Science, Tech, Math › Science Tornadoes: An Introduction to Nature's Most Violent Storms Share Flipboard Email Print Science Weather & Climate Storms & Other Phenomena Understanding Your Forecast Chemistry Biology Physics Geology Astronomy By Rachelle Oblack Rachelle Oblack is a K-12 science educator and Holt McDougal science textbook writer. She specializes in climate and weather. our editorial process Rachelle Oblack Updated January 29, 2020 Roughly 1,300 tornadoes occur across the United States every year. Explore the basics of tornadoes, one of nature's most unpredictable storms. 01 of 06 Spawned from Severe Thunderstorms Cultura RM/Jason Persoff Stromdoctor/Getty Images There are four key ingredients are needed to spin up severe storms capable of producing a tornado: Warm, moist airCool, dry airA strong jet streamFlatlands Warm, moist air clashing with the cool, dry air creates the instability and lift needed to trigger thunderstorm development. The jet stream provides the twisting motion. When you have a strong jet high in the atmosphere and weaker winds near the surface, it produces wind shear. Topography also plays a major role, with flatlands allowing the ingredients to mix best. How strong of a tornado you get depends on how extreme each ingredient is. 02 of 06 Tornado Alleys: Hotspots of Tornado Activity Dan Craggs/Wikimedia Commons Tornado Alley is a nickname given to an area that experiences a high frequency of tornadoes each year. Within the U.S., there are four such "alleys": Tornado Alley of the southern Plains region that includes the states of Iowa, Nebraska, Texas, Oklahoma, and KansasDixie Alley in the Gulf Coast area, including GeorgiaHoosier Alley includes Kentucky, Illinois, Indiana, and OhioFlorida Don't live in an "alley" state? You're still not 100 percent safe from tornadoes. Tornado alleys are the regions most affected by twisters, but twisters can and do form anywhere. While the weather conditions and topography of the United States make tops for tornadoes of any country in the world, they can and do form in other places such as Canada, the UK, Europe, Bangladesh, and New Zealand. The only continent without a documented tornado is Antarctica. 03 of 06 Tornado Season: When it Peaks in Your State NOAA NCDC Unlike hurricanes, tornadoes do not have a set start and end date during which they occur. If the conditions are right for a tornado, they can occur anytime throughout the year. Of course, there are certain times of the year when they're more likely to occur, depending on where you live. Why is spring considered to be peak tornado season? Spring tornadoes occur most often across the southern Plains and southeastern regions of the United States. If you live in Dixie Alley or anywhere along the Mississippi to Tennessee river valleys, you're more likely to see tornadoes in the fall, winter, and spring months. Along Hoosier Alley, tornado activity peaks in the spring and early summer. The farther north you live, the more likely tornadoes are to occur in the latter parts of summer. 04 of 06 Tornado Strength: The Enhanced Fujita Scale Guenther Dr. Hollaender/E+/Getty Images When a tornado does form, its strength is measured using a scale known as the Enhanced Fujita (EF) scale. This scale estimates what the wind speed was during the tornado, by taking into consideration the type of structures that were damaged and the degree of damage they sustained. The scale is as follows: EF0: 65 to 85 mph windsEF1: 86 to 110 mph windsEF2: 111 to 135 mph windsEF3: 136 to 165 mph windsEF4: 166 to 200 mph windsEF5: Over 200 mph winds Stronger than a Hurricane Wind speeds in a tornado are higher than wind speeds in a hurricane. Hurricane wind speeds in a category 5 hurricane are defined as having sustained winds over 155 mph. Tornado wind speeds can almost double that, the strongest exceeding 300 mph. Even so, hurricanes produce far more property damage, because they are larger storm systems and travel over much greater distances. 05 of 06 Tornadoes and Climate Change john finney photography / Moment / Getty Images Climatologists studying the historical record of tornado reports in the United States have identified evidence for changes that may be related to global warming. Since 1974, there has been a decrease in the number of tornado days with one or two tornadoes called "low-frequency tornado days," and an increase in the number of high-frequency days. Tornadoes that occur on high-frequency days tend to be located further east than those that occur on lower-frequency days. Another change thought to have been the result of global warming is that the tornado seasons have shifted earlier in the year by an average of 12–13 days since the 1950s. 06 of 06 Tornado Safety James Brey/Getty Images According to the NOAA National Weather Service, tornadoes were the leading cause of weather fatalities between 2007 and 2016, with an average of 105 deaths per year. Heat and flooding are the other leading causes of weather-related deaths, and both surpass tornadoes over a 30-year time frame. Most of the deaths are not due to the rotating winds, but the debris rotating inside the tornado. Chunks of flying debris can be carried many miles away as lighter material is lifted high into the atmosphere. To protect yourself be sure to know your tornado risks, alerts, and safe locations in your area. Sources Davies-Jones, Robert. "A Review of Supercell and Tornado Dynamics." Atmospheric Research 158-159 (2015): 274–91. Print.Elsner, James B., Svetoslava C. Elsner, and Thomas H. Jagger. "The Increasing Efficiency of Tornado Days in the United States." Climate Dynamics 45.3 (2015): 651–59. Print.Long, John A., Paul C. Stoy, and Tobias Gerken. "Tornado Seasonality in the Southeastern United States." Weather and Climate Extremes 20 (2018): 81–-91. Print.Moore, Todd W. "On the Temporal and Spatial Characteristics of Tornado Days in the United States." Atmospheric Research 184 (2017): 56–65. Print.