# Swelter: Heat Index & WBGT — Wet Bulb Temperature & Safety

> Calculates WBGT heat stress for outdoor activity safety. (iPhone/iPad app by RBRN Labs LLC.)

- Source: https://appshunter.io/ios/app/swelter-heat-index-and-wbgt/id6794481096 (this page in markdown: same URL + `.md`)
- Developer: [RBRN Labs LLC](https://appshunter.io/developer/6794481098)
- Category: Weather, Health & Fitness
- Price: Free with in-app purchases
- Rating: 5.00/5 from 11 App Store ratings · 2 written reviews indexed
- Age rating: +4
- Requires: iOS 17.0 · 6 MB
- Languages: American English
- Released: 2026-08-05
- Data updated: 2026-09-11
- Monetization: freemium
- User reviews in markdown: https://appshunter.io/ios/app/swelter-heat-index-and-wbgt/id6794481096/reviews.md

## What is Swelter?

How risky is today's heat? Swelter tracks the heat index and WBGT 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 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/

## Key features

- WBGT heat stress calculation
- Hourly heat risk timeline
- Best window for outdoor activity
- Seven-day WBGT forecast
- Severe heat and storm alerts
- Home and lock screen widgets
- Global location support

## Pricing and in-app purchases

Base price: Free.

**Subscriptions**

| Subscription | Price | Period |
| --- | --- | --- |
| Swelter Pro Monthly | $2.99 | per month |
| Swelter Pro Yearly | $14.99 | per year |

**One-time purchases**

| Purchase | Price | Type |
| --- | --- | --- |
| Swelter Pro Lifetime | $39.99 | one-time |


## Recent user reviews (2 of 2)

All indexed reviews: https://appshunter.io/ios/app/swelter-heat-index-and-wbgt/id6794481096/reviews.md

### 5/5 — Love this app!

*2026-08-31*

Great work Swoog!

### 5/5 — Helpful app

*2026-08-19*

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! 😎🐈

## Frequently asked questions about Swelter

### What is Swelter's WBGT calculation based on?

Swelter calculates WBGT by considering air temperature, humidity, wind, and sun strength. This provides a more accurate measure of heat stress than air temperature and heat index alone.

### Does Swelter have ads?

No, Swelter does not contain advertisements. The app is supported entirely by Swelter Pro subscriptions.

### What features are included in Swelter Pro?

Swelter Pro unlocks full week charts, hourly UV and precipitation data, additional alert types, year-over-year trends, saved locations, and the complete widget set. Free users retain core safety features.

### How accurate is Swelter's WBGT data?

Swelter is validated against a large network of WBGT stations, with 99.94% of readings within one flag level compared to 10,760 station-hours of data.

### What is the age rating for Swelter?

Swelter has an age rating of 12+, making it suitable for users aged 12 and older.

### How often is Swelter updated?

Swelter was last updated on August 6, 2026, with the latest version being 1.1. This indicates regular maintenance and potential feature additions.

### What devices does Swelter support?

Swelter is available for iPhone and iPad devices, allowing users on Apple's mobile platforms to access its heat safety features.

### What are the subscription options for Swelter Pro?

Swelter Pro offers monthly subscriptions at $2.99 per month and yearly subscriptions at $14.99 per year. The yearly plan includes a 7-day free trial.

## Version history (last 5 releases)

### 1.4.8 — 2026-09-10

Fixed a cosmetic bug with temperature readings displayed in Celsius. Small refinements to the heat index / feels like / wind chill labels in the app.

### 1.4.7 — 2026-09-09

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.

### 1.4.6 — 2026-09-07

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.

### 1.4.5 — 2026-09-05

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.

### 1.4.4 — 2026-09-04

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.

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*Data collected daily from the US App Store and indexed by [AppsHunter](https://appshunter.io/). User reviews are verbatim App Store reviews. Ratings, prices and chart positions refresh continuously; this snapshot is from 2026-09-11.*