NSSL | Education | Ideas

Ideas for Teaching

• Stereo photography
• Why radar can't (usually) see tornadoes
• Graphing information
• Vectors for wind speed
• Calculating %
• Simple math
• Not so simple math
• Wildflowers & rainfall

Calculating Percentages

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Introduction

If you have been to the Tornado Project web site and read about the Fujita Scale you saw a pie chart showing the frequency of various strengths of tornadoes. They are, in tabular form:

The percentages above were calculated by looking at all recorded tornadoes from 1950-1994. The data is certainly noisy, since tornadoes are a rare event. It's also certainly noisy since tornadoes occuring over open land may not be reported. Is 44 years of data enough to see an accurate distribution? We can't really say, but this is the longest dataset we have. Looking at these tornado reports over time shows an increase in reports of nearly two orders of magnitude since 1950 to the present. Much of the increase is likely due to increased efforts to collect the data. A scientist at the National Severe Storms Lab only uses data from 1980 onward for all tornado ratings, and reports from 1921-present for F2 and greater. [For a slightly longer discussion on this topic, see how he collected and processed data.]

Sample Size

We can look at tornado reports for any county in the US and calculate the percentages for that county. But when starting work like this a question to ask is how much data do I need to look at to get an accurate result? There is no clear answer: you need to have enough data in your sample to have the same distribution as if all data were used. Although the above data has some uncertainty, its the best anyone knows so we will use it.

Let's calculate some percentages! On to page 2...

The dataset started as the official one used by the National Oceanic and Atmospheric Administration and archived at the National Climatic Data Center. The Tornado Project went back to read descriptions of tornadoes prior to the creation of the Fujita Scale around 1970 to rate older tornadoes. In fact, they went back before 1950 and published their results in a book, Significant Tornadoes--1680-1991, and supplement, Significant Tornadoes Update--1992-1995.

Last updated: March 26, 2002
Created by: DSZ