Swelter: Heat Stress Index
Wet bulb temperature & alerts
About
This app calculates the Wet Bulb Globe Temperature (WBGT) to assess outdoor heat risk for activities. It provides clear safety flags and hourly forecasts to help you plan outdoor events safely. Get personalized alerts and find the best times for your activities.
What's New in Swelter
1.2.5
August 18, 2026
Several small stability enhancements and improvements.
Subscription plans
Swelter Pro Monthly
Every Swelter Pro feature, billed monthly.
In-App Purchases
$39.99
Swelter Pro Lifetime
Every Swelter Pro feature, forever. One time purchase.
Charts
FAQ
What is Swelter's WBGT calculation based on?
Swelter calculates WBGT by considering air temperature, humidity, wind, and sun strength. This provides a more accurate measure of heat stress than air temperature and heat index alone.
Does Swelter have ads?
No, Swelter does not contain advertisements. The app is supported entirely by Swelter Pro subscriptions.
What features are included in Swelter Pro?
Swelter Pro unlocks full week charts, hourly UV and precipitation data, additional alert types, year-over-year trends, saved locations, and the complete widget set. Free users retain core safety features.
How accurate is Swelter's WBGT data?
Swelter is validated against a large network of WBGT stations, with 99.94% of readings within one flag level compared to 10,760 station-hours of data.
What is the age rating for Swelter?
Swelter has an age rating of 12+, making it suitable for users aged 12 and older.
How often is Swelter updated?
Swelter was last updated on August 6, 2026, with the latest version being 1.1. This indicates regular maintenance and potential feature additions.
What devices does Swelter support?
Swelter is available for iPhone and iPad devices, allowing users on Apple's mobile platforms to access its heat safety features.
What are the subscription options for Swelter Pro?
Swelter Pro offers monthly subscriptions at $2.99 per month and yearly subscriptions at $14.99 per year. The yearly plan includes a 7-day free trial.