Why doesn't SPF 15 simply mean fifteen hours before you burn?

Answer: It compares how much sunlight reddens protected and bare skin.

It counts how long the lotion keeps working before it wears off. — That treats the label as a wear-off timer. Sunscreen does wear off, so time outdoors matters, but SPF is not a countdown for how long the lotion keeps working. The SPF number comes from a redness test that compares protected and bare skin.

It compares how much sunlight reddens protected and bare skin. ✓ — Right: the number comes from comparing how much sunlight reddens protected skin with how much reddens bare skin. In testing, SPF 15 means protected skin needed about fifteen times as much exposure to redden. That makes it a protection ratio, not a schedule for hours outdoors.

It tracks how warm the skin feels while you are in the sun. — Warmth is a feeling on the skin, not the thing SPF is tested against. The label is based on sunlight and the redness response, so a hot face or a cool breeze does not change what the number means. Feeling comfortable in the sun can still leave you exposed to burning ultraviolet light.