Swelter is currently in App Store review. The links on this page will be updated to point to the real App Store listing once it's available.

Announcing Swelter

Screenshot of the Swelter iOS app showing the WBGT heat-risk gauge and daily timeline

Available on the App Store ↗

Swelter is my brand new iOS app, built to answer a simple question: how risky is it to be active outdoors right now? It estimates wet-bulb globe temperature (WBGT), the standard measure of heat stress, entirely on your device using Apple Weather data, and turns it into a clear heat-risk flag and guidance for your day.

Key functionality and features:

  • Current WBGT, heat-risk flag, and weather conditions (a.k.a. how favorable or how risky is it for a run or outdoor activity right now?)
  • Heat, storm, and precipitation alerts via push notifications
  • Week-ahead WBGT forecasts and detailed weather insights
  • A best-window planner that finds the most favorable time to be outdoors today
  • Home Screen and Lock Screen widgets
  • Year-over-year trend comparisons
  • A live animated background, dark/light mode, and iOS 26+ Liquid Glass design
  • On-device WBGT calculation, so your location data stays private
  • Fahrenheit/Celsius units and a location picker for anywhere on Earth

I built Swelter because I’m a runner and I wanted one app that quickly tells me how risky current conditions are for doing cardio outdoors. Here in Richmond, VA (and around the world), summers are getting hotter. It can be difficult to know how risky it actually is outdoors from a standard weather app, since air temperature alone does not take into account humidity, sun, and wind (the conditions which actually determine how well evaporative cooling from sweat works). WBGT (wet-bulb globe temperature) takes all of these factors into account and categorizes the results into published threshold values (heat-risk flag). That is why WBGT is the gold-standard metric for assessing heat risk, rather than simply using air temperature or heat index (feels-like). Heat index is useful (and calculated/displayed in the app), but WBGT is the superior metric for heat risk, and it’s what Swelter is powered by.

When building Swelter’s core algorithm/engine, I used published guidelines and research from industry-leading organizations and scientific papers to create an algorithm that can estimate WBGT from Apple Weather data. No WBGT algorithm will ever be as accurate as a physical hardware sensor due to variance from forecast data (weather forecasts and WBGT are estimates and simulations of real conditions, not measurements from a physical WBGT sensor), however, I tested and validated the core algorithm against Japan’s national network of thousands of physical WBGT sensors. Across thousands of hours of compared data, Swelter reached 89% exact flag-for-flag agreement and 99.97% agreement within one boundary step (the app displays a “Near …” line when the current WBGT is near a boundary).

Swelter is an independent product of RBRN Labs LLC, my indie app studio. The core heat-risk feature is free forever; some features require a Swelter Pro subscription.

Find Swelter on the App Store.

For the full launch announcement and feature rundown, read Welcome to Swelter on the RBRN Labs site.

Swelter is a product of RBRN Labs LLC, and the Swelter name is a trademark of RBRN Labs LLC. Swelter is not affiliated with, endorsed by, or sponsored by any of the organizations, agencies, or authors whose published research is referenced here. Accuracy figures are from a July 2026 comparison using measured station inputs; day-to-day readings in the app come from forecast data and can vary more. Weather data from Apple Weather.