What causes tornadoes and how to decipher watches and warnings

When severe weather hits the United States, there are ways people across the country can prepare for the potential impact of hail, rain, damaging wind and more.

Tornadoes can take down power for thousands of people, cause catastrophic property damage and kill. The U.S. sees about 1,200 tornadoes each year.

Experts say watching forecasts and knowing the meaning of different stages of warnings are key to safety.

Here's what to know about the causes for these storms, and the alerts for watches, warnings and emergencies.

What can cause a tornado?

The large areas of low atmospheric pressure that come with some storm systems can spawn tornadoes. Low pressure is associated with stormy weather, and the lower it is, the stronger a storm can be. Low pressure systems draw air into them, creating those rains and winds.

“In the spring and the transition seasons — spring and fall and even winter — we get these really strong, what we call extratropical low pressure systems,” said Victor Gensini, a professor of atmospheric sciences at Northern Illinois University. “These are just storm systems that occur in the mid-latitudes of each of the hemispheres.”

“If you think about a map of the United States and you draw a big U across the map, that would be a big trough,” he added. “That big trough helps promote the creation of this area of low pressure and strong storm system.”

How do storm systems turn into tornadoes?

These storm systems cause very strong winds and hail.

Combine that with the side of the low-pressure storm system that has warm and humid air that wants to rise, and there's possibility of thunderstorms.

In the right conditions — warm moist air close to the ground, cooler dry air above and wind changes — the storms could lead to tornadoes, according to the National Weather Service.

“In that type of atmosphere where it’s warm down low and cold aloft, and it’s also humid down low, you can get air to rise through thunderstorm updrafts at a pretty good clip, and that leads to potentially severe weather,” said Paul Markowski, professor of meteorology at Penn State University. "Usually the stronger the thunderstorm updraft — the updraft is just the term for where the air is rising — the more potential you have for things like damaging winds, hail, and potentially even tornadoes.

“The other ingredient you need in addition to that temperature variation with height is what’s called wind shear," he said, "which is the term that refers to how much the winds change as you go from the lower atmosphere to the upper atmosphere.”

What are differences between a tornado watch, warning and emergency?

The weather service's Storm Prediction Center issues a tornado watch in an area, up to 10 hours ahead of a storm, when conditions suggest possible tornadoes. People in that area, which usually covers several counties or states, should then be prepared for a potential warning.

A warning is issued by local forecast offices when weather radar indicates a tornado is coming or has already been seen. This typically encompasses a smaller area such as a city or county. People in the area need to take action, such as moving to a shelter on the lowest floor of a building, and by avoiding windows.

Tornado emergencies are the weather service's highest alert and are issued when a tornado has touched down in an area and catastrophic damage has been confirmed. They are rare, but require immediate shelter in place.

What’s the best way to prepare?

People should have multiple ways to get watches and warnings and know the definitions of each, experts say. Weather radios can be especially helpful if power is lost and TVs and ordinary radios no longer work.

It’s also important to have safe, enclosed and underground shelters — or rooms above ground that are equipped to handle tornado-level winds — prepared in advance.

“In a lot of the parts of the country affected by tornadoes, there is no underground option," Markowski said. "You’re not going to be any better off leaving your house, frankly. So if you’re in your house, just put as many walls between you and the tornado. Get away from windows. An interior room like a bathroom is a good spot.”

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