When the Skies Turn Dark

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Why We Love It

See hundreds of tornado warnings together with geography and seasonality—and sense the terror of weather phenomenon! We love that this map shows how some warnings create a quilt-like pattern across county lines, and others are uniquely shaped to a specific storm. As you click on the map, notice certain locations have dozens of warnings while others, just a few miles away, have almost none. Such is the randomness of these storms.

Why It Works

This map works by giving insight into something reckless and frightening. The top tabs show how time of year affects storm location and frequency. Notice that the further into the future a warning is aimed, the less certain we can be of where the tornado will land. Many warnings are cone-shaped and wider at one end. Interestingly, most cone-shaped warnings trend from southwest to northeast due to humid hot air from the south colliding with dry air from the west.

Important Steps

When working in ArcGIS Online, use smart mapping and pick “single symbol” to show each warning as a solid colored box.

Adjust the transparency slider so each individual symbol is very faint, allowing the patterns to build up.

To isolate warnings by season, apply a filter to certain time ranges.

To create optimal performance in a browser, consider utilizing a hosted tile layer.



The data comes from the rich archive at the Iowa Environmental Mesonet.


Group individual warnings by season, showing three months at a time.


Making this map takes less than 20 minutes in ArcGIS Online. Filtering the data by month is the most time-consuming task.



Since terrain plays a big role in where tornadoes can form and travel, the World Terrain basemap is a perfect choice.

Five Different Maps


Presenting five different maps is easier using the various Story Map templates. Here we used the Map Series template.

Not all maps


Not all maps need legends, especially if the information is as simple and self-explanatory as it is here.

More Information

Map Author

Jeremy Bartley

Jeremy Bartley


Geographer who loves to make maps and visualize data. Interested in simplifying the map-making process. Likes to pick the strings on his banjo.

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