Rain and hail are commonplace in a thunderstorm, but when the pressure and temperature changes are significant, high winds are concentrated and accelerated, and often result in a tornado. As the colliding fronts intensify, specific regions of thunderstorms will develop a mesocyclone high up in the atmosphere.
This massive rotation can be anywhere from a couple miles wide, to as much as 10 miles in diameter. This giant system is the tornado. One interesting theory with mesocyclones is that the rapid changes in wind speed, and wind shear from rising warm air drawn up in to the mesocyclone system, cause horizontal tube-like vortexes to form within a severe thunderstorm.
As the warm updraft continues, the tube-like vortex gradually turns vertical. Severe tornadoes often form when three very different types of air come together in a particular way. Near the ground surface southerly or southeasterly transport warm, humid air from the Gulf of Mexico into the Plains. As you go up to the middle layers of the atmosphere above the humid air, the winds veer to the southwest and transport hot, dry air from the Mexico highlands and deserts over the Plains. This forms a layer of hot, dry air in which the temperature often increases with height.
The hot, dry layer is very stable and inhibits any convection that tries to develop. The winds continue to veer as you go up into the higher layers of the atmosphere.
At the high layers of the atmosphere, westerly winds transport cool, moist air from the Pacific Ocean over the Rocky Mountains and into the Plains above the convective cap. This sets up a scenario that has warm, humid air near the surface with hot dry air above it and cool, moist air at the higher levels above the dry air.
These three air streams also arrive from different directions, which provides the necessary wind shear. This scenario highly favors severe weather, including supercells and tornadoes. The existence of the mid-level convective cap, which will be referred to as an inversion layer , is a key ingredient.
An inversion layer is a layer in the atmosphere where the temperature increases with increasing height. Recall that typically temperature decreases with increasing height. An inversion layer is extremely stable for rising motion.
At first an inversion layer will inhibit thunderstorm formation because rising parcels from the surface will not be able to penetrate through this layer. However, as the day progresses and the atmosphere gradually warms from below the inversion layer erodes away, and surface parcels become warm enough to "break through" the old inversion layer and reach the unstable atmosphere above.
All day the sun's energy was used to heat the lower atmosphere and increase the water vapor content by evaporation essentially adding more energy to the lower atmosphere.
When parcels are finally able to "break through", it is like blowing the lid off of a pressure cooker. This is depicted graphically on this convection cap page from USA Today An illustrative diagram of a convective cap will be discussed in lecture. Click here to view the diagram. A comprehensive explanation behind the formation of tornadoes is given in the NOAA web page: tornadoes The three figures on below display where tornadoes of differing strengths are most common in the United States.
Click on the thumbnails to see larger image. The EF scale is defined in the next section. When that happens the moist air is pushed up. What happens to a blob of moist air as it rises? It cools off and after a while, some of the water vapor turns into liquid drops that we see as clouds. That warms up the rest of the air in the blob the heat of vaporization is returned to us so that it doesn't cool off as fast as it would if the air was dry.
When that blob of air gets to the part of the atmosphere where it is very cold, it will be warmer and less dense than the air around it. Since it is less dense, it will start to rise faster without being pushed, just like a balloon filled with helium does. Conditions in the atmosphere change a lot over a small distance in the vicinity of thunderstorms.
Where the rain is falling, the pressure goes up by a few millibars about 0. This is because as the rain falls, some of it evaporates, which makes the air cooler and heavier. Another process is going on, however, that makes the picture complicated. This low pressure region is also typically a few millibars lower than the environment of the storm.
At the top of the storm the pressure is high compared to places far away from the storm and air is blown out. Thunderstorms can be seen with a variety of tools. They also cause the air pressure in the tornado to drop below normal atmospheric pressure by over millibars the normal day-to-day pressure variations we experience are about 15 millibars.
The air around the vortex is pulled into this low pressure zone where it expands and cools rapidly. This causes water droplets to condense from the air, making the outlines of the vortex visible as the characteristic funnel shaped cloud. The low pressure inside the vortex picks up debris such as soil particles, which may give the tornado an ominous dark color.
A tornado can act as a giant vacuum cleaner sweeping over anything unlucky enough to be in its path. The damage path of a tornado may range from ft m to over 0. Tornadoes move with the thunderstorm that they are attached to, traveling at average speeds of about mph kph , although some tornadoes have been seen to stand still, while other tornadoes have been clocked at 60 mph 90 kph.
Since a typical tornado has a lifetime of about five to 10 minutes, it may stay on the ground for mi km. Occasionally, a severe tornado may cut a path of destruction over mi km long.
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