Estimate time to sunburn by UV index, skin type, and SPF using live weather.
These estimates are based on dermatological research on UV exposure and erythema (skin reddening). Here's how it works:
The UV Index measures the intensity of skin-damaging radiation. Higher number means faster burn.
A 0.8× geometry factor is applied because you're a person standing and moving, not a flat sensor pointing at the sky.
Each skin type has a Minimal Erythemal Dose (MED), the UV energy needed to cause visible reddening. Measured in J/m² (joules per square meter).
These are population averages. Your actual threshold varies with genetics, recent exposure, and other factors.
SPF divides the UV reaching your skin. SPF 30 means only 1/30th of the UV energy gets through.
Real-world sunscreen rarely performs at lab-rated SPF due to application thickness and sweat. We model this degradation over time.
Think of it as filling a bucket. Each minute adds UV damage, and at 100% you're burnt.
This runs minute-by-minute using hourly UV forecasts, interpolating between data points.
For low UV (UVI < 3), a smoothing curve accounts for reduced effective exposure at dawn/dusk sun angles.
Sweat Index is an outdoor comfort score based on temperature and dew point. Higher values mean hotter, muggier air where sweat doesn't evaporate as easily.
This does not change UV strength or your calculated sunburn time. Use it as context for outdoor comfort, sunscreen reapplication, and deciding whether conditions feel manageable for time outside.