Swelter: Heat Safety & WBGT
Heat risk for outdoor activity
Free · In‑App Purchases · Designed for iPad. Not verified for macOS.
The only consumer heat-safety app validated against the world's largest network of measured WBGT stations.
Swelter answers one question: how risky is it to be active outdoors right now?
Swelter computes WBGT (wet-bulb globe temperature), the standard measure of heat stress used worldwide for decades, right on your phone, and turns it into four flags:
GOOD TO GO · CAUTION · HIGH RISK · CANCEL.
Air temperature and heat index (feels-like temp) alone undersell dangerous heat. They leave out direct sun and wind, the factors that decide how well your body can actually cool itself. Swelter runs on WBGT, which takes into account temperature, humidity, wind, and sun strength (based on real solar geometry for your exact location and hour).
Along with clear guidance on whether you're good to go or should postpone outdoor activity, Swelter provides useful insights and features:
TODAY
Your current heat flag, the number behind it, and an hour-by-hour timeline. Pick any hour to see how the risk changes throughout the day.
BEST WINDOW
Swelter finds the least risky stretch of your day for a run, a hike, practice, or a shift outdoors, and shows when it opens and closes.
WEEK AHEAD
Seven days of hourly WBGT, so you can plan around the heat in advance.
ALERTS
A heads-up before extreme heat windows and storms arrive. Severe-heat and storm alerts are free for everyone.
WIDGETS
The current flag and today's timeline, on your home screen and lock screen.
LOCATION
Pick any location around the world to view current WBGT conditions, shown in that place's own local time. Save up to 10 favorite locations to quickly switch between.
VALIDATED
Swelter is the only consumer heat-safety app validated against the world's largest network of measured WBGT stations: 10,760 station-hours compared so far, with 99.94% of readings within one flag level.
PRIVATE BY DESIGN
No account, no third-party analytics, and no selling your data; we are supported entirely by Swelter Pro subscriptions. Your location is used only to retrieve Apple Weather data.
SWELTER PRO
Unlocks every chart and screen: the full week charts, hourly UV and precipitation, extra alert types, year-over-year trends, saved locations, and the complete widget set. Free users keep the core safety read: today's flag, the gauge, the timeline, current conditions, and severe-heat and storm alerts.
Swelter is a planning tool, not a medical device. Conditions can change quickly. When in doubt, choose the cooler option and use your best judgment.
Weather data provided by Apple Weather.
Terms of Use (EULA): https://rbrn-labs.com/swelter/terms/
Privacy Policy: https://rbrn-labs.com/swelter/privacy-policy/
Ratings & Reviews
Swelter now runs on your chosen location's time zone, so places around the globe show the day from their own perspective.
Improvements to alerts; notifications now name what location they're for, and won't repeat for a warning you've already seen.
Faster background updates, instant loading for saved places, and backend improvements to keep the app stable.
The developer, RBRN Labs LLC, indicated that the app’s privacy practices may include handling of data as described below. For more information, see the developer’s privacy policy .
Data Not Collected
The developer does not collect any data from this app.
Accessibility
The developer has not yet indicated which accessibility features this app supports. Learn More
Information
- Seller
- RBRN Labs LLC
- Size
- 6.1 MB
- Category
- Weather
- Compatibility
Requires iOS 17.0 or later.
- iPhone
Requires iOS 17.0 or later. - iPad
Requires iPadOS 17.0 or later. - Mac
Requires macOS 14.0 or later and a Mac with Apple M1 chip or later.
- iPhone
- Languages
- English
- Age Rating
13+
- 13+
- In-App Purchases
Yes
- Swelter Pro Lifetime $39.99
- Swelter Pro Yearly $14.99
- Swelter Pro Monthly $2.99
- Copyright
- © 2026 RBRN Labs LLC