Tornadoes are the most violent and damaging part of a thunderstorm. Although not in the center of the tornado-producing region of the country, Iowa has a number of tornadoes annually, especially in the western and southwestern parts of the state. Figure 12.23 depicts the annual number of tornadoes per state. The region of most tornado occurrence is to the south and west of Iowa, peaking in Kansas, Oklahoma, and Texas. This is the prime area for tornado researchers and tornado chasers in the spring and early summer.

tornadoaverageweak Fig 12.23 Number of tornadoes per state.

Tornadoes in Iowa occur primarily in the months of May, the peak of the Iowa tornado season, and June. Numbers fall off from these peaks through the summer and into the early spring. During the cold season they are very rare. But they can occur and have occurred in any month. Why is May the peak of the tornado season? We have the greatest contrast between the cold air of winter and the warm, moist air of the approaching summer. Warm air is advected from the south. Cold air is transported aloft from the north. The contrast of air masses produces a very unstable situation.

The most likely time to have tornadoes in Iowa is 6 p.m. There are very few before noon. After noon the number increases to 6 p.m. The numbers drop off after 10 p.m. (Figure 12.24). The number of tornadoes over night is very small compared to the thunderstorm frequency at night. Intense thunderstorms do occur at night. But the tornado-producing ones usually occur in the late afternoon and evening.

Fig 12.24. Iowa tornado frequency distribution (1959-71)(Stanford, 1987).

What is the prime direction of movement? Two-thirds of the tornadoes move in from the southwest; about one-third of them from the west or northwest, and less than 2% of them come from an easterly direction. But they can move any direction. The vast majority that are coming toward you are coming from the southwest or at least from the western half of the circle.

Tornadoes form only when there is strong thunderstorm activity in the area. The day most likely to have tornadoes occurs when there is a strong cold front, with the cold front running almost north and south. Warm moist air will be flowing northward ahead of the cold front. Storms will usually begin over the Plains and move eastward. Nation-wide the tornadoes are most likely in May, but deaths are most likely in April. There are fewer tornadoes in April, but deaths are most likely. Fewer total tornadoes occur in April, but the percentage of severe tornadoes is higher. This is probably due to a few well known April tornado outbreaks.

One outbreak that I experienced, was the outbreak of tornadoes on April 3 of 1974. I was a National Weather Service employee at the time, stationed in Alabama. As we came to work that morning, we saw that there was a cold front laying just like the weather situation is pictured on pg. 116 of your book. I remember Clarence came to work, glanced at the weather map, and said, "Oh, tornado weather." It was no news to any of us. As the day started to develop, the tornadoes began to develop all the way from Illinois to Alabama. This particular one that you see making the long path (on page 116) went through the National Weather Service office at Huntsville. That's not where I was. But I was watching these guys on the computer typing the message: "Tornado on the runway, coming toward tower. Evacu..." They didn't get the word finished. They left the tower. The tower was destroyed. We didn't hear anything more from those guys and we didn't know if they were hurt or not. We didn't hear from there for two weeks until they got their weather office back open up in Huntsville. It was essentially completely destroyed, but no one was hurt.

The super outbreak was the largest single outbreak of tornadoes on a single day. The length of the path of these was great. There were 127 tornadoes in that particular outbreak, 315 deaths, and $600 million in claim damages. Another famous tornado was the Tri-state tornado of March 15, 1925. Reportedly a single tornado, but more likely a series of tornadoes produced a line of damage which extended from southeastern Missouri to southern Indiana. There were 695 dead and 2027 injured in this event.

To learn more about this go to the Tornado Project Online.

Tornados are classified by the fujita scale, a scale developed by a retired professor from the University of Chicago, Theodore Fujita. More about him can be seen in the text on page 126-127. The scale is based on the damage done by the tornado.

Most tornados care classified as F0-F1, which are fairly weak tornadoes with winds less than 112 mph (Figure 12.25).

fujitascale Fig 12.25 Breakdown of tornado strengths.

But the strongest tornadoes are the largest killers (Figure 12.26).

fujitadeaths Fig 12.26 Breakdown of deaths by tornado class.

A number of tornado myths exist. That large cities do not get hit by tornadoes or that rivers protect places are not true. Cases that debunk these myths can be found at the Tornado Project Online site.

Lesson 12 Reflection
Why reflect?
Submit your answers to the following questions in the Student Notebook System.
  1. In your own words, write a short summary (< 150 words) for this lesson.
  2. What is the most valuable concept that you learned from the lesson? Why is this concept valuable to you?
  3. What concepts in the lesson are still unclear/the least clear to you?
  4. What learning strategies did you use in this lesson?