly 2,000 thunderstorms are in progress over the Earth's surface, and lightning strikes the Earth 100 times each second. There are about 45,000 thunderstorms daily and 16 million annually around the world.
  • The sky is filled with electric charge. In a calm sky, the + and - charges are evenly interspersed throughout the atmosphere. Therefore, a calm sky has a neutral charge.
    Inside a thunderstorm, electric charge is spread out differently. A thunderstorm consists of ice crystals and hailstones. The ice crystals have a + charge, while the hailstones have a - charge. The ice crystals are pushed to the top of the thunderstorm cloud by an updraft. Meanwhile, the hailstones 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 - charge at the bottom of the cloud gets large enough to overcome air resistance, a flow of - charge rushes toward the earth. This is known as a stepped leader. The + charges of the Earth are attracted to this stepped leader, so a flow of + charge moves into the air. When the stepped leader and the + charge from the earth meet, a strong electric current carries + charge up into the cloud. This electric current is known as the return stroke of lightning and is visible to the human eye.
  • Side Note: You can work out how far away the thunderstorm is like this: As soon as you see the lightning, start counting the seconds. Stop when you hear the thunder, and divide the number you have counted by three. The answer you get tells you roughly how far you are, in kilometers, from the thunderstorm
  • Warm moist tropical air shoots upward as it meets colder, dryer polar air. As the warm moist air rises, it may meet varying wind directions at different altitudes due to a strong westerly jet stream. If these varying winds are staggered in just the right manner with sufficient speed, they will act on the upward rising air, spinning it like a top.
  • Transcript

    • 1. Thunderstorms and Tornadoes Chapter 15 Thunderstorms and Tornadoes EXTREME WEATHER PHOTOS
    • 2. Thunderstorms Violent, local atmospheric disturbance that mostly last only 30 minutes, but can create several dangerous phenomena: Torrential rain: usually intense, but short in duration - flash flooding and slope failure in mountainous terrain is often associated with this type of precipitation. High wind: Winds at the surface beneath a thunderstorm can reach well over 50 mph (80 kph). Hail: falling at several meters per second, hail stones can result in extensive damage to crops and property in just a few minutes and can injure or kill. Lightning: every year in the U.S., lightning is responsible for an average of almost 100 deaths and 300 injuries. Lightning frequently starts fires which threaten homes, businesses, and lives. Power and communication outages caused by lightning (as well as wind) can result in large scale disruption of everyday activities. Tornadoes:
    • 3. Thunderstorms Ingredients for Cooking up the Storm Sources of Moisture, A Lifting Mechanism, and Instability. All thunderstorms require three ingredients for their formation: Above and Right: Life Cycle of a Thunderstorm
    • 4. Sources of moisture It is estimated that there are as many as 40,000 thunderstorm occurrences each day world-wide, most occurring in latitudes near the equator. Late afternoon hours in Spring and summer bring the greatest risk of thunderstorms to most of North America. Right: The figure to the right shows the average number of thunderstorm days each year throughout the U.S. The greatest frequency of occurrence is in the southeastern states, with Florida having the highest incidence. Thunderstorms Ingredients for Cooking up the Storm Left: Typical sources of moisture are large bodies of water such as the Atlantic and Pacific oceans as well as the Gulf of Mexico. The southeastern U.S. has access to two moisture sources in the Atlantic ocean and the Gulf of Mexico which helps explain why there are so many thunderstorms in that region.
    • 5. Thunderstorms Ingredients for Cooking up the Storm A Lifting Mechanism a. Convergent, b. Convective, c. Orographic, d. Frontal
    • 6. Thunderstorms Ingredients for Cooking up the Storm Cold air pushing under warm air along a cold front is a common lifting mechanism. Below: A squall line is a line of thunderstorm which typically forms along the leading edge of a cold front.
    • 7. Thunderstorms Ingredients for Cooking up the Storm Atmospheric Instability Above: Unstable air - air parcel forced aloft cools slower than the surrounding environment and continues to rise because it is warmer and more buoyant than its surroundings. Right: Clouds of vertical development form.
    • 8. Life Cycle of A Thunderstorm Cumulus Stage A cumulus cloud begins to grow vertically. Air within the cloud is dominated by updraft. Below: Thunderstorms begin when a parcel of warm, moist air rises and cools adiabatically until it reaches saturation and forms a fluffy cumulus cloud. As this moisture condenses, it releases heat and warms the air parcel more, causing more uplifting.
    • 9. Mature Stage The storm is at its most intense stage of development and is now a cumulonimbus cloud. The top of the cloud reaches the tropopause where upward motion is hindered and the cloud spreads out horizontally. Life Cycle of A Thunderstorm Right: Mature thunderstorm cloud with typical anvil shaped cloud.
    • 10. Mature Stage Strong updrafts and downdrafts coexist. The mature air mass thunderstorm contains heavy rain, thunder, lightning, and produces wind gusts at the surface. Life Cycle of A Thunderstorm Right: When the updrafts reach their maximum altitude precipitation begins to form. The falling rain causes a drag on the air, and a downdraft is created.
    • 11. Dissipating Stage The thunderstorm enters the dissipating stage as the supply of warm moist air is depleted. Air currents within the convective storm are now mainly downdrafts. Light rain and weak outflow winds may remain for a while during this stage, before leaving behind just a remnant anvil top (below).                                                                       Life Cycle of A Thunderstorm
    • 12. Lightning Lightning occurs when thunderstorms concentrate positive electrical charges in the upper part of cumulonimbus clouds and negative charges in the lower part. When the difference in the charge become great enough to overcome air resistance, a sudden and visible violent electrical discharge occurs in the form of a lightning stroke.
    • 13. These two brothers didn't recognize the danger they were in. Their hair is standing on end from a thunderstorm-induced electrical charge. The two brothers, and their sister, were killed moments later when lightning struck. Lightning Images The electrical discharge can be within clouds and/or between the cloud and ground. Left: Cloud-to-ground lightning is initiated by the downward-moving leader stroke. Multiple cloud-to- ground and cloud-to-cloud lightning strokes are observed during a night-time thunderstorm at left. Right: Ground-to-cloud lightning is a lightning discharge between the ground and a cumulonimbus cloud from an upward-moving leader stroke.
    • 14. Thunder The lightning heats the surrounding air to about 30, 000 degrees Celsius, which expands at supersonic speeds, creating the mighty crashes we recognize as thunder. Lightning and thunder happen at exactly the same time, but you see the lightning before you hear the thunder because light travels faster than sound.
    • 15. Hail Hailstones appear when warm humid air in a thunderstorm rises rapidly into the upper atmosphere and freezes forming tiny ice crystals. If the updraft is strong enough, the ice crystals can rise and fall many times, with the hail getting new coats of water which then freeze, making the hail grow. When the hail is too heavy to be sustained in the air by the updraft, it falls to earth.
    • 16. Flash Floods A short lived upstream flood characterized by rapidly rising, high velocity water.
    • 17. Tornadoes A tornado is a violently rotating funnel of air spawned from supercell thunderstorms. 3 main atmospheric conditions must occur simultaneously to produce a tornado 1) a northerly flow of warm, moist air from the Gulf of Mexico 2) a cold, dry air mass rapidly moving southward from Canada or the Rocky Mountains
    • 18. Tornadoes These three air masses moving in different directions produce shearing conditions that give thunderstorm clouds a "spin" 3) strong easterly jet stream Left: Directional wind shear is the change in wind direction with height. Left: Speed shear is the change in wind speed with height.
    • 19. The rotational cell sags below the cloud base to form a distinctive slowly rotating wall cloud. Strong tornadoes form within and then descend from the wall cloud. −−→
    • 20. Tornadoes The momentum of the mesocyclone is what generates sufficient strength to extend a funnel below the cloud base to the ground. Forming a Tornado An
    • 21. When viewed from the top on weather radar screens, the counter-clockwise rotation of the mesocyclone gives the supercell a characteristic "hook echo".
    • 22. The descending funnel is generally white or clear, made visible because water vapor within it condenses into liquid droplets. After the funnel touches the ground and becomes a tornado, the color of the funnel will change, often depending upon the type of dirt and debris. As a tornado matures, it becomes wider and more intense.
    • 23. As the tornado gradually loses intensity, the funnel decreases in size and takes on a contorted, rope-like appearance before it completely dissipates.
    • 24. Tornado Characteristics Occur mostly in late spring - early summer when conditions are best for tornado formation; but can occur anytime. As the season progresses, tornado formation migrates northward. Most tornadoes travel northeast. They can move across the ground at speeds up to ~60 mph and have max wind speeds of >300 mph. Tornadoes can last from several seconds to more than an hour, but most last less than 10 minutes. About 70% of all tornadoes on Earth occur in the central and southern US. One section of the nation is best at producing tornadoes. This area is called "Tornado Alley," with TX and OK having the most (see map)
    • 25. The severity of a tornado is categorized based on wind speed estimated from the damage that they cause on the Fujita Scale. Tornado width is estimated by the path of destruction -- which can be up to one mile. Dr. Ted Fujita developed the F- scale for tornadoes by examining damage and evaluating the wind speeds that caused such damage. He used this set of photos at left as his standard for comparison.
    • 26. Wind speeds and damages to be expected in different strength buildings.
    • 27. The size and/or shape of a tornado is no measure of its strength. Wind speed increases along with the difference between atmospheric pressure inside the funnel, and the pressure outside it. Don’t believe it…See for yourself…. tweaking a twister
    • 28. Tornado Hazards Extreme winds and flying debris cause almost all of the destruction. A strong tornado will completely demolish houses and everything in them.
    • 29. Tornado Safety
    • 30. Tornado Outbreak Left: Super Outbreak of 1974 The largest known tornado outbreak to date. Started just after noon on April 3, 1974 and lasted 17 hours with a total of 148 tornadoes spanning 13 states, killing 315 people and injuring 5,484 others. A tornado outbreak is a series of tornadoes spawned by a group of storms.