How a Tornado Forms



The first thing that is needed to produce a tornado is a thunderstorm. To create a thunderstorm you need to have three things, the first of which is a mass of unstable air, which is the result of warm moist air in the lower atmosphere and cold dry air in the upper atmosphere. The second thing needed for a thunderstorm to develop is the force, which mixes the warm and cold air causing the instability. This force can be wind from mountains, fronts or in air convergence like that found where bodies of water and land meet. The warm air rises from the lower atmosphere causing an updraft, which mixes with the heavier cold air. The third factor that must be present for the creation of a thunderstorm is moisture in the atmosphere. This happens when the moisture in the air turns into water droplets and forms clouds. These three things together make up the cumulus stage or formation of a thunderstorm.


The second phase of a thunderstorms life cycle is the mature thunderstorm phase which is when the moisture formed in the clouds is released when it becomes to heavy to stay up in the atmosphere and falls to earth in the form rain, snow, sleet or hail. During this phase there is an updraft of the lighter moist warm air from the ground, which keeps the thunderstorm from dissipating. While the warm air is rising there is cold dry air that is causing a down draft causing the heavy water droplets to fall. This is also the period when there is thunder and lightning, which is one of the more spectacular aspects of thunderstorm. 


These thunderstorms are classified into several categories including single cell, multicell cluster, multicell line, and supercell storms. The first of these and the weakest is the single cell storm, which is not as common and does not produce much if any damage. The second type of storm I listed is the multicell cluster, which is the most common type of storm we experience and can last several hours due to its ability to continually develop. The multicell line storm is stronger than the two previously mentioned types of storms and can lead to hail as well as some small tornadoes.


The most violent storms are known as supercell storms, which are also the most likely to produce tornadoes. During this type of storm there is an extremely strong updraft of warm moist Gulf air with temperatures that are usually above 75 degrees F. This gulf air interacts with the fast moving cooler northern air moving south. The last factor needed to produce the tornado is the jet stream to be moving at least 150 mph. This sheering action of the three air masses colliding forms the tornado in the part of the storm known as the mesocyclone. If, in this area of the tornado, the wind that is rising up in the mesocyclone starts to change direction the chances of a tornado forming are very likely since this is the main force which causes the spinning action of the tornado. This directional change in wind is known as the vertical wind sheer. The more developed and faster these winds rotate the more compact they become. As they become very compact they will spin faster and faster like a figure skater, who holds her arms close to her body to increase her rotation speed. Most tornadoes spin counterclockwise in the northern hemisphere and clockwise in the southern hemisphere, although they can also reverse these by spinning anticyclonically. Because of this tendency to spin in different directions and relatively small size of a tornado most believe that the coriolos affect has very little to do with the directional spin of a tornado. Although these super cell storms with or without producing tornados are extremely destructive they are the least common of all of the storms.      

The last stage of a thunderstorm is the dissipation stage. When the cold air causing the down draft which pushes the moisture out of the clouds starts to become more dominant than the warm air carrying the moisture up the storm will begin to dissipate.  


Although it is not known why in one storm there is a tornado and in another similar storm there isnít one by studying patterns in the weather we are becoming better able to predict the likelihood of a tornado being produced from a severe weather storm. 


Fujita Scale