Wet bulb temperature measures heat stress by combining air temperature and humidity. It shows how well the human body can cool itself through sweating. This matters because sweating only works when moisture can evaporate from the skin. When humidity is too high, that cooling process breaks down. At around 95°F wet bulb temperature — roughly 35°C — the human body can no longer reliably regulate its internal temperature, even in shade, because sweat can no longer cool the body effectively. At that point, prolonged exposure can lead to overheating, heat stroke, organ failure, and death without access to active cooling. In other words, wet bulb temperature is not just another weather metric. It is a way to understand when heat becomes physically unsustainable for human life.
The goal of this project was to design and build a simple weather-style app that shows the wet bulb temperature for any selected location. The app provides clear warnings and recommended actions when conditions become unsafe, helping users understand when heat and humidity may require them to seek shade, reduce activity, hydrate, or find active cooling.
1. Keeping the Experience Familiar
I wanted the app to feel familiar rather than technical. The concept was to avoid reinventing the weather app experience and instead focus on making one important climate metric easier to understand.
The app includes automatic location detection, the ability to select and save specific locations, a clear current-condition card, and a 12-hour forecast. Each location is presented with the most important information first, including wet bulb temperature, temperature, humidity, severity level, and practical guidance.
2. Data and Severity Levels
Weather data comes from the Open-Meteo API, including current temperature, relative humidity, and hourly forecast data. The app then calculates the wet bulb temperature using a standard approximation formula. Location search and place selection use Apple’s MapKit, while current-location detection uses Core Location.
To make the information easier to understand, the app groups wet bulb conditions into four severity levels:
Normal
Conditions are generally safe, but users should still stay aware of heat and hydration.
Warning
Heat stress may become uncomfortable or risky during extended outdoor activity. Users should take breaks, hydrate, and limit intense activity.
Danger
Conditions may become unsafe, especially for vulnerable people or anyone exercising or working outdoors. Users should seek shade, reduce exposure, and look for cooling.
Deadly
Conditions are life-threatening without access to cooling. Users should avoid outdoor exposure and seek air conditioning or another safe cooling option immediately.
3. Design and Build
The app was built in Xcode using SwiftUI, with ChatGPT assisting throughout the development process. This included help with structuring the app, connecting weather and location data, calculating wet bulb temperature, building the forecast UI, and troubleshooting issues during development.
The idea for the app came from reading Kim Stanley Robinson’s climate fiction novel The Ministry for the Future, which opens with a catastrophic heat event where wet bulb temperatures reach levels that make survival impossible without cooling. That scenario made the concept feel urgent, practical, and worth translating into a simple tool that people could understand at a glance.
4. Release Strategy
The app is still in development. My priority is to make sure it works reliably before releasing it publicly. I am open to making it free, but I also want to think carefully about positioning, visibility, and how to communicate its value. Since wet bulb temperature is still not commonly understood by the general public, the release strategy will need to focus not just on the app itself, but also on education and awareness.