When the Skies Turn Dark

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

Pick a symbol to show each warning

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

Learn styling and symbol tips
Make the symbol transparent

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

Change transparency of a layer
Apply filter to time ranges

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

How to apply/work with existing filters
Use hosted tile layer for faster usage

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

Publish hosted tile layers from files

Data and software

The data comes from the rich archive at the Iowa Environmental Mesonet. Software used: ArcGIS Online.

View IEM data
Analysis

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

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

Provide a neutral background

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

View World Terrain basemap
Browse Map Series templates

Presenting five different maps is easier using the various Story Map templates.

Build a Map Series
No need for legends

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