Wet Bulb Temperature: Swelter
WBGT & Heat Safety Forecasts
Free · In‑App Purchases · Designed for iPad. Not verified for macOS.
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/
more Gives me a better understanding of the best times to get outside to exercise or do yard work. The alert for high risk conditions is especially helpful.
Developer Response Thanks for using Swelter, enjoy the time outdoors! 😎🐈
Gives me a better understanding of the best times to get outside to exercise or do yard work. The alert for high risk conditions is especially helpful.
Thanks for using Swelter, enjoy the time outdoors! 😎🐈
Small enhancements to keep the app running smoothly.
1.6.3 17h ago
MAJOR UPDATE
Introducing a brand new Apple Watch app. Swelter is now on your wrist, with a full companion app, watch face complications, and Smart Stack support. There is also a new Apple Watch screen in Settings where you can check sync status, troubleshoot, and see details about how the watch app works. Some complications and features are free, and some are part of Swelter Pro.
v1.6.2:
- App Store product page updates.
v1.6.1:
- Widgets and complications can now call WeatherKit directly, you should see the "stale" state (Open Swelter to Update) significantly less often now on widgets.
v1.6.0:
- A beautiful Apple Watch app with full screen color gradients, bold typography, current WBGT guidance, and useful weather metrics at a glance.
- Screens include: core WBGT guidance, Quick Metrics, Best Window, Heat Index / Feels Like / Wind Chill, UV Index, wind speed, precipitation, and sunrise/sunset.
- Complications include small circles for WBGT and heat index, corner complications for analog watch faces, inline text complications, and a large complication for modular faces.
- A Smart Stack widget shows current WBGT, the current verdict, and heat index/feels-like temperature at a glance.
- This update also improves the icons and colors of the Air Temperature and Heat Index / Feels Like / Wind Chill readings in the app and widgets. They now follow the full temperature color spectrum, so the color always matches the current reading.
The watch app runs a full copy of the same physics engine as the iPhone app and fetches Apple Weather data directly, so it keeps working on its own, away from your phone.
If you run into any issues or have questions, email gavin@rbrn-labs.com or use Contact in the app.
Thanks for using Swelter!
1.6.2 2d ago
MAJOR UPDATE
Introducing a brand new Apple Watch app. Swelter is now on your wrist, with a full companion app, watch face complications, and Smart Stack support. There is also a new Apple Watch screen in Settings where you can check sync status, troubleshoot, and see details about how the watch app works. Some complications and features are free, and some are part of Swelter Pro.
v1.6.1:
- Widgets and complications can now call WeatherKit directly, you should see the "stale" state (Open Swelter to Update) significantly less often now on widgets.
v1.6.0:
- A beautiful Apple Watch app with full screen color gradients, bold typography, current WBGT guidance, and useful weather metrics at a glance.
- Screens include: core WBGT guidance, Quick Metrics, Best Window, Heat Index / Feels Like / Wind Chill, UV Index, wind speed, precipitation, and sunrise/sunset.
- Complications include small circles for WBGT and heat index, corner complications for analog watch faces, inline text complications, and a large complication for modular faces.
- A Smart Stack widget shows current WBGT, the current verdict, and heat index/feels-like temperature at a glance.
- This update also improves the icons and colors of the Air Temperature and Heat Index / Feels Like / Wind Chill readings in the app and widgets. They now follow the full temperature color spectrum, so the color always matches the current reading.
The watch app runs a full copy of the same physics engine as the iPhone app and fetches Apple Weather data directly, so it keeps working on its own, away from your phone.
If you run into any issues or have questions, email gavin@rbrn-labs.com or use Contact in the app.
Thanks for using Swelter!
1.6.1 3d ago
MAJOR UPDATE
Introducing a brand new Apple Watch app. Swelter is now on your wrist, with a full companion app, watch face complications, and Smart Stack support. There is also a new Apple Watch screen in Settings where you can check sync status, troubleshoot, and see details about how the watch app works. Some complications and features are free, and some are part of Swelter Pro.
- A beautiful Apple Watch app with full screen color gradients, bold typography, current WBGT guidance, and useful weather metrics at a glance.
- Screens include: core WBGT guidance, Quick Metrics, Best Window, Heat Index / Feels Like / Wind Chill, UV Index, wind speed, precipitation, and sunrise/sunset.
- Complications include small circles for WBGT and heat index, corner complications for analog watch faces, inline text complications, and a large complication for modular faces.
- A Smart Stack widget shows current WBGT, the current verdict, and heat index/feels-like temperature at a glance.
- This update also improves the icons and colors of the Air Temperature and Heat Index / Feels Like / Wind Chill readings in the app and widgets. They now follow the full temperature color spectrum, so the color always matches the current reading.
The watch app runs a full copy of the same physics engine as the iPhone app and fetches Apple Weather data directly, so it keeps working on its own, away from your phone.
If you run into any issues or have questions, email gavin@rbrn-labs.com or use Contact in the app.
Thanks for using Swelter!
1.6.0 5d ago
Bug fixes and improvements.
- Fixed a visual glitch where the animated background could briefly jump when navigating back from the Tomorrow screen. The background effects now fade out and back in cleanly during navigation.
1.5.8 Sep 29
New in version 1.5.7:
- Wind card (Pro): view current wind conditions, speed of wind gusts, direction the wind is blowing from, and the direction you are facing; powered by the built-in compass/magnetometer.
1.5.7 Sep 25
MAJOR UPDATE
Our largest update yet. New features, plus design and performance improvements.
New in version 1.5.6:
- Share button: Spotted some interesting weather conditions? You can now tap the "Share" button in the top left nav bar to generate a light or dark mode image of today's forecast and save it to your photo gallery, or share with your friends via iMessage.
- If you prefer a cleaner look, you can also disable this Share button in Settings > Appearance > Hide Share Button (Pro)
New in version 1.5.5:
- Tomorrow View (Pro): the full detailed Today screen, for tomorrow. The gauge, the timeline, and every chart for the day ahead. Simply hit the “>” button in the top left nav bar.
- Today's Pressure (Pro): a new barometric pressure chart with a Rising/Falling/Steady indicator and your choice of inHg or hPa.
- The WBGT chart on the Week screen is now free for all users.
- The Today screen now dims past hours, so you can see where "Now" is at a glance.
- Rebuilt the rain animation from the ground up. It's smoother, more realistic, and runs at 120hz on supported devices.
- New Appearance option (Pro): disable the rain, thunder, and snow effects while keeping the ambient background.
- Week chart improvements (Pro): cleaner 2-hour scale, color gradients under the lines, and fixed bugs with chart layout.
- Performance improvements (found and fixed bugs using profiler). Redesigned the “Pro” lock for free users, the gaussian blur was causing performance issues, we now use a simpler cover design.
Questions or ideas? Email gavin@rbrn-labs.com or use Contact in the app.
Thanks for using Swelter!
1.5.6 Sep 24
MAJOR UPDATE
Our largest update yet. New features, plus design and performance improvements.
- Tomorrow View (Pro): the full detailed Today screen, for tomorrow. The gauge, the timeline, and every chart for the day ahead. Simply hit the “>” button in the top left nav bar.
- Today's Pressure (Pro): a new barometric pressure chart with a Rising/Falling/Steady indicator and your choice of inHg or hPa.
- The WBGT chart on the Week screen is now free for all users.
- The Today screen now dims past hours, so you can see where "Now" is at a glance.
- Rebuilt the rain animation from the ground up. It's smoother, more realistic, and runs at 120hz on supported devices.
- New Appearance option (Pro): disable the rain, thunder, and snow effects while keeping the ambient background.
- Week chart improvements (Pro): cleaner 2-hour scale, color gradients under the lines, and fixed bugs with chart layout.
- Performance improvements (found and fixed bugs using profiler). Redesigned the “Pro” lock for free users, the gaussian blur was causing performance issues, we now use a simpler cover design.
Questions or ideas? Email gavin@rbrn-labs.com or use Contact in the app.
Thanks for using Swelter!
Note:
If you see "No Pressure Data" on the new Pressure card, this will become available once your 5 minute local cache passes (the "Updated" time on the top left of the Today screen).
1.5.5 Sep 24
You can now customize the Today screen. There is a new Layout screen in Settings that lets you drag the cards into any order, or uncheck a card to hide it entirely.
1.5.4 Sep 21
Added another new widget set: current air temperature. The small variant of this widget displays the current air temperature, UV index, and weather conditions. The medium variant adds today’s high temperature, the current heat index/feels-like temp/wind-chill, precipitation %, and relative humidity. Available in the widget gallery, and you can add to your Home Screen. The background color of this widget is a gradient which uses the same color scale the app uses to display air temperature, ranging from deep blue (frigid) to dark red (hot).
1.5.3 Sep 20
NEW: This update adds new widgets for your home and lock screen.
The flagship new widget for your Home Screen is a new full color gradient widget which shows the current WBGT and coach verdict. GOOD TO GO? Or caution, high risk, cancel, etc. now rendered in beautiful full color, with a more minimalist layout than some of the more information dense widgets.
We also added a handful of new information dense widgets if you want more info at a glance: “Now & Best Window” - a widget which combines current conditions along with today’s best window. And “Heat index” widgets for viewing the current air temp / humidity.
Also new: 2 widgets for your lock screen. A small and large variant of the same Heat Index widget, which is calculated from the current air temperature / humidity, and shows how hot the air feels at rest in the shade.
The Heat Index widgets follow the same dynamic system that the app itself uses. When it is hot outside the heat index is displayed, when it’s comfortable this switches to showing the “feels-like” temp. And in cold, windy conditions, this displays the wind chill.
1.5.2 Sep 18
NEW: Heat Stress Guidance. Tap "Additional Guidance" under the coach sentence to see published NWS guidance for your current reading, including how quickly heat stress builds in direct sun and how many minutes of breaks to take each hour.
The WBGT gauge now places its marker more precisely within each color band.
Located and fixed a difficult to reproduce bug, on some small screen devices, the X-axis could overflow, leading to the screen becoming horizontally scrollable. We fixed this by ensuring X-axis scroll is never possible. If you run into this or similar bugs/issues, please email me at gavin@rbrn-labs.com
1.5.1 Sep 15
Small enhancements to keep the app running smoothly.
1.5.0 Sep 13
Small update to the science: WBGT readings during sunny hours now run a touch higher (up to about 1°F / 0.5°C); nights and cloudy hours are unchanged.
We tuned the WBGT algorithm slightly to account for weather forecast data inputs. The globe temperature term now follows spherical geometry instead of a horizontal-surface sine, the Kasten-Czeplak cloud coefficient was recalibrated very slightly, and the wind cooling credit was modulated so that it now fades under strong sun, not just high humidity. Every change was replayed against tens of thousands of station-hours we have banked. These come from the same WBGT monitoring network we compare against. The end result: sunny hours now read a touch higher; most readings are unchanged. The difference is simply a very marginal increase in WBGT during very sunny hours. We continue to refine our algorithm against weather station data, and in-the-field reports from athletes using the app!
Core accuracy holds: across more than 20,000 compared station-hours, readings average within 0.7°F (0.4°C) of the monitoring network, with 90% exact flag agreement and 99.96% within one flag. The app shows a "Near" line whenever a reading sits close to a boundary.
1.4.9 Sep 11
Fixed a cosmetic bug with temperature readings displayed in Celsius. Small refinements to the heat index / feels like / wind chill labels in the app.
1.4.8 Sep 10
Added a new Heat Index card to the Today screen. It uses our new dynamic tri-state label system. At 80°F and above, the number is the NWS heat index. Below that, it is the feels-like temperature. And in cold, windy conditions (below 50°F with wind above 3 mph), it becomes the wind chill. Anywhere you see this number in the app or widgets, the label now matches the conditions: Heat Index when it is hot, Feels Like when it is comfortable, Wind Chill when it is cold.
Added a quick-glance precipitation chance tile to the Today screen.
Improved the weather condition icon colors in light mode.
Fixed a crash in the Mac version of Swelter when opening the location picker or the upgrade screen.
Several other small fixes and improvements throughout the app.
1.4.7 Sep 9
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.
v1.4.5 adds support for meters per second (m/s) in the Units settings for wind speed.
1.4.6 Sep 7
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.
v1.4.5 improves how Swelter works around the world.
There is a new Units screen in Settings. You can now choose your wind speed unit (mph or km/h) separately from your temperature unit (°F or °C), so the app can match the way your region reads the weather.
Swelter also now fully supports 24-hour time. Every time shown in the app follows your device's 12-hour or 24-hour clock setting.
1.4.5 Sep 5
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.
v1.4.4 Includes a few minor refinements to the Guide screen.
1.4.4 Sep 4
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.
1.4.3 Sep 3
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.
v1.4.1 brings a new heat index chart to the Week tab for Pro users. You can now view the heat index for the week ahead, to track how hot the air feels.
v1.4.2 brings improvements to the Today screen. The UV index chart no longer shows empty gray bars for UV 0, and there is a new cloud cover chart which shows how overcast or clear today's skies are.
1.4.2 Sep 2
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.
v1.4.1 brings a new heat index chart to the Week tab for Pro users. You can now view the heat index for the week ahead, to track how hot the air feels.
1.4.1 Aug 31
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.
Sources & Algorithm screen updated with insights into how Swelter calculates heat index.
Improved WBGT accuracy under cloud cover. Our latest validation studies against measured weather-station data showed cloudy hours reading slightly low (by about 1 °F), so we tuned how Swelter converts cloud cover into sun strength. Most readings are unchanged; the improvement shows up during mid-day hours under heavy cloud cover.
Bug fix: In frigid conditions the WBGT calculator now opens within proper bounds.
1.4.0 Aug 31
Improved the yellow text color used in many spots across in the app in light mode. This color is now easier to read in light mode, matching the contrast ratio used for the dark mode variant. This fix applies to the widgets as well.
1.3.1 Aug 27
Completely reworked the in-app guide to be easier to follow and more useful. Minor refinements to various UI elements throughout the app.
Above 80F the "feels like" temperature now renders with the label "HEAT INDEX" - this is the official start of heat index scale (>80F), and Swelter now surfaces this label more cleanly.
Fixed a bug where the heat risk (WBGT) calculator loaded with the incorrect cloud cover %
1.3.0 Aug 27
Small enhancements to keep the app running smoothly.
more Version 1.6.3 17h ago
Data Not Collected The developer does not collect any data from this app.
Supported Features
Larger Text
Dark Interface
Reduced Motion