Playing with Data

Personal Views Expressed in Data

Population Affected by Tornadoes and Tornado Warnings

I have been curious as to the number of people warned for tornado warnings during the Storm Based Warning Era for awhile. Today I needed a mental break from my dissertation so I spent 20 minutes and calculated the numbers. Then, as it typically happens, I started to ask additional questions, such as "What's the number of people impacted?" I decided to do that as well. Below are the results.

To compute the total population that was warned, I took each warning, gridded it on to the 5-km population grid I have, and then counted the number of people inside that polygon. The population grid is taken from the 2010 census, so the number of people warned is an approximation and is also normalized to 2010 numbers. To calculate the number of people impacted, I gridded each tornado track, using the data from the Storm Prediction Center's WCM Data Page, onto the population grid. This resulted in a tornado track that was 5-km wide. Assuming a tornado width of 1km (which is a large tornado!) this gives me the number of people who were approximately within 2-km of the tornado track. I repeated the process with 3 grid point line-width (+/- 7-km) and with a 5 grid point line width (+/- 12-km). I then summed the population totals for each of these scenarios. Lastly, I calculated the percentage of people warned that were impacted for each of the radii thresholds (+/- 2-km, +/- 7-km, +/- 12-km). This is presented in the tables below as the percentage in parentheses.


Please note these are approximations. The population counts are based on census data and are only as good as the census data. It does not account for transients (i.e., people who were just passing through the area). Furthermore, if an area was warned twice, those people were counted twice. The number of people warned (presented here) is not the same as the number of unique people warned.

One last, important caveat. A tornado that occurred, but was not warned, will still count toward the population impacted. The population impacted by tornadoes is exactly what it says it is. It is not a conditional count requiring a warning to have been issued.

In any event, here's the results. I'm interested in what you guys think. Feel free to leave suggestions as well, but just know that I'm headed back to working on my dissertation so it might be a week or so before I can act on any of these suggestions!

Number of People Warned For
Tornado Warnings

  • 2008: 134,180,576
  • 2009: 99,676,144
  • 2010: 133,951,616
  • 2011: 144,896,400
  • 2012: 90,455,328

Number of People Impacted By
Tornadoes (+/- 2 kilometers)

  • 2008: 5,772,576 (4.3 %)
  • 2009: 3,120,432 (3.1 %)
  • 2010: 4,935,376 (3.6 %)
  • 2011: 7,068,976 (4.8 %)
  • 2012: Official Tornado Data Unavailable At This Time

Number of People Impacted By
Tornadoes (+/- 7 kilometers)

  • 2008: 15,949,392 (11.8 %)
  • 2009: 8,370,304 (8.3 %)
  • 2010: 15,872,896 (11.8 %)
  • 2011: 22,155,984 (15.2 %)
  • 2012: Official Tornado Data Unavailable At This Time

Number of People Impacted By
Tornadoes (+/- 12 kilometers)

  • 2008: 23,074,960 (17.2 %)
  • 2009: 12,152,944 (12.2 %)
  • 2010: 22,187,520 (16.5 %)
  • 2011: 33,982,416 (23.5 %)
  • 2012: Official Tornado Data Unavailable At This Time