This app tracks wet bulb globe temperature (WBGT) and heat index hour by hour to assess heat risk for outdoor activities. It provides daily flags, best window recommendations, and alerts for extreme heat, validated against extensive station data.
1.7.0
October 31, 2026
This update adds support for iPhone Duo. - Swelter now fills the cover screen when folded, and expands to use the full width of the inner display when unfolded. - Fixed layout bugs when resizing the app mid-session, including folding and unfolding during use. Thanks for using Swelter!
MoreEvery Swelter Pro feature, billed monthly.
Heat safety, UV, and weather for athletes.
Oct 4, 2026 — Nov 3, 2026
Ask Siri for the current heat risk outdoors.
Aug 31, 2026 — Sep 30, 2026
Swelter uses the Wet Bulb Globe Temperature (WBGT) to assess heat stress, considering direct sun and wind. It categorizes conditions into GOOD TO GO, CAUTION, HIGH RISK, or CANCEL based on ACSM thresholds.
No, Swelter is private by design and does not contain advertisements. Your data is also not sold, and location is only used for weather data retrieval.
Swelter's WBGT algorithm was validated against over 20,000 hours of weather station data from Japan's largest WBGT monitoring network. The full methodology and sources are documented within the app.
Swelter Pro unlocks full week charts, the 10-day forecast, hourly UV and precipitation data, additional alert types, saved locations, and year-over-year trends. A 7-day free trial is available with the yearly plan.
Yes, Swelter is designed for privacy and does not require an account. Your location data is used solely for retrieving local weather information.
Swelter is available on Apple Watch, iPhone, and iPad, allowing you to check heat safety conditions across your devices.
The app was last updated on October 31, 2026, indicating regular maintenance and updates from the developer, RBRN Labs LLC.
No, Swelter is a planning tool and not a medical device. Its readings are estimates from weather data, and users should use their best judgment for heat safety.
