Tornado Season

Spring for many parts of the country means tornado season.  Looking at the map below you can see where tornadoes occur most frequently.  You will notice that eastern Colorado is included in “Tornado Alley”.  Here along the Front Range we also see tornadoes, but due to our elevation they tend to be the weaker variety, instead of some of the monsters that the Plains states see.  

Although tornadoes occur throughout the world, including India and Bangladesh, they are most intense and devastating in the United States. Tornadoes can strike at any time of day, but are much more frequent in the afternoon and evening, after the heat of the day has produced the hot air that powers a “tornadic thunderstorm” — a thunderstorm that produces a tornado.

Tornadoes are common in Tornado Alley because of the Rocky Mountains to the west and the Gulf of Mexico to the south. In spring, a strong westerly jet stream flows across the Alley, creating instability and a trough of low pressure that draws warm, moist air in from the Gulf. “Conditions for the supercells [large, powerful thunderstorms] that spawn tornadoes require strong vertical wind shear [changes in wind speed and direction with height] and lots of instability. And that’s exactly what happens in Tornado Alley.

The fact that supercells occur where warm, moist air meets cold, dry air suggests the source of energy for both t-storm and tornado: latent heat in the warm, moist air. Latent heat is heat you can’t detect with a thermometer. An air-conditioner struggles to remove humidity from Houston’s soggy July air because the air holds so much latent heat. Latent heat is the heat that is released to the atmosphere when water vapor condenses into liquid water.  Heat is added to liquid water to turn it into a vapor.  Thunderstorms release massive amounts of this latent heat as large amounts of air are lifted, cooled, forming clouds, rain and hail.

A tornadic thunderstorm can form where moist, warm air gets trapped beneath warm, dry air under a stable layer of cold, dry air. This air sandwich is called an inversion.

If the cap is disturbed by a front or movement in the upper atmosphere, the warm, moist air can punch through the stable air above it. The condensing of water vapor releases latent heat, and the warm air starts to spiral upward.  Aided by different winds at different levels of the atmosphere, the rotating updraft gains velocity.

We look for several ingredients when looking for potential tornado formation:

1. A large, layered sandwich develops in the atmosphere. Hot, humid air is trapped beneath cold, dry air.

2. The “cap” (a layer of stable air between the hot and cold air) is disturbed by winds in the upper atmosphere or by the arrival of a weather front.

3. Lower-level air rises and expands in the reduced air pressure aloft.

4. As the air cools, moisture condenses, releasing latent heat which warms the air, making it buoyant, and causing it to rise at speeds up to 150 mph. By now, the cloud is a thunderstorm. Upper-level winds tilt the thunderhead, creating the anvil shape.

5. The thunderstorm may die out in intense rain and/or hail. Or it may spawn a tornado.

6. Interactions between air at various altitudes, humidities and temperatures cause rain, lightning, air circulation and strengthening of the rotating updraft, now called a “mesocyclone.” Low-level wind helps cause this rotation, which is almost always counter-clockwise (seen from above) in the Northern Hemisphere.

7. A tornado may form below the mesocyclone. As the spinning air column narrows, it rotates faster and extends higher into the storm.

That, much simplified, is one theory of the origin of tornadoes. Scientists do not fully understand the first stage of formation, however. Some suspect that wind shear — differences in wind speed at different altitudes – may start the vortex. Just as, when you roll a wad of clay in your hands, it eventually becomes a rotating column of clay, when the different bodies of air move against each other, they create wind shear and then a tornado.

In the picture to the below, stronger west winds aloft start a rotating column of air.  As strong updrafts develop in a thunderstorm this column is brought vertical and we have the start of a tornado.

Much more common along the Front Range are funnel clouds.  Many people see these and think that they are tornadoes.  A funnel clouds never touches the ground, while a tornado stretches from the cloud base all the way to the ground.  This is not to say that if you see a funnel cloud it is safe, most tornadoes start out as funnel clouds.

Although we do not see the big tornadoes that the eastern plains see, they certainly can cause damage.  Even the weaker tornados have winds speeds in excess of 100 mph.  By comparison the strongest tornados on the plains have winds in excess of 200mph, some approach 300mph.