Burn time is Shade’s estimate of how long you can stay in the sun before your skin is likely to start burning. It’s built from three things: the UV index where you are right now, your skin type, and whether you’re wearing sunscreen or covered up.
How the estimate works
The UV index measures how much skin-burning UV reaches the ground. Each skin type has a typical dose at which it starts to redden, known as the minimal erythemal dose. Shade divides that dose by the UV you’re getting, keeps a margin below it, and gives you a single number in minutes.
When you start a session, the countdown follows the hourly UV forecast instead of a fixed value. That matters because UV climbs towards solar noon and falls off in the afternoon, so the same 30 minutes at 11:00 and at 16:00 are not the same dose.
What you see
- Today screen: the current UV, your burn time and the recommended SPF in one card.
- During a session: a live countdown, turn-over cues and a heads-up before your estimated burn time.
- Earlier sessions count: time you already spent in the sun today is taken off your remaining budget.
Where the numbers come from
UV and weather data come from Apple Weather. Burn-time guidance follows the WHO UV Index and standard Fitzpatrick skin-type values. Shade shows estimates, not limits.
Frequently asked questions
How does Shade estimate burn time?
It takes the live UV index from Apple Weather, your Fitzpatrick skin type and whether you’re wearing sunscreen, and estimates how long until your skin is likely to receive a burning dose. The estimate follows WHO UV Index guidance and standard minimal erythemal dose values, with a margin below the threshold.
Why does Shade count sunscreen so conservatively?
Most people apply far less sunscreen than the amount used to test SPF, and skip reapplying. Shade counts sunscreen as a few-fold extension, not the full SPF number, so the estimate doesn’t run long.
Is the burn time a guarantee?
No. It’s an estimate. Altitude, reflection from sand or water, medication and your own skin all change the real answer.