About this app
Swelter answers one question: how risky is it to be active outdoors right now? Track the current heat index, and the number that goes beyond it: WBGT. Swelter computes WBGT (wet-bulb globe temperature) right on your phone. It is the standard measure of heat stress, used worldwide for decades to decide when to limit outdoor activity. Your local conditions become one of four flags: GOOD TO GO, CAUTION, HIGH RISK, or CANCEL. Swelter's four flags follow the American College of Sports Medicine's published WBGT thresholds for outdoor activity. GOOD TO GO runs below 82°F (27.8°C). CAUTION begins at 82°F (27.8°C), HIGH RISK at 87°F (30.6°C), and CANCEL at 90°F (32.2°C). The heat index tells you how hot the air feels, but it is measured in the shade and leaves out direct sun and wind. WBGT, the number that powers Swelter's flags, takes air temperature, humidity, wind, and sun strength into account, so it reflects how well your body can actually cool itself (via the evaporation of sweat). Swelter follows published scientific guidelines from leading organizations and research in the field of outdoor exertion & heat safety, and we validated our algorithm against measured weather-station data. TODAY The current heat-risk flag, the number behind it (WBGT), and an hour-by-hour timeline. Also: heat index, wet bulb temperature, humidity, UV, and wind at a glance. BEST WINDOW Swelter finds the least risky stretch of your day for a run, a hike, practice, or a shift outdoors. WEEK AHEAD Seven days of hourly WBGT so you can plan around the heat. Plus a 10 Day Forecast for seeing upcoming temperature trends and heat waves. ALERTS A heads-up before extreme heat 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. LOCATIONS Check any place in the world, in its own local time. Save up to 10 favorites (Pro feature). HEAT RISK CALCULATOR Set the temperature, humidity, sun, cloud cover, and wind yourself, and Swelter estimates the WBGT and its risk flag. It opens with your current conditions so you can start from right now. SIRI & SHORTCUTS Swelter supports App Intents so you can ask Siri for the current heat risk, or build your own custom Shortcuts from data in the app. VALIDATED Swelter is validated against the world's largest WBGT monitoring network: 21,370 station-hours compared so far, with 90% exact flag agreement and 99.96% of readings within one flag level (the app calls out when it is near a boundary). The full methodology and sources are documented in the app. PRIVATE BY DESIGN Supported by Pro subscriptions, not ads. No account needed, and your data is never sold. Your location is used only to retrieve Apple Weather data. SWELTER PRO Try Pro free for 7 days with the yearly plan. Unlocks every chart and screen: full week charts, 10 day forecast, 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/
Latest release
What’s New
Introducing a major new feature: Swelter now integrates with Siri & Shortcuts. Ask Siri for current heat conditions and Siri will reply with the current heat risk and WBGT from the app's forecast data. Power users can build custom Shortcuts using the WBGT and heat risk flag data. v1.4.3 brings a new feature for Pro users. Tap the Week tab and there is a new 10 Day Forecast button which lets you view weather conditions, high/low air temp, and peak WBGT + heat risk for the next 10 days! This is useful for seeing upcoming temperature trends and heat waves. Further out forecasts may change and update.
Audience growth
Ratings growth
Baseline captured. Growth appears after the crawler observes a changed rating count.
Momentum comes from crawler snapshots for the tracked storefront. “Viral growth signal” means unusually fast recent rating gains; it is an indicator, not download data.
Price tracking
Price intelligence
First snapshot captured. The trend line starts after the next crawler price observation.
- First observed Sep 4, 2026 · 5:20 AM Free
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