The forecast calls for thunderstorms, so you’re on edge, watching the skies and waiting.
Late afternoon brings those familiar billowing towers with flashes of lightning and far-off rumbles of thunder. As they draw closer, you begin to worry that the storms might bring more than rain and a light show.
Could they produce a tornado?
Tornadoes Are Born From (Some) Thunderstorms
Tornadoes come from thunderstorms, but only a few storms will ever produce one. A specific set of conditions must occur to create a tornado.
Thunderstorms form when warm, humid air near the ground is covered by cold, dry air at higher altitudes. This most often occurs when a cold front pushes into a warm region.
The warm air rises, bubbling up into the colder air aloft. The cold air causes the humidity to condense into raindrops, and those raindrops build up an electrical charge.
When that charge becomes large enough, electricity (lightning) arcs between the ground and cloud, and thunder follows.
Isolated thunderstorms sometimes form, but storms usually form along a cold front. Such a line of storms is sometimes called a storm front or a squall line.
Why Do Some Thunderstorms Spawn Tornadoes?
As thunderstorms form and grow, warm air flows in and up, while cold air and rain fall downward and out. Some thunderstorms develop a spin, or rotation.
If this spin becomes strong enough, a funnel cloud can drop from the bottom of the cloud. When that funnel reaches the ground, it becomes a tornado.
It’s All About the Shear
Wind shear is the primary reason that some thunderstorms rotate. Wind shear happens when winds at different altitudes are blowing from different directions.
For example, on a warm Tornado Alley afternoon, strong south winds bring moist air up from the Gulf of Mexico. At the same time, a cold-front bears down from the northwest, driven by the jet stream. A line of thunderstorms forms along and ahead of the front.
As a column of warm, moist air rises, the low-altitude south wind and high-altitude wind from the northwest cause it to spin. As the storm grows, the spin increases. Finally, the whole thunderstorm is rotating.
Professional stormchasers know that this rotation is a very good indication that the thunderstorm could produce a tornado.
Other Types of Tornadoes
In addition to storm fronts, tornadoes can be created in other ways.
Waterspouts are tornadoes that form over open water. They are usually weak (EF0 or EF1), but they can cause damage if they come ashore.
Spin-Up tornadoes are short-lived tornadoes that pop up in fast-moving storms. They are usually weak and last only a few minutes.
Hurricanes often produce thunderstorms with tornadoes as they come ashore. These tornadoes can be moderate strength, producing significant damage.
So-called Front Range tornadoes come from thunderstorms that form east of the Rocky Mountains. Cold, dry air flowing down from the mountains creates wind shear, causing thunderstorms to spin.
The largest thunderstorms, called supercells, can produce multi-vortex tornadoes. This is when multiple funnels drop from the same storm at the same time. For instance, the 2013 EF5 El Reno tornado produced a main funnel and up to three smaller ones.
Tornadoes come from thunderstorms, but only a few will ever produce one. Weather forecasting and warning systems (such as Doppler radar) have improved greatly over the years. But nobody can predict exactly when a tornado will form.
Pay attention to the weather forecast and warnings when storms form, and be prepared to go to your safe place if a tornado should strike.
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