Sun, skin and UV — explained

Cloud cuts brightness and heat far more than it cuts ultraviolet — which is exactly why cloudy days catch people out.

Skin-only means sunburn; skin plus how you feel means get checked. The symptoms that separate the two, and the ones that are an emergency.

One discipline decides the outcome: stop at half your burn threshold. The full method, in the order you would actually do it.

Same tan, far more margin for error. Why the shoulder hours beat the peak, and how to find solar noon for your location instead of guessing.

Almost no app measures UV; they all forecast it. A comparison by what each one is actually for, and what to look for before you trust any of them.

Sunscreen slows tanning rather than stopping it, and removes the part of the dose that burns you. What SPF numbers actually mean in practice.

Peak at 12–24 hours, peeling from day three, discolouration for weeks. What to expect, what to do, and the point at which it needs medical attention.

Vitamin D synthesis needs UVB, and UVB needs a high sun. Why the UV index decides it, roughly how long it takes, and when supplements are the honest answer.

The Fitzpatrick scale measures how your skin reacts to UV, not what colour it is. How to place yourself correctly, and what your type predicts.

Two different tans, two different clocks. Why the colour you see the same day is not the one that lasts, and how long a real tan actually takes to build.

The UV index is not a temperature and not a percentage. What the scale measures, band by band, and the five factors that move it where you are.

There is no single best UV index for tanning — there is a range, and a personal limit. What UV 3, 5, 8 and 11 actually mean in minutes for your skin type.