Article - September 19, 2019

An Expert Meteorologist Answers Your Tornado Questions


While tornado season is generally considered to be in April and May, fall is the second most common time of year for tornadoes. With 30 years of forecasting experience, Senior Meteorologist Dave Gorham shares insight into tornadoes, how to forecast them, and how to stay safe.

Tornadic supercell in the American Plains

What conditions does a tornado need to develop?

Severe weather, including tornadoes, is often caused by cold air colliding with warm air, which can happen any time of year. It’s often thought that hot air rises, but actually, hot air only rises when it’s hotter than the air surrounding it. As air rises into a cool atmosphere, it condenses to create clouds—small cumulus clouds on a summer afternoon, or monstrous cumulonimbus clouds along a cold front. The greater the contrast between temperatures, the more rapidly the air rises. When the air rushes upwards fast, it can be strong enough to rip the wings off an airplane. When updrafts are at their peak, cumulonimbus clouds can exceed 55,000 feet high (more than 10 miles)!

As air rises inside the cumulonimbus supercell cloud, it turns counterclockwise. This rotation combined with winds and updrafts/downdrafts creates an environment conducive to tornado formation.

How strong can tornadoes become?

A tornado on the ground is one of nature’s most destructive forces. Violently rotating winds can reach up to 300 mph (480 kph). Supercells are massive and can create tornadoes as wide as two miles (three km) and if the exact conditions persist long enough, a tornado can travel dozens of miles—creating an extensive path of destruction. Most tornadoes are much smaller, with winds of around 110 mph (180 kph), a width of 250 ft (80 m) and travel only a mile or two (2-3 km). These smaller or weaker tornadoes can still be very destructive.

How long does a tornado typically last?

Tornadoes can last from several seconds to more than an hour, though they typically last for about ten minutes. The average distance tornadoes travel is about 3.5 miles (5.6 km).

How are tornadoes forecast?

Forecasting a specific tornado is difficult even an hour before it’s visible on weather radar. Where exactly a tornado forms, when it will touch down and for how long it will remain on the ground are some of the most difficult considerations a weather forecaster will face. This is because the dynamics within a supercell cumulonimbus are complicated and not entirely understood by meteorologists. Conditions that are ideal for tornado formation do not always produce a tornado. However, the atmospheric conditions that allow cumulonimbus supercell development are well understood and allow weather forecasters to identify suspect regions days or even a week in advance of an actual tornado reaching the ground.

That being said, research in this area continues. Some have found that the location and frequency of lightning in the cloud can be used to predict tornado development. This technique is especially useful in remote areas beyond the range of weather radar.

What should I do to prepare for a tornado? What can I do once the tornado warning is made?

Being weather aware and tuned into local media and weather reports is the best way to prepare for a tornado. If you’re aware of the potential for tornadoes, you increase the margin of safety for you, your family and your employees. In the United States, tornadoes have occurred in every state and month of the year. Though most common during the hottest time of day (late afternoon), tornadoes can form at any time of day or night. Your awareness toolkit should include social media, including Facebook and Twitter—where your network and local authorities are posting critical storm information. Be sure your smart phone has weather apps that include radar and information from the National Weather Service.

Tornado safety tips:

Seeking proper shelter is the most important tip. However, in order to do this, you must identify what the nearest proper shelter is.

  • In a house: Identify a safe place in your home where household members and pets can gather during a tornado. This may be a storm cellar, basement, or an interior room on the lowest floor that does not have windows.
  • In a high-rise building: If you do not have enough time to get to the lowest floor, pick a hallway in the center of the building.
  • In a building: If in a school, church, theater, office building or similar facility, go directly to an enclosed, windowless area in the center of the building—away from glass and on the lowest floor possible. Then, crouch down and cover your head. Using the elevator or descending multiple levels of stairs uses precious time and it may be best to seek interior shelter on the floor where you are located.
  • In a mobile home: Choose a safe place in a nearby sturdy building. If your mobile home park has a designated shelter, make that your safe place. No mobile home, no matter how it is configured, is safe in a tornado.
  • If outside: Find a nearby building preferably with a basement. If you are in a car, do not try to outrun a tornado. If no shelter is nearby, lie flat and face-down on low ground, protecting the back of your head with your arms. Get as far away from trees and cars as you can.

For added protection, get under something sturdy such as a heavy table or workbench. Cover your body with a blanket, sleeping bag or mattress. Protect your head with anything available.

What are some common myths about tornadoes?

Myths related to tornadoes and tornado safety originated long ago – well before we understood the meteorology and science of tornado formation and movement. Here are some that continue to be believed:

  • Opening windows reduces tornado damage. This theory suggests that opening the window just before a tornado strikes equalizes the inside pressure with the outside pressure, thus preventing the house from exploding. Even with the most violent tornadoes, there is only a pressure drop of about 10%. This pressure difference will be quickly equalized without damage to the house but even if the pressure difference is great, the windows will break first, thus equalizing the pressure. If your house is going to be damaged by a tornado, it will not be from unequal pressure. Don’t waste time opening windows in advance of a tornado that could be better spent seeking shelter.

  • Use highway overpasses as shelter. Most overpasses provide little or no shelter to tornado winds and flying debris. Though there are documented cases of people surviving under highway overpasses, experts warn against using them for protection. In fact, evidence suggests that overpasses may be the worst place to be as they may actually create a "wind-tunnel" effect under the span, further increasing the wind speed and debris.

  • You can outrun a tornado in your car. Although cars can travel faster than the average tornado, the directive from the National Weather Service is for house-dwellers in the path of a tornado to take shelter at home rather than risk an escape by vehicle. Cars can be heavily damaged by even weak tornadoes, and in violent tornadoes they can be thrown long distances. Additionally, the same thunderstorms which produce tornadoes can also produce flooding rains, hail, lightning and strong winds far from the tornado-producing area, all of which can make driving difficult and dangerous. Instead, leave your car parked safely and seek shelter in substantial shelter or a ditch if no shelter is nearby. However, distant tornadoes can be successfully avoided by driving at a right angle (90-degrees) from the tornado’s direction of apparent movement.

  • Green clouds indicate the presence of hail and/or tornado formation. Maybe, maybe not. Sunlight passing through rain can be reflected in such a way as to make clouds appear green. Hail or tornadoes do not always form in these conditions, though it is possible.

  • Tornadoes only form on flat land. Unfortunately, this appears to be wishful thinking by mountain realtors. Tornadoes can form anywhere and are not impeded by mountains, hills, forests, rivers or even large cities.

  • Springtime is tornado season. It is true that springtime is the most common time of year for tornadoes and more tornadoes occur in the spring than in any other time of the year. However, tornadoes have occurred in every month of the year.

  • Nighttime tornadoes are more dangerous. Nighttime tornadoes are influenced by the same meteorological conditions as daytime tornadoes and so are not inherently stronger (or weaker) based on the time of day. However, they are considered to be more dangerous because they can strike when people are sleeping and not tuned into local weather or other media reports. These “surprise” tornadoes strike when people haven’t had time to seek adequate shelter, meaning that the amount of injury and death can be higher.

  • Tornadoes only occur in the southern United States. Tornadoes have occurred in every state in the US. However, southern states typically have the greatest combination of the meteorological and geographic features that favor or enhance tornado development.

  • Climate change will increase and intensify tornadoes. This is unknown. The warming global atmosphere will add additional moisture into the atmosphere, which would help fuel the formation of more clouds, more rain, more severe weather and – by inclusion – more tornadoes. However, a warming atmosphere could also will reduce the temperature contrast that is critical for tornado development, thus reducing their frequency. It’s reasonable to suggest that climate shifts may impact tornado development and frequency, but there are far too many unknowns to be decisive. Additionally, any impacts would vary greatly by region.

  • We know that Sharknado is fictional, but is it true that animals have been sucked up by tornadoes and deposited miles away? Fish, frogs and other animals including, famously, cows, have been “relocated” by tornadoes. Strong updrafts that feed the supercell cumulonimbus clouds can lift animals up into the storm and deposit them elsewhere. This is true for more than just animals—there are reports of vehicles, even large tractors, being found miles away from where they were before the storm.

waterspout at sea

What is a waterspout?

A waterspout is a tornado over water and are common along the southeast US coast. They can happen over seas, bays and lakes worldwide. Although waterspouts are always tornadoes by definition; they don't officially count in tornado records unless they hit land. They are smaller and weaker than the most intense Great Plains tornadoes, but still can be quite dangerous. Waterspouts can overturn boats, damage larger ships, do significant damage when hitting land, and kill people. The National Weather Service will often issue special marine warnings when waterspouts are likely or have been sighted over coastal waters, or tornado warnings when waterspouts can move onshore.

Tornado Alert Terms:

Tornado warning: A tornado warning is issued when a tornado has been sighted or has been indicated on radar in your area. When this happens, anyone in the area must immediately take shelter in a safe, sturdy structure. Ideally, this shelter area has a battery-operated radio so that you can stay informed of the up-to-the-minute conditions from the media and your local weather office.

Tornado Watch: A tornado watch is issued to alert people to the possibility of a tornado forming in their area. There has been no tornado sighted, but conditions are favorable for tornado development. Stay alert, listen to local news and weather reports, and make sure your family and co-workers are aware of the threat and ready to quickly seek shelter.