One of the most severe forms of weather in Iowa is the tornado. Tornadoes cause extensive property and crop damage, injuries and even death. Iowa ranks sixth in the United States for tornado frequency, averaging 46 tornadoes each year.
Warm, Moist Meets Cold, Dry
Tornadoes form in powerful storms usually created when warm moist air from the Gulf of Mexico meets cold, dry air from the north or northwest. The warm, moist air is held below the stable, cooler layer until a disturbance, such as a front or strong crosswind, helps the lower air mass break free. The warm moist air then spirals upward, fueled by varying wind velocities at different levels of the storm. Gradually, this updraft gains speed and a funnel cloud is formed. Once the funnel cloud touches down and comes into contact with the ground, it becomes a tornado.
Worse Than a Hurricane
The average tornado is a quarter-mile wide and the damage is 16 miles long. Tornadoes produce some of the most destructive winds on earth. Because of its high winds and severe weather, it is difficult to directly analyze a tornado with wind speeds, however based on inferred wind speed and the amount of damage, it is estimated some tornados a can produce wind speeds up to 200 miles per hour (mph) or more. In contrast, Hurricane Katrina in 2006, one of the largest and most destructive hurricanes ever recorded, had maximum winds recorded at 175 mph.
Predicting a Tornado
Although tornadoes tend to be unpredictable, a few generalities can be made. Most tornadoes emerge from afternoon thunderstorms between the hours of 3 p.m. and 9 p.m. During this time period the heat of the day creates the warm, moist air needed for tornadoes to form. Tornadoes usually travel northeast at an average of 30 mph, but range from nearly stationary to traveling as fast as 70 mph.
In 1860, a tornado killed 141 people in Comanche. In 1882, 100 were killed in Grinnell, while in 1893, 71 were killed in Pomeroy. Though these numbers are estimates, they show the devastating nature of tornados when early warning was not possible. Tornado forecasting saw dramatic improvements beginning in 1953. Since then, Doppler radar has been developed, allowing meteorologists to detect increasing wind rotations within storms. Meteorologists can then issue storm warnings, giving people more time to find shelter. Between 1950 and 1995 there were 1,416 tornados and only 61 deaths, showing how early warning can greatly improve the ability to survive a tornado.
- Pamela Holz, Ed., "Iowa Weather" Iowa Natural Resource Heritage Series. (1999) Iowa Association of Naturalists: 18.