When the Skies Turn Dark

View Live Map

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 coloured 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 utilising a hosted tile layer.
    Publish hosted tile layers from files
Previous
Next

Requirements

  • 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.
Previous
Next

Tips and tricks

  • 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.
Previous
Next

Resources

Previous
Next
Jeremy Bartley, Geographer

Map Author

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.

Start making maps with a free trial of ArcGIS

Try ArcGIS