WBGT App for iPhone
Swelter shows wet-bulb globe temperature (WBGT) for your exact location, computed on your iPhone from Apple Weather data. It turns the number into a clear flag, so you can see how risky it is to be active outdoors right now and find the best window today.
Why WBGT instead of the heat index
Air temperature and feels-like readings undersell dangerous heat. They leave out direct sun and wind, the factors that decide whether sweat can actually cool you.
The heat index assumes you are resting in the shade with a light breeze. WBGT is different. It accounts for temperature, humidity, sun, and wind together, and it is built for exertion in direct sun. It has been the standard heat stress index worldwide for decades, used to decide when to limit outdoor activity.
What Swelter shows you
Swelter turns WBGT into one of four flags: GOOD TO GO, CAUTION, HIGH RISK, or CANCEL. The Today screen shows the current WBGT, the current flag with plain-language guidance, and an hourly timeline for the rest of the day. A best window planner points out the most favorable time to be outdoors today, and the week view shows WBGT charts for the days ahead. The flags follow the American College of Sports Medicine's published WBGT thresholds, a scale built for exertion outdoors in the sun. The week view also includes a 10 day forecast for spotting heat waves.
Notifications give you a heads-up before severe heat, storms, or precipitation. Widgets put the current flag and today's best window on your Home Screen and Lock Screen. You can also ask Siri for the current heat risk or today's best window.
Weather data shown from Apple Weather.
Who Swelter is for
Swelter is built for runners, hikers, cyclists, coaches and athletes, outdoor workers, and anyone planning a day in the heat.
Along with WBGT, Swelter tracks the heat index, air temperature, wet bulb temperature, humidity, UV index, wind, precipitation and storms, and sunrise and sunset. Conditions update hour by hour, up to ten days ahead.
Tested against measured station data
Swelter is validated against the world's largest WBGT monitoring network. We compared the engine's output against tens of thousands of station-hours of values computed from measured weather-station data, and we published the methodology, the results, and the limitations.
Read how we validated Swelter's WBGT engine for the full story and the numbers.
Frequently asked questions
What is WBGT?
WBGT stands for wet-bulb globe temperature. It combines air temperature, humidity, sun, and wind into a single number that reflects how much heat stress your body faces outdoors. It has been used worldwide for decades to decide when to limit outdoor activity.
What is the difference between WBGT and the heat index?
The heat index combines air temperature and humidity, and it assumes you are resting in the shade with a light breeze. WBGT also accounts for direct sun and wind, and it is built for exertion. That is why two days with the same heat index can carry very different risk once you are working or exercising in the sun.
What WBGT levels do the flags change at?
The four flags change at WBGT values of 82, 87, and 90 °F (27.8, 30.6, and 32.2 °C), following the American College of Sports Medicine's published thresholds. When a reading sits close to a boundary, Swelter calls it out with a "Near" line.
How can I check WBGT on my iPhone?
Swelter shows the current WBGT for your location, along with an hourly timeline for the rest of the day and the week ahead. Everything is computed on your iPhone from Apple Weather data, so there is no account and no extra hardware.
Does Swelter need a WBGT sensor?
No. Swelter estimates WBGT from local weather data, so it works anywhere your iPhone does. No estimate can match a calibrated instrument. To understand how close the estimates come, we compared the engine against tens of thousands of station-hours of reference values computed from measured weather-station data, and we published the methodology, the results, and the limitations.
Is Swelter free?
The core of Swelter is free: today's flag, the WBGT gauge, the hourly timeline, current conditions, and severe-heat and storm alerts. Swelter Pro unlocks deeper charts, additional alert types, and the full widget set. Pro is $2.99 per month, $14.99 per year with a 7-day free trial, or $39.99 for lifetime access.
How do I cancel Swelter Pro?
Subscriptions are billed and managed by Apple through your Apple ID, not by us. You can cancel anytime in Settings, under your Apple ID's Subscriptions section, and Pro stays active until the end of the period you already paid for. Deleting the app does not cancel a subscription. Refund requests are handled by Apple.
Does Swelter need an account?
No. There are no accounts, no tracking across apps, and your data is never sold. Your location is used only to retrieve Apple Weather data, and WBGT is computed on your device.
Does Swelter work outside the United States?
Yes. Swelter covers any location Apple Weather covers, and readings display in Fahrenheit or Celsius. A location picker also lets you view conditions anywhere on Earth.
How do I get help or suggest a feature?
If you have any questions, suggestions, or need to report a bug, you can reach us through the Support page. You can also use the Contact Us option in the app's settings.
Disclaimer
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. Guidance in the app comes from published, publicly available materials, and Swelter is not affiliated with, endorsed by, or sponsored by the organizations behind them.
Check the heat before you head out
Swelter for iPhone. Available now on the App Store.