UV Index Explained: Scale, Chart & Sun Protection Guide

What is the UV index?

The UV index measures the strength of skin-damaging ultraviolet radiation reaching the ground, on a scale that starts at 0 and is open-ended (11+ is “extreme”). It peaks around solar noon, rises with altitude by roughly 10% per 1,000 m, and reflects strongly off snow, sand, and water. Clouds help less than people assume - up to 80% of UV passes through thin cloud cover.

UV index chart

WHO UV index categories and recommended protection
UV index	Category	Burn time (fair skin)	Protection needed
0–2	Low	60+ minutes	Sunglasses on bright days
3–5	Moderate	30–45 minutes	SPF 30, hat, shade at midday
6–7	High	15–25 minutes	SPF 30+, hat, sunglasses, seek shade 10am–4pm
8–10	Very high	10–15 minutes	SPF 50, cover up, minimize midday sun
11+	Extreme	Under 10 minutes	Avoid midday sun entirely; full protection

Reading the UV forecast like a pro

Every city page on this site shows the current UV index. Four rules of thumb cover most situations:

Temperature is not UV. A cool, bright spring day can burn you just as fast as a hot one.
Reflection doubles exposure. Water, sand and snow bounce UV back up at you - beach and ski days count twice.
Some medications amplify it. Certain antibiotics and retinoids increase sun sensitivity - check the label.
Reapply every two hours. One morning application of sunscreen does not last a full day, especially after swimming.

To turn any UV index value into irradiance and estimated burn times, use the UV index converter. For places where UV runs high year-round, check current levels in Honolulu, Sydney or Cape Town.

UV by season, latitude and altitude

Three geometry rules explain most surprise sunburns:

Season and sun height: UV scales with how high the sun climbs. Midsummer midday UV can be 8-10 where winter midday manages 1-2 - but spring catches people out, because UV recovers months before warmth does.
Latitude: closer to the equator, the sun stays high all year. Singapore sits at UV 9-12 in every month; Stockholm never escapes winter UV 0-1.
Altitude: thinner air filters less - UV rises roughly 10% per 1,000 m, which is why mile-high Denver burns faster than sea-level cities. A 3,000 m ski slope adds a third more UV, then snow reflects up to 80% of it back at your face.

Sunscreen numbers, decoded

SPF measures how much longer protected skin takes to burn: SPF 30 filters about 97% of UVB, SPF 50 about 98%. The gap between them is small; applying enough matters far more. Most people use a third of the tested amount - a full shot-glass worth covers an adult body, reapplied every two hours.

Look for “broad spectrum”, which adds UVA protection (the aging-and-deep-damage wavelengths SPF alone ignores), and remember that UV passes through clouds and reflects off water, sand and snow - shade and cover still beat any bottle.

Frequently asked questions

Is SPF 100 twice as good as SPF 50?

No. SPF 50 filters about 98% of UVB and SPF 100 about 99% - a one-point gain. Applying a generous, even layer and reapplying every two hours matters far more than the number on the bottle.

Can you still make vitamin D when the UV index is low?

Barely. Below UV 3 the skin makes very little vitamin D, which is why health agencies in northern countries recommend dietary vitamin D through winter rather than sun exposure.

What UV index is safe for tanning or being outside?

No UV level produces a “safe” tan - A tan is skin damage. At UV 0–2 unprotected time is low-risk for most people; from UV 3 upward, sun protection is recommended.

Can you burn on a cloudy day?

Yes. Up to 80% of UV radiation penetrates thin clouds, so a UV index of 6 under haze burns nearly as fast as under clear skies.

When is the UV index highest?

Within about two hours either side of solar noon, in late spring and summer, at high altitude, and near reflective surfaces like water, sand, or snow.