As Iowa enters spring and storm season with May and June accounting for 60-to-70 percent of the year's tornados, the familiar anvils that indicate thunderstorms will become more common.

Thunderstorms require three factors in order to form — convective (or unstable) air, moisture and updraft/downdraft caused by a warm front rapidly rising over a cold front.

A storm's strength can be determined by how well defined its tower or "anvil" is. A weaker storm will have a less definite tower, whereas a stronger storm will have a well-defined anvil shape above it. If this anvil has a dome above it, the storm is even stronger. The anvil or tower is also the location of the strongest updraft and is usually the place where tornados will form, said Jeff Johnson from the National Weather Service.

The anvil or tower is the location of the strongest updraft in the storm. The rising warm, moist air billows up above the rest of the storm, and the strength of the updraft determines how well-defined the anvil is. A stronger updraft correlates with a better-defined anvil, while a weaker updraft correlates with a wispier anvil.

There are several types of thunderstorms, Johnson said. Multi-cell clusters, which are multiple small thunderstorms; squall lines, which have great rainfall and can have damaging winds; supercell storms, which are the strong isolated storms that spawn most tornados; and  high-precipitation supercells, which also have a great amount of rainfall and can produce tornados.

Some thunderstorms can spawn tornados, some of which are small spouts that cause little to no damage, while others are massive walls of clouds and debris up to 1 1/2 miles wide that can leave communities in ruins.

"If it's a weak tornado, you can see the funnel, and it's just like a vortex," said T.C. Chen, professor of Dynamic Meteorology. "When it's strong, it has a very strong convergence of air."

 The vortex of the tornado is comparatively small, but the reason that some can cover up to a mile and half is because stronger tornados have a faster-spinning vortex. Because of this, more air is drawn in creating the intense winds, which create part of the debris field often seen at the base of tornados, Chen said.

In very intense tornados like the F-5 in Parkersburg, most of the damage is caused by the air being pulled into the vortex rather than the vortex itself.

"The damage of a tornado looks very destructive and very impressive, but the scale is so small," Chen said. "But one drought can affect the economy of Iowa."

Those who spot severe weather are encouraged to call the Severe Weather Spotter Hotline at 1-800-SKYWARN.

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