Summers are getting hotter. Swelter shows how risky it is to be
active outdoors right now, using WBGT, wet-bulb temperature, and
heat index readings computed on your iPhone.
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. Swelter shows the
heat index too, since it is still useful for how hot the air
feels at rest.
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.
Pick any location
Away from home? Swelter lets you view current conditions for
any location around the globe.
Ask Siri
Ask Siri for the current heat risk, the WBGT, or today's best
window. Swelter also works with Shortcuts, so you can build
your own automations with its data.
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.
Why Swelter
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.
WBGT is estimated from Apple Weather data, not measured by a
calibrated device. Swelter is a planning tool, not a medical
device, and is not suitable for regulatory or organizational
compliance (OSHA, athletic governing bodies, military). If you
feel signs of heat illness (dizziness, confusion, nausea, or you
stop sweating), stop activity, get to shade, and cool down right
away. If someone becomes confused or passes out, call emergency
services.
Forecasts can be wrong — when in doubt, choose the cooler option
and use your best judgment.
