Estimate sunburn risk from skin type and UV.
Sunburn is a dose problem: risk rises with planned exposure measured against the time your skin can tolerate at the current UV index. Sunscreen extends that tolerance, but real-world protection is far below the label — modelling it as the square root of the stated SPF reflects typical under-application. Once planned exposure reaches the tolerance time, erythema is essentially certain and additional time increases severity rather than probability.
Dose-based burn risk
Protected burn time = (200 x skin type factor / (UV x 10)) x sqrt(SPF); risk = exposure / burn time x 90%, rising above 90% once the ratio exceeds one
Sunburn is an acute injury and cumulative UV exposure raises skin cancer risk. These estimates are population averages and do not account for medication, photosensitising conditions or personal history — consult a healthcare professional.
Unprotected fair skin can burn in fifteen minutes or less. With realistically applied SPF 15 that extends to roughly an hour, which is still short for a day outdoors.
Very little — a tan provides an SPF equivalent of about 3 at best, and the tanning process itself is DNA damage.
Yes. Thin cloud transmits most UV, and broken cloud can even scatter it to slightly above clear-sky levels.