The UV index and when to wear sunglasses: a number, a time of day, and a rule of thumb

The UV index is a forecast of how strong ultraviolet light will be, on a scale that starts at 0 and in most places tops out around 11 or 12. From an index of 3, the World Health Organization advises eye and skin protection. The index peaks around solar noon, is higher in summer, at altitude, closer to the equator and over reflective surfaces like snow, water and sand, and it has nothing to do with how warm or bright the day feels. If you want a rule that needs no forecast: when your shadow is shorter than you are tall, the sun is high enough that the UV is strong, and sunglasses are the right call.

The scale

The index is calculated from the ultraviolet that reaches the ground, weighted for its effect on skin, and forecast for the peak of the day. Weather services publish it daily; most weather apps show it.

What moves the index

Time of day. UV rises through the morning, peaks around solar noon, which is about 1pm in summer where clocks are on daylight time, and falls through the afternoon. The window from roughly 10am to 4pm carries most of the day's UV.

Season. The sun is higher in summer, so its light passes through less atmosphere and less UV is absorbed. Summer indices are several times winter ones at temperate latitudes.

Latitude. Closer to the equator, higher sun, higher index, year round.

Altitude. About 10 percent more UV per 1,000 metres. A mountain in summer regularly reaches extreme.

Cloud. Thin cloud barely reduces UV. Broken cloud can briefly increase it by scattering. Only thick, dark cloud reduces it much. Our cloudy days post covers this.

Reflection. Snow reflects up to 80 percent of UV, sand around 15, water around 10 at most angles and more at low sun. Over any of them you receive UV from below as well as above.

Ozone. Thinner ozone, more UV. This is why the index in parts of the southern hemisphere runs higher than at the same latitude in the north.

What does not move it

Temperature. UV is not heat. A cool windy day at altitude or by the sea can carry an extreme index; a hot humid evening carries almost none.

Brightness. Your eyes judge visible light, which cloud and haze reduce far more than they reduce UV. A hazy day can look dim and carry a high index.

That mismatch is why the index exists: it tells you what you cannot feel.

When to wear sunglasses

The short version: any time the index is 3 or above, which in summer at temperate latitudes is most of the day, and in winter is around midday on clear days.

The rule that needs no forecast is the shadow rule. Stand in the sun. If your shadow is shorter than your height, the sun is more than 45 degrees above the horizon, and at that angle the UV is strong enough to protect against. If your shadow is longer than you, the sun is lower and the index is usually moderate or below.

Add the reflective cases regardless of the index: water, snow and sand send UV up from below, and a low index over snow still burns the cornea. Our snow blindness post covers the worst case.

What the sunglasses need

For the eye, the protection is UV blocking, not darkness. A stated UV claim is what matters; our UV400 vs polarized post explains the labels, and our cheap sunglasses post explains why a dark lens without it is worse than none. At high and extreme indices, coverage at the sides and a hat add to what the lens does.

Our lenses carry a UV-protective coating, stated on every product page with its limits; the frame maker does not state UV400. For an extreme-index environment, altitude, snow, the tropics at noon, ask any seller for the UV claim in writing.

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