How do severe storms and tornadoes form?

How do tornadoes and thunder storms form?

• Answer:

  THUNDERSTORMS The Necessary Ingredients for Thunderstorms All thunderstorms require three ingredients for their formation: Moisture, Instability, and a lifting mechanism. Sources of moisture Typical sources of moisture are large bodies of water such as the Atlantic and Pacific oceans as well as the Gulf of Mexico. Water temperature plays a large role in how much moisture is in the atmosphere. Recall from the Ocean Section that warm ocean currents occur along east coasts of continents and cool ocean currents occur along west coasts. The amount of ocean water evaporation into the atmosphere is higher in warm ocean currents and therefore put more moisture into the atmopsphere than with cold ocean currents at the same latitude. 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 much rain in that region. Instability Air is considered unstable if it continues to rise when given a nudge upward (or continues to sink if given a nudge downward). An unstable air mass is characterized by warm moist air near the surface and cold dry air aloft. In these situations, if a bubble or parcel of air is forced upward it will continue to rise on its own. As it rises it cools and some of the water vapor will condense, forming the familiar tall cumulonimbus cloud that is the thunderstorm. Characteristics of an unstable air mass with warm moist air near the surface with colder and drier air aloft. Air that is forced upward will continue to rise, and air that is forced downward will continue to sink. Sources of Lift (upward) Typically, for a thunderstorm to develop, there needs to be a mechanism which initiates the upward motion, something that will give the air a nudge upward. This is done by several methods. Differential Heating This heating of the ground and lower atmosphere is not uniform. For example, a grassy field will heat at a slower rate than a paved street. The warmest air, called thermals, tends to rise. In the image (right) a wildfire provided the differential heating for a cumulus cloud to form over the smoke plum. Fronts, Drylines and Outflow Boundaries Fronts are the boundary between two air masses of different temperatures. Fronts lift warm moist air. Cold fronts lift air the most abruptly. If the air is moist and unstable thunderstorms will form along the cold front. Drylines are the boundary between two air masses of different moisture content and separate warm moist air from hot dry air. While the temperature may be different across the dryline, the main difference is the rapid decrease in moisture behind the dryline. It is the lack of moisture which allows the temperatures to occasionally be higher than ahead of the dryline. However, the result is the same as the warm moist air is lifted along the dryline forming thunderstorms. This is common over the plains in the spring and early summer. Outflow boundaries are a result of the rush of cold air as a thunderstorm moves overhead. The rain-cooled air acts as a "mini cold front", called an outflow boundary. Like fronts, this boundary lifts warm moist air and can cause new thunderstorms to form. Terrain As air encounters a mountain it is forced up the slope of the terrain. Upslope thunderstorms are common in the Rocky Mountain west during the summer. TORNADOS A tornado is a violently rotating (usually counterclockwise in the northern hemisphere) column of air descending from a thunderstorm and in contact with the ground. Although tornadoes are usually brief, lasting only a few minutes, they can sometimes last for more than an hour and travel several miles causing considerable damage. The United States experiences more tornadoes by far than any other country. In a typical year about 1000 tornadoes will strike the United States. The peak of the tornado season is April through June and more tornadoes strike the central United States than any other place in the world. This area has been nicknamed "tornado alley." Most tornadoes are spawned from supercell thunderstorms. Supercell thunderstorms are characterized by a persistent rotating updraft and form in environments of strong vertical wind shear. Wind shear is the change in wind speed and/or direction with height. (For more about shear - click on images at left). The updraft lifts the rotating column of air created by the speed shear. This provides two different rotations to the supercell; cyclonic or counter clockwise rotation and an anti-cyclonic of clockwise rotation. The directional shear amplifies the cyclonic rotation and diminishes the anti-cyclonic rotation (the rotation on the right side of the of the updraft in the illustration at left). All that remains is the cyclonic rotation called a mesocyclone. By definition a supercell is a rotating thunderstorm. When viewed from the top, the counter-clockwise rotation of the mesocyclone gives the supercell its classic "hook" appearence when seen by radar. As the air rises in the storm, it becomes stretched and more narrow with time. The image (below right) is from the doppler radar in Fort Worth, Texas, March 28, 2000. This image was taken at 7:10 p.m., about one hour after a tornado moved through downtown Fort Worth, TX. This image shows the second tornado that day in the Metroplex. The colors indicate the intensity of the rain with green representing light rain, the yellow and orange for moderate rain and reds for the heaviest rain and hail. The classic "hook" pattern of the supercell from which a tornado was observed can be clearly seen. See if you can spot the hook. Hint: look where the rain is most intense (the different shades of red). To see an overlay of the hook, move the your mouse over the image. The exact processes for the formation of a funnel are not known yet. Recent theories suggest that once a mesocyclone is underway, tornado development is related to the temperature differences across the edge of downdraft air wrapping around the mesocyclone. However, mathematical modelling studies of tornado formation also indicate that it can happen without such temperature patterns; and in fact, very little temperature variation was observed near some of the most destructive tornadoes in history on May 3, 1999 in Oklahoma. The funnel cloud of a tornado consists of moist air. As the funnel descends the water vapor within it condenses into liquid droplets. The liquid droplets are identical to cloud droplets yet are not considered part of the cloud since they form within the funnel. The descending funnel is made visible because of the water droplets. The funnel takes on the color of the cloud droplets, which is white. Due to the air movement, dust and debris on the ground will begin rotating, often becoming several feet high and hundreds of yards wide. After the funnel touches the ground and becomes a tornado, the color of the funnel will change. The color often depends upon the type of dirt and debris is moves over (red dirt produces a red tornado, black dirt a black tornado, etc.). Tornadoes can last from several seconds to more than an hour but most last less than 10 minutes. The size and/or shape of a tornado is no measure of its strength. Occasionally, small tornadoes do major damage and some very large tornadoes, over a quarter-mile wide, have produced only light damage. The tornado will gradually lose intensity. The condensation funnel decreases in size, the tornado becomes tilted with height, and it takes on a contorted, rope-like appearance before it completely dissipates. Learn more about tornadoes from the More FAQ's and Answers about Thunderstorms & Tornados http://www.srh.noaa.gov/srh/jetstream/tstorms/tornado.htm http://www.spc.noaa.gov/faq/tornado/ http://www.nssl.noaa.gov/faq/faq_tor.php http://www.usatoday.com/weather/tornado/wtfaq.htm http://library.thinkquest.org/03oct/00758/en/disaster/tornado.html http://www.tornadohistoryproject.com/ http://www.nssl.noaa.gov/hazard/ map of peak tornado season http://www.erh.noaa.gov/cae/svrwx/when.htm http://www.srh.noaa.gov/srh/jetstream/lightning/lightning_intro.htm http://www.nssl.noaa.gov/primer/lightning/ltg_faq.shtml http://newweb.wrh.noaa.gov/hnx/LightningMyths-1.pdf http://library.thinkquest.org/03oct/00758/en/disaster/lightning.html http://www.ucar.edu/communications/infopack/lightning/faq.html http://www.srh.weather.gov/srh/jetstream/lightning/lightning_faq.htm http://www.lightningmaster.com/FAQ.htm http://www.ourbetternature.org/lightning2.htm