# Swelter: Heat Index & Safety — WBGT, wet bulb & heat alerts

> 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-safety/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 6 App Store ratings · 1 written reviews indexed
- Age rating: +4
- Requires: iOS 17.0 · 6 MB
- Languages: American English
- Released: 2026-08-05
- Data updated: 2026-08-31
- Monetization: freemium
- User reviews in markdown: https://appshunter.io/ios/app/swelter-heat-index-and-safety/id6794481096/reviews.md

## What is Swelter?

Swelter answers one question: how risky is it to be active outdoors right now? Track the current heat index, and the number that goes beyond it: WBGT.

Swelter computes WBGT (wet-bulb globe temperature) right on your phone. It is the standard measure of heat stress, used worldwide for decades to decide when to limit outdoor activity. Your local conditions become one of four flags: GOOD TO GO, CAUTION, HIGH RISK, or CANCEL.

Swelter's four flags follow the American College of Sports Medicine's published WBGT thresholds for outdoor activity. GOOD TO GO runs below 82°F (27.8°C). CAUTION begins at 82°F (27.8°C), HIGH RISK at 87°F (30.6°C), and CANCEL at 90°F (32.2°C).

The heat index tells you how hot the air feels, but it is measured in the shade and leaves out direct sun and wind. WBGT, the number that powers Swelter's flags, takes air temperature, humidity, wind, and sun strength into account, so it reflects how well your body can actually cool itself (via the evaporation of sweat). Swelter follows published scientific guidelines from leading organizations and research in the field of outdoor exertion & heat safety, and we validated our algorithm against physical sensor data.

TODAY
The current heat-risk flag, the number behind it (WBGT), and an hour-by-hour timeline. Also: heat index, wet bulb temperature, humidity, UV, and wind at a glance.

BEST WINDOW
Swelter finds the least risky stretch of your day for a run, a hike, practice, or a shift outdoors.

WEEK AHEAD
Seven days of hourly WBGT so you can plan around the heat.

ALERTS
A heads-up before extreme heat and storms arrive. Severe-heat and storm alerts are free for everyone.

WIDGETS
The current flag and today's timeline on your home screen and lock screen.

LOCATIONS
Check any place in the world, in its own local time. Save up to 10 favorites (Pro feature).

HEAT RISK CALCULATOR
Set the temperature, humidity, sun, cloud cover, and wind yourself, and Swelter estimates the WBGT and its risk flag. It opens with your current conditions so you can start from right now.

VALIDATED
Swelter is the only consumer heat-safety app validated against the world's largest network of measured WBGT stations: 20,230 station-hours compared so far, with 90% exact flag agreement and 99.96% of readings within one flag level (the app calls out when it is near a boundary). The full methodology and sources are documented in the app.

PRIVATE BY DESIGN
Supported by Pro subscriptions, not ads. No account needed, and your data is never sold. Your location is used only to retrieve Apple Weather data.

SWELTER PRO
Unlocks every chart and screen: full week charts, hourly UV and precipitation, extra alert types, year-over-year trends, saved locations, and the complete widget set. Free users keep the core safety 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. 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 |


## Chart rankings

- #152 in Top Free · Weather (US App Store)

## Recent user reviews (1 of 1)

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

### 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)

### 1.3.1 — 2026-08-27

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.

### 1.3.0 — 2026-08-27

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 %

### 1.2.9 — 2026-08-26

New feature: The WBGT calculator. Set your own temperature, humidity, sun, and wind readings, and Swelter estimates the WBGT and its risk flag. Find it at the bottom of the Today screen (Pro feature).

Minor refresh to light mode app icon, a new Guide section, and updated validation numbers.

### 1.2.8 — 2026-08-24

Added a new storm indicator on the "Heat Risk - Next Hours" widget at the bottom of the Today screen. You can now easily see storms ahead.

The "Try the Algorithm" feature (Settings > About > Sources & Algorithm) now functions as a proper WBGT calculator. You can drag sliders between the normal WBGT range (40°F to 120°F air temperature) along with humidity, sun angle, cloud cover, and wind to calculate WBGT from your own readings.

Several small stability enhancements.

Thank you for using Swelter! We have a bunch of new features for athletes & outdoor enthusiasts under construction that will release in the near future.

### 1.2.7 — 2026-08-22

Added a new section in the in-app Guide explaining how the app schedules notifications.

Improved UI edge case behavior on small screen devices with max size dynamic type enabled (Accessibility).

A few other small enhancements around the app.

We have some big features for athletes & outdoor enthusiasts coming soon, thank you for using Swelter!

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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 2026-08-31.*