Wet Bulb Temperature: Swelter

How risky is today's heat? Swelter tracks the WBGT, wet bulb temperature, and heat index hour by hour, finds the best window in your day, and alerts you before extreme heat arrives. Validated against 20k+ hours of station data.

Your local conditions become one of four flags: GOOD TO GO, CAUTION, HIGH RISK, or CANCEL. The flags follow the American College of Sports Medicine's published WBGT thresholds, a scale built for heat safety during outdoor exertion in the sun.

The heat index tells you how hot it feels in the shade. WBGT (wet-bulb globe temperature) also counts direct sun and wind, so it reflects how well your body can actually cool itself. It is the standard measure of heat stress, used worldwide for decades to decide when to limit outdoor activity. Swelter computes it on your phone from Apple Weather data.

Built for runners, hikers, coaches and athletes, outdoor workers, and anyone planning a day in the heat.

WHAT SWELTER DOES
- Today: the current flag, the WBGT behind it, and an hour-by-hour timeline
- Best Window: the least risky stretch of your day for a run, practice, or a shift outdoors
- Week Ahead: seven days of hourly WBGT, plus a 10 Day Forecast for spotting heat waves
- Alerts: a heads-up before extreme heat and storms arrive
- Widgets: your flag and timeline on the home and lock screen
- Any location worldwide, in its own local time
- Siri & Shortcuts: ask for the current heat risk or build your own automations

Also at a glance: NWS heat index, air temperature, wet bulb temperature, humidity, UV index, wind, precipitation and storms, sunrise and sunset.

When building Swelter's WBGT algorithm, we validated it against over 20,000 hours of weather station data from the world's largest WBGT monitoring network in Japan. The full methodology and sources are documented in the app.

Swelter is private by design: no account, no advertisements, and your data is never sold. Your location is used only to retrieve Apple Weather data.

Try Pro free for 7 days with the yearly plan. Pro unlocks full week charts, the 10 day forecast, hourly UV and precipitation, extra alert types, saved locations, and year-over-year trends. Free users keep the core 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. Its readings are estimates from weather data, not measurements from a physical 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/