Severe Weather Phenomenon

Thunderstorms, Tornadoes


Thunderstorms affect relatively small areas when compared with hurricanes. The typical thunderstorm is 15 miles in diameter and lasts nearly 30 minutes. Nearly 1,800 thunderstorms are occurring at any moment around the world. That's 16 million a year!

Despite their small size, all thunderstorms are dangerous. A thunderstorm is basically a cumulonimbus cloud which has developed to the point of producing lightning, and thus thunder, and lightning kills more people each year than tornadoes. Heavy rain from thunderstorms can lead to flash flooding. Strong winds, hail, and tornadoes are also dangers associated with thunderstorms. Of the estimated 100,000 thunderstorms that occur each year in the United States, only about 10% are classified as severe. The National Weather Service considers a thunderstorm severe if it produces hail at least 3/4-inch in diameter, wind 58 mph or higher, or tornadoes.

Problem 1.
A thunderstorm classified as approaching severe levels meets what criteria? Record your answer on your answer sheet.

What Makes A Thunderstorm?

Every thunderstorm needs:
  • Moisture - Moisture is needed to form clouds and rain.
  • Unstable Air - Relatively warm air near the surface and relatively cool air at mid-levels of the troposphere will create unstable conditions. A high moisture content of the near surface air will increase the instability of the air.
  • Lifting mechanism - Listed below are four basic mechanisms active in the atmosphere which will cause air near ground to be lifted. Besides these four, lifting of surface air may be influenced by processes occurring at mid to upper levels of the troposphere, such as divergence of air and lifting associated with jet stream regions of maximum wind flow; a jet max.
    Orographic Lifting - occurs when air flowing horizontally encounters the earth's surface rising in elevation. The air is forced to move upward with the land elevation.
    This is most noticeable in mountainous regions but occurs everywhere that the air is directed against increasing land elevations.
    Frontal Lifting - occurs when air is forced to flow upward over a more dense air mass as shown to the right. Essentially, the more dense air is acting as a land surface increasing in elevation.
    The less dense air flows upward as it encounters the more dense air. The dividing line between these two masses of air of different densities is called a front.
    Convergence Lifting - occurs when the flow of air is forced to move closer together. The converging air must move either upward or downward. Near the earth's surface, the land and ocean surface acts as a physical barrier so the air cannot move down, it must move upward.
    Convection lifting - occurs when air near the ground becomes warmed by heating. During the summer and near lower latitudes, the land surface can become greatly warmed by daytime heating. Parcels of air in contact with the surface become warmed, become less dense and begin to rise.

    Problem 2.
    We have stated that unstable air is needed for thunderstorm formation. What characteristics does stable air have? Record your answer on your answer sheet.

    Problem 3.
    Convergence of air near ground level can aid in thunderstorm formation. Where can divergence of air aid in thunderstorm formation? Record your answer on your answer sheet.

    How Does A Thunderstorm Form?

    Before thunderstorms develop, a change in wind direction with height 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. This begins the Developing Stage of the thunderstorm. Only rising air is occurring in the cloud. The cloud builds from a small Cumulus cloud to Cumulus Congestus to Towering Cumulus type.

    At the onset of precipitation, the storm is considered to have moved to the Mature Stage. The cloud develops an anvil and becomes a cumulonimbus type cloud. In this stage, the cloud grows to its maximum height, sometimes reaching to near 55,000 feet or greater.

    An area of updraft rotation, 2-6 miles wide, the mesocyclone, now extends through much of the storm accompanied by a downdraft in the region of precipitation. The most intense precipitation, lightning, strong winds, and possible hail and violent tornadoes occur in the mature stage.

    It is in the most intense portion of the mesocyclone-updraft region, an area called the cyclonic vortex that tornadoes will develop.

    The region of precipitation will develop a strong cool to cold downdraft which when it hits the ground will spread outward forming a gust front. The strong winds of the downdraft, called microbursts, and the gust front may cause as much damage as a tornado. Notice the tornado forms beneath the upward rising air of the cyclonic vortex, while precipitation is occurring beneath the downdraft region.

    Problem 4.
    What is a downdraft?

    Record your answer on your answer sheet.

    Problem 5.
    How large (horizontal cross-section) is a microburst?

    The wind speeds in a microburst can achieve what magnitude?

    Record your answers on your answer sheet.

    The final stage of the thunderstorm is the Dissipating Stage.

    Rainfall decreases in intensity and finally stops falling from this thunderstorm, although other thunderstorms may have developed and the severe weather continues.

    In the dissipating stage, some thunderstorms can produce strong gusts of wind and occasional lightning, although this activity is ending.

    Thunderstorms frequently develop in warm, moist, unstable 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 dry line, which separates very warm, moist (mT) air to the east from the hot, dry (cT) 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 may grow to severe strength producing hail, strong winds and tornadoes.

    Notice the synoptic pattern in the image to the right. This is a classical pattern for development of severe thunderstorms and tornadoes in Texas and the Central Plains, and should be easily recognized, (pattern recognition), by any forecaster as a pattern in which severe thunderstorms and tornadoes may occur and close evaluation of weather observations in the area should be made. However, it is not the only condition in which severe thunderstorms and tornadoes may occur.

    In the above synoptic pattern,
    a cold front is moving toward the east and southeast across Texas.
    A low pressure center is located in Nebraska, although the low pressure center may be located anywhere to the north. Air is moving toward the low pressure center and if it is located north of our area, then maritime tropical (mT) air will flow from the Gulf of Mexico toward the low and across our area. The large red arrow indicates a low-level jet of rapidly moving mT air moving northward towards the low pressure center.
    A dry line is located between the mT air and continental tropical (cT) air which is moving toward the north and northeast out of northern Mexico.
    The cT air is located east and southeast of the cold front.

    With this situation,
    the more dense continental polar (cP) air is lifting the less dense continental tropical air (frontal lifting). However, since the cT air is relatively dry, few clouds form along the cold front.
    To the east along the dry line, the cT air, being more dense than the mT air, is lifting the mT air. Again, a type of frontal lifting. Here we see a significant amount of clouds because of the moisture of the mT air.
    To the north, the cT air and the mT air masses are converging, being squeezed together; another cause (convergent lifting) for the air in this warm sector to be lifted.

    Notice the purple arrow labeled High tropospheric jet. The jet, region of fastest moving air in the upper troposphere, is lying across the low level jet of mT air. This high tropospheric jet will further aid in lifting the air since it is moving air at upper levels away from the region as air from low levels is rising.

    This high tropospheric jet acts similar to a vacuum sweeper at upper levels, drawing air upward and into it. Additionally, This jet is advecting cool to cold air at upper levels. The cool air aloft and the warm, moist air at lower levels serves to make the atmosphere very unstable, a necessary ingredient for severe weather and tornadoes.

    Open the image, May4Surface.gif and the May4-300mb.gif images, in the Atmo 202 folder.

    Considering the classical pattern for severe weather, answer the following question.

    Problem 6.
    On the surface map on your answer sheet, draw a line enclosing only that area where severe thunderstorms and tornadoes would be expected to occur with the synoptic pattern shown for May 4. Do not enclose regions where severe thunderstorms and tornadoes would not be expected to occur.

    Record your answers on your answer sheet.


    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. 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!

    Problem 7.
    What is the definition of a tornado?

    What is a condensation funnel?

    Record your answers on your answer sheet.

    What causes tornadoes?

    Besides the lifting and unstable air mentioned above which helps form thunderstorms, tornadoes are found to frequently form under the above conditions when there is also an inversion aloft, often near 800 millibars.

    This inversion acts as a cap, or lid, to the near-surface air below the inversion, preventing the unstable air from rising above this cap.

    As the day progresses, radiation further warms the ground which in turn warms the near-surface air making the atmosphere even more unstable.

    As the heating continues, rising parcels of air are finally able to break through this inversion cap, similar to holes being punched in the lid and the hot, moist, unstable air is able to continue to ascend into the cooler air above the inversion.

    An explosive growth of cloud occurs with intense updrafts of air shooting through these "holes" in the inversion. The action of air moving rapidly away above the inversion and the unstable air rising through the inversion serves to create a column of air which has lower pressure near the central vertical axis of the column than is found outward from the axis. This causes near-surface air to move rapidly toward the column while air is being withdrawn from the column by the jet stream at upper levels.

    This is the mesocyclone, the spinning, updraft region of the cumulonimbus cloud. As this mesocyclone extends vertically as the cumulonimbus cloud develops, a cyclonic vortex begins developing which if sufficiently strong, will gradually extend downward below the cloud base to the ground.

    Take a look at this site to see several Texas tornado pictures, information about Texas tornadoes and plans for building safe rooms.

    Notice the wall cloud in this image by Tim Marshall. Notice how the cloud in the center seems to drop from the base of the cumulonimbus cloud and the tornado extends from the center. Wall clouds apparently form as warm, moist air is being drawn toward the low pressure of the mesocyclone, resulting in expansion and sufficient cooling to reach saturation and thus production of liquid water droplets to form the cloud.

    Sometimes, several spinning, suction vortices may form within the main funnel cloud.

    Problem 8.
    What is the definition of a suction vortex? Record your answer on your answer sheet.

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    Copyright © 1996-2007 Texas A&M University, Texas A&M Meteorology Department and Marion Alcorn.