Chapter 10 THE HISTORICAL RECORD OF TORNADOES Goals: To use data derived from the official tornado archives of the Storm Prediction Center to study the characteristics of tornadoes in many states during the period 1950-1995.

Data:

Tabular information about each tornado that was observed during the period 1950-1995 in the following states:

Arkansas, Colorado, Connecticut, Florida, Louisiana, Michigan, Minnesota, Montana, New Hampshire, New Mexico, New York, North Carolina, Ohio, Pennsylvania, South Carolina, South Dakota, Wisconsin.

Procedure:

1. For a state (or states) of your instructor's choosing, graph the total number of tornadoes as a function of year. Also, graph the number of tornadoes rated F2 or higher as a function of year. Do the trends that you see in the graphs agree with the national trends in tornado frequency?

2. Obtain a county map of the state(s) with which you are working. In the tabular data, the last column is called "COUNTIES." In this column, the county (or counties) in which a tornado touched down is indicated by a number (or numbers): use the county code tables and count the number of tornadoes in each county. Then plot these numbers on your county map. Are there any regions in the state(s) that have a higher tornado frequency?

3. Because states vary in size, a more accurate measure of tornado threat in each county would require dividing by the area of the county. Obtain this area data for the state(s) with which you are working. Compute tornado frequency per unit area for each county (an appropriate unit might be "tornadoes per 100 square miles"). With this new measure of tornado threat, are the regions of highest risk different than in the last question?

4. To compare tornado frequency in Southern States with that in Northern States, let's use Louisiana, Arkansas, Wisconsin, Ohio, and New York (in this order, the states first move north and then east, which in general parallels the progression of tornado season). For each state, plot the number of tornadoes as a function of half-month. That is, count the number of tornadoes in each state during the period January 1-15, January 16-31, February 1-15, February 16-29, ... , November 16-30, December 1-15, December 16-31. Does the peak time of year for tornado frequency occur at different times of the year in different parts of the country? If so, explain.