Iowans are quite familiar with the severe weather that can rake across the state in the spring and summer months. Located at the northern end of Tornado Alley, Iowa typically sees about 50 tornadoes a year, along with frequent severe storms that can unleash dangerous hail and straight-line winds, and hundreds of additional thunderstorms that can produce deadly lightning. However, the chances that a tornado will strike your exact location are very low, and most people will only experience a few severe thunderstorms in a given year. Therefore, it is crucial that you are ready each year for severe weather and have a safety plan in place. Hopefully, you will never need to implement your safety plan, but having one can mean the difference between life and death in a dangerous weather event.
The remainder of this page contains a quick introduction to the various types of severe weather one can expect in Iowa, while the links at the top will help you develop a safety plan, become familiar with weather forecasts and warnings, and provide tips on staying safe when severe weather threatens.
An Introduction to Iowa Severe Weather
A tornado is defined as a violently rotating column of air generated from a thunderstorm in contact with the ground. Winds in a tornado can range anywhere from 50 to 200+ mph. Tornadoes take on a variety of shapes, sometimes resembling a thin rope that is only a few feet wide, other times appearing as a monstrous mile-wide wedge that fills the horizon. Rain sometimes surrounds a tornado and makes it impossible to see until it is too late. In addition, tornadoes may last only a minute or so, while others may stay on the ground for over an hour.
Iowa has experienced tornadoes during every month of the year, but they are most frequent from March to July, with the climatological peak in activity occurring in June. This does not mean that violent and deadly tornadoes are confined to this five month window. Deadly tornado outbreaks have occurred in Iowa as early as late January and as late as mid November. Tornadoes tend to touch down during the late afternoon and early evening hours, but overnight and morning tornadoes are also possible.
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Straight-line wind is a term used to describe non-tornadic winds generated by severe thunderstorms. These winds have their origins aloft in the thunderstorm, where rain cools the air in its immediate vicinity. This cold air accelerates downward because it is denser than the surrounding air mass, and spreads out across the ground upon reaching the surface. These winds are typically in the 50 to 70 mph range, but in rare cases can exceed 100 or even 115 mph (similar to a Category 3 hurricane). Unlike tornadoes, downed trees and other debris trails are oriented in a single direction, hence the term "straight-line" winds.
Even though straight-line winds are not as strong as large tornadoes, they still have the ability to uproot trees, down power lines, damage buildings (especially grain bins, storage sheds, and other similar structures), and flatten crops. High profile vehicles are also vulnerable and can be flipped or forced off the road by these winds. Falling trees and other debris pose a hazard to anyone in their path; many deaths in straight-line wind storms are attributed to trees falling onto people in their cars or homes. Downed live power lines can easily electrocute and possibly kill anyone who comes in contact with them. Straight-line winds are responsible for most thunderstorm wind damage, and can cause damage equivalent to an EF-2 tornado. However, while a tornado damage track is relatively short and narrow, the damage swath from a straight-line wind event can be tens of miles wide and affect thousands of square miles.
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Hail is frozen precipitation that falls from a thunderstorm and can grow to the size of softballs or larger, but is generally less than two inches in diameter. A strong thunderstorm updraft (rising air in a storm) is key for the production of hail. As rain near the base of the storm gets caught in the updraft, these drops get lofted high into the storm and freeze into small ice pellets. These ice pellets cycle through the updraft, repeatedly collecting more water and growing larger as they are lofted into the storm and freeze. Once the thunderstorm updraft can no longer support the hailstones, they fall to the ground. A number of meteorological factors can influence hail sizes, but the strength of the thunderstorm updraft (therefore, the strength of the storm itself) is the most critical.
Hail poses a serious threat to anyone outside and outdoor property. Large hailstones can fall at speeds of over 100 mph and easily injure or kill anyone caught in their path. Pets and livestock are also susceptible to injury or death by hail. In addition, strong winds during a hailstorm can amplify the effects of small hail and damage the sides of buildings. Close to a billion dollars in property damage is caused by hail each year, mostly to automobiles, house roofs, and crops.
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While undoubtedly the most commonly experienced severe weather threat, lightning is by far the deadliest of these phenomena. An average of 58 people are killed and over 300 injured in the United States each year by lightning, making it the third deadliest weather phenomenon (behind heat and floods). Lightning develops as the result of an electrostatic charge build-up in a thunderstorm, which is caused by colliding ice crystals high in the storm. This charge continues to build over time, with the bottom of the storm becoming negatively charged and the ground becoming positively charged. When the electric charge becomes too great, a large bolt of electricity, a lightning bolt, travels from one charge source to the other.
The temperature of lightning can exceed 50,000°F, over five times hotter than the surface of the sun. This super-heats the air around the bolt, producing a shock wave that we hear as thunder. Because sound waves travel much slower than light waves, the distance from a lightning bolt can be calculated using the time that elapses from when a lightning bolt is seen to when thunder is heard. Sound waves travel approximately one mile every five seconds, so one can divide the elapsed time by five to find the distance (in miles) between them and the lightning bolt. For example, if the time between the lightning strike and thunder is 20 seconds, 20 divided by 5 would be 4 miles.
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