Analyzing Tornadoes from a Spatial Perspective Using GIS

Spring in North America brings not only new flowers, but a new crop of tornadoes. As we are all aware, the 2011 tornado season has already been horrific, and our hearts go out to all those affected.

Like most natural phenomena, tornadoes exhibit a spatial pattern on a continental, regional, and local scale, and can be examined and understood with the use of GIS.

A lesson that includes a dataset of tornadoes spanning a 40 year period that I developed on ArcLessons (visit and search on “tornado”) easily gives one a sense of the distribution of tornadoes. Downloading this free lesson and data and examining the dataset within ArcGIS desktop indicates that while far more common on the Great Plains and interior lowlands, tornadoes have occurred in nearly every state, and are not as uncommon in the mountain west as one might think. Contrary to popular opinion, the data also reveals that Kansas is not the area with the highest density of tornado outbreaks. Do you know what state has the highest density? See below for the answer.

Selecting the tornadoes month by month shows the seasonal ebb and flow of the outbreak of tornadoes, starting from coastal areas near the Gulf of Mexico in January and increasing to a spatial maximum in July of each year. Interestingly, the numeric maximum occurs in April, three months earlier than the spatial maximum. Examining the data by time reveals still more patterns. During which six hour period do you think the most tornadoes touch down—between midnight and 6:00am, 6:00am to Noon, Noon to 6:00pm, or 6:00pm to midnight? Examining the data from 1950 to 1990 reveals that the tornado causing the most injuries (1,740) occurred in northern Texas on 10 April 1979 and the one causing the most fatalities (116) occurred in northeast Michigan on 8 June 1953. The lesson also invites you to discover in which elevation range tornadoes are most common.

Can current and recent tornadoes be examined within a GIS environment? Absolutely. One way is to scroll down on the NOAA Storm Prediction Center ( and enter a date. I wanted to map the tornadoes that occurred the day when one touched down at the St Louis airport, so I entered 110422 for 22 April 2011. I obtained the following page: From this page I obtained a map and a CSV file of these data, which I analyzed via the “AddXY” tool in ArcMap. This revealed that the airport tornado was only one point along an entire line of tornadoes that day in that region. This same NOAA page provides data allowing you to compare the pattern of tornadoes to other weather phenomena, such as hail and wind:

Another way of analyzing current and recent tornadoes is with ArcGIS Online. Access the Esri disaster response page ( and select severe weather. This is only one of many related resources housed at ArcGIS Online. In another example, access and search on “Tuscaloosa owner:jjkerski.” Opening up this map reveals pre- and post-tornado imagery that you can toggle on and off or adjust the opacity. You can also measure the width and length of the Tuscaloosa tornado and assess which buildings were affected and which ones were not. With this very sobering data set, you can use the tools available to measure the width and length of the track of this violent tornado.

Joseph Kerski, Geographer, has a background in government, academia, and industry. He serves as Education Manager for Environmental Systems Research Institute (Esri) in Colorado, USA. He served for 21 years as Geographer at the US Geological Survey and at the US Census Bureau. Follow Joseph on Twitter @josephkerski

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