Plan around the heatwith Swelter

Summers are getting hotter. Swelter shows how risky it is to be active outdoors right now.

Swelter Today screen: WBGT gauge at 75°F with a GOOD TO GO flag and an all-green hourly timeline

Why it's high risk: humidity, sun strength, air temperature, and wind cooling, with current condition tiles

Swelter Today screen: WBGT gauge reading 88°F with a HIGH RISK flag and today's hourly timeline

Today's conditions and Next Hours list showing hourly WBGT flags from GOOD TO GO to HIGH RISK

Rainy ambient backdrop with a storm warning: storms override heat guidance and canceling outdoor activity is recommended

Weather data shown from Apple Weather.

Go beyond the heat index

Swelter runs on wet-bulb globe temperature (WBGT), the gold-standard heat stress index, used worldwide for decades to decide when to limit outdoor activity. Heat index combines air temperature and humidity, but it assumes you are at rest in the shade, with a light breeze. WBGT, the metric that powers Swelter, is built for exertion in direct sun.

See why it's risky

Humidity, sun strength, air temperature, and wind cooling, broken down in plain language.

Swelter shows how risky conditions are for a run or any outdoor activity, and when to postpone or cancel.

A heads-up when it matters

View the entire day ahead to see the best windows for outdoor activities. Receive useful notifications ahead of heat or weather events, such as precipitation or storms.

Beautiful ambient weather visuals

Live background animations mirror current weather conditions. Calming ambient visuals update with current heat or precipitation conditions.

Widgets for your Home Screen

Glanceable WBGT, today's best window, and current conditions without opening the app.

Swelter Home Screen widgets: WBGT gauge with GOOD TO GO, best window today, and current condition dials

Pick any location

Away from home? Swelter lets you view current conditions for any location around the globe.

Dubai at 87°F WBGT: HIGH RISK flag, feels like 114°F, with guidance to avoid hard effort in direct sun

Privacy first

Supported entirely by Pro subscriptions. No account needed, and your data is never sold.

On-device

WBGT is estimated from local weather data right on your iPhone.

A planning tool

Swelter helps you plan around the heat. It is a planning tool, not a medical device.

Accuracy, Measured

Swelter is validated against the world's largest WBGT monitoring network.

We tested and refined Swelter's engine against hourly values from Japan's national heat-stroke monitoring system.

21,370station hours
0.7 °Favg. error
90%flag agreement
99.96%within one flag level

90% is exact flag-for-flag agreement. Nearly every difference was a single boundary step: the same borderline readings the app calls out with its “Near …” line. Within one adjacent flag level, agreement was 99.96%.

July–August 2026 comparison, using measured station inputs. Day-to-day readings come from forecast data and can vary more; WBGT is always an estimate from Apple Weather data, not a reading from a calibrated device.

Check the heat before you head out

Swelter for iPhone. Available now on the App Store.

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