How a Tornado Forms During Severe Weather

Forming tornado by Roger Attrill
Tornadoes are extremely dangerous and fascinating at the same time. They are awe inspiring to watch, just not too close. There can one tornado or there can be several at one time. You can even see smaller tornadoes dancing around a bigger tornado.

Tornadoes form in severe thunderstorms, peaking in June, starting first in the southern states then moving north. A severe thunderstorm will usually develop along frontal boundaries, such as when a cold front moves into an area. Behind the front is cold polar air and in front is the warm moist humid air, the two colliding different air masses can form large thunderstorms which tornadoes come from.

Tornadoes also form along what is called a dryline. This also occurs when two different air masses meet, usually on the west side of this line it is warm and very dry, to the east of the dryline, it is hot, moist and humid. You can look on and look at the dew point map. Usually in the western high plains you will see an area where the dew points are possibly as low as the teens and just to the east of this area, the dew points are in the 50s and higher. In between is what is called the dry line, and sooner or later thunderstorms will start to form there. If the dryline is in your area, you can actually look in that direction and see the thunderhead clouds actually billowing up right along the dryline.

Another area where thunderstorms can develop and turn severe is along outflow boundaries. These boundaries are created when a thunderstorm dies out or rains out. When this happens the cold outflow of the storm created by the rain and the storm dissipating pushes this cold outflow out away from the storm. When this boundary collides with warmer and moist air, another batch of thunderstorms can develop.

You can also get two or more outflow boundaries colliding. You can see this on your local National Weather Service (NWS) radar, set it to loop. If a thunderstorm in your area has rained out and is diminishing and does push out an outflow boundary, it will appear as a thin line moving on the radar loop. You can watch this when two boundaries collide and watch for new storms to develop in that area.

Before a thunderstorm develops, there are horizontal spinning winds blowing, due to changing wind directions, speeds and heights. When the two differing air masses meet and the storm starts to build, the warm moist air rushes upward where it meets cold dry air, which creates strong updrafts in the storm. These updrafts can encounter winds blowing at different directions within the cloud, which is called wind shear. It is the updrafts that cause the horizontal winds to start to tilt to a vertical motion.

The updrafts also cause the storm to build and that is what you see as it towers higher. It is also these updrafts and downdrafts that cause the hail to form, freezing a little more each time they go back up. If the winds are blowing right and there is enough wind shear it will cause the storm to rotate. This rotation can cause a wall cloud to form, clouds that are a lowering at the base of a thunderstorm. Usually towards the back of the storm or the southwest backside of the storm you will see a lowering of clouds. From this wall cloud is most likely where you will see a funnel. Some wall clouds are very well defined and others you have to really look for.
Wall cloud
A funnel is rotation, a vortex with swirling condensation and clouds. This is produced by the violent up and downdrafts in the storm and the wind shear acting on it turning like a top. A misconception is that this funnel comes all the way to the ground and then becomes a tornado doing its damage. If you see a funnel coming out of the base of a storm, you can bet there is a vortex swirling on the ground underneath it as well. It just might not yet be strong enough to show. If the funnel is at least half way down to the ground, there are probably tornadic winds on the ground.

A tornado doesn’t really touch down it actually spins up. Spinning up as an ice skater moves faster when they draw their arms in. The vortex on the ground starts to spin up and then it starts to pick up sand, dirt, leaves and debris that rise up and meets the funnel and then you have the familiar looking tornado. Just because you cannot see the funnel all the way to the ground doesn’t mean that there isn’t an actual tornado occurring. You can’t always see the connection.

If the tornado is in the middle of rain or hail, you might not see it approaching, this is what is called “rain wrapped”. At night you can’t see them expect in flashes of lightening. Some tornadoes are on the ground a long time and you know they are approaching, other times they just start up suddenly and violently.
Funnel over Manitoba by Justin Hobson
Several times when a thunderstorm was overhead, I could look up and see the green, turquoise and dark roiling base of the storm, I might not see a funnel of any kind, but looking down the street all of a sudden I could see swirling what looked like white mist, then the trees start to blow violently and then small branches and leaves were blowing in a circle. This time it didn’t amount to much as it only lasted a minute. Other times that could have become a full-blown tornado.

Tornadoes usually move towards the northeast from the southwest, but not always. In May 2008, two outflow boundaries met and rapidly formed a severe thunderstorm near Denver’s airport. This storm produced one and maybe more large tornadoes that destroyed part of Windsor, Colorado. This tornado moved from the southeast to the northwest, which is rather unusual.
F5 Tornado near Elie, Manitoba by Justin Hobson

Just because it is calm doesn’t mean there isn’t a tornado nearby. I was in an apartment building and I could only see east. On the radio they were saying a funnel was just to my west. It was as calm as could be and then all of a sudden there were loud pops all over the building as the winds hit suddenly. The pops and bangs were everyone’s doors slamming shut. Then I saw in the parking lot the swirling of dust, leaves and branches. Tornadoes can form and strike that fast. You can also read about How Tornadoes Are Predicted.

Copyright © Sam Montana 2009-2014

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