Tornadoes: How They Form
When and Where are They Most Likely to Occur
And
The Devastation Left Behind

Donna Moritz

GENS 320 Physical Geography
David Pares
February 26, 2015 A tornado is a pugnaciously spinning column of air that is in contact with the earth’s land surface. All tornados form from a weather phenomenon known as thunderstorms. Let’s first take a brief look at thunderstorms. A thunderstorm is a storm with lightning and thunder. It’s produced by a cumulonimbus cloud, which is a cloud having a low base and often spread out in the shape of an anvil extending to great heights, usually producing gusty winds, heavy rain and sometimes hail. The basic components needed for a thunderstorm to form are moisture, unstable air and lift. You need moisture to form clouds and rain, unstable air that is relatively warm and can rise rapidly and lastly you need lift. All of these components make the weather favorable for a tornado day. A tornado day will have low-level southerly winds and southwesterly jet stream winds which causes turbulence on the boundary of the two systems. When a tornado forms from a thunderstorm, it is because of a strong updraft in a supercell thunderstorm that caused the rotating air to tilt vertically causing a mesocyclone. A mesocyclone is a rotating whirlwind of air within a supercell thunderstorm. A supercell thunderstorm is a long lived storm whose updrafts and downdrafts are in near balance. These storms have the highest propensity to produce tornados that stay on the ground for extended periods of time. Tornados produced by a supercell thunderstorm have the ability to produce winds exceeding 200 mph. In order to be classified as a tornado, there must be contact with the earth’s surface. If there is no contact with the ground, then it is simply a funnel cloud. A funnel cloud is a whirling cone shaped column of air that extends downward from the bottom of a thunderstorm but does not actually touch the surface of the earth. Several circumstances are essential for the formation of tornadoes and the thunderstorm clouds with which most tornadoes are developed from. 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. (Wicker, 2015) There are some tall tale signs to watch for when observing severe weather that is possible of developing tornados. A wall cloud is an unforeseen lowering of a rain-free cumulonimbus base into a low-hanging adjunct cloud. A wall cloud is usually positioned in the southwest portion of the storm. A rotating wall cloud usually develops before tornadoes or funnel clouds. Another sign is the formation of a funnel cloud, which was mentioned earlier.

Funnel Cloud http://www.wunderground.com/wximage/newokie/25 Wall Cloud http://www.erh.noaa.gov/iln/spotters/guide/ Tornados are so destructive because they can literally occur at any time of day or night and any time during the year. However, in the northern states peak tornado season is during the summer months. In the southern states, peak season is from March through May. There are a few southern states that have a second peak season in the fall. The most likely time of day for a tornado is between 3:00 p.m. and 9:00 p.m., although a tornado can develop in the darkest of night and these night time tornados are the scariest because they are