What is the UV index?

Updated September 20, 2026. Sources are listed at the end. See how we write our guides.

The UV index is a scale from 0 to 11+ that tells you how strongly the sun can burn unprotected skin at a particular place and time. The higher the number, the less time it takes to burn.

The UV index scale at a glance

Level	UV index	What to do
Low	0 to 2	Little risk for most people. Sunglasses on bright days. Take extra care on snow.
Moderate	3 to 5	Sun protection recommended: hat, sunglasses and sunscreen. Seek shade around midday.
High	6 to 7	Protection needed. Cut back on midday sun, cover up and use SPF 30+ sunscreen.
Very high	8 to 10	Extra protection needed. Unprotected skin can burn quickly. Avoid the midday sun.
Extreme	11+	Take every precaution. Unprotected skin can burn in minutes.

The World Health Organization (WHO), the U.S. EPA and many national weather services use this same scale, so a 7 means the same thing in Denver, Sydney or Zermatt. The scale is open-ended: the "11+" category covers everything from 11 upward.

What the number measures

UV radiation is invisible light from the sun. The UV index weights the different UV wavelengths by how effectively they damage skin and turns the result into one number for a given place and moment. By definition, each index point equals 25 milliwatts of sunburn-weighted UV per square metre, but you never need to do that maths. Just remember that higher means faster damage.

A reading of 1 at 8 am and a reading of 7 at 1 pm are the same measurement taken when the sun was low and when it was high. That is why the UV index rises and falls through the day, and why UV by Hour draws it as a curve.

UVA, UVB and UVC in plain terms

UVC is almost entirely absorbed by the atmosphere and doesn’t reach the ground from the sun.
UVB is the main cause of sunburn. It mostly affects the outer layers of skin and the surface of the eye.
UVA penetrates deeper. It contributes to skin ageing and to skin cancer risk, and it reaches you through clouds and glass more easily than UVB.

The UV index is weighted toward the wavelengths that cause sunburn, which is why it tracks well with burn risk. But UVA matters for long-term skin health too. That is why dermatology groups recommend broad-spectrum sunscreen and clothing that blocks both.

Why 3 is the number to remember

The WHO says sun protection should be used when the index reaches 3 or above. Below 3, most people can be outdoors without sunscreen or extra covering, though people with very fair or sensitive skin may want to be more careful, and snow or high altitude can add more UV than the index suggests.

Quick rule

Check the day’s peak. If it’s 3 or more, plan for a hat, sunglasses and sunscreen. If it’s 6 or more, cover up as well and take breaks in the shade around midday. At 8 or more, plan your day around the midday peak.

What changes the UV index

Time of day. UV peaks when the sun is highest, around solar noon. See when UV is strongest.
Season and latitude. A higher sun means stronger UV, so summer and places nearer the equator get more.
Altitude. The WHO says UV increases by about 10% to 12% for every 1,000 metres (3,300 feet) you climb, because thinner air filters less. More in UV at altitude, snow and clouds.
Clouds. UV is highest under cloudless skies but can still be high with cloud cover. Thin or broken cloud often lets most of it through.
Ozone. The ozone layer absorbs part of the UV before it reaches the ground, so a thinner layer lets more through.
Reflection. Fresh snow can reflect as much as 80% of UV, dry sand about 15% and sea foam about 25%, according to the WHO.

Forecast, measured and clear-sky values

Different sources report the index in different ways, and that’s why two apps can disagree:

A forecast is a computer model’s estimate, usually including expected cloud cover.
A measured value comes from an instrument at a fixed spot and is the ground truth for that spot.
A clear-sky value ignores clouds and shows the most UV the sky could deliver that day.

UV by Hour shows a model forecast for the place you pick. A cloudy day can show a low peak, and if the clouds break, the real UV can end up higher than forecast. Treat any single number as a good guide, not a guarantee.

The UV index isn’t the temperature

You can’t feel UV. A cool, breezy day at altitude can have a higher UV index than a hot day at sea level, and a sunburn often shows up hours after the exposure with no warning at the time. Go by the number, not by how warm it feels.

Who needs to be extra careful

Everyone can be harmed by UV, and the guidance in the table applies to all skin tones. Some people burn faster or face higher risk: people with fair skin, children, people who have had skin cancer, and people who take medicines or use products that make skin more sensitive to sunlight. If any of that applies to you, ask a doctor or pharmacist how much protection you need.

Common myths

“I can’t burn on a cloudy day.” You can. Cloud cuts UV less than most people expect, especially thin or broken cloud.
“A base tan protects me.” A tan is a sign that skin has already been damaged, and it gives little real protection.
“Darker skin doesn’t need protection.” Darker skin has more natural protection but can still be damaged, and sun protection is recommended for everyone.
“I’m in the shade, so I’m safe.” Shade lowers your exposure but doesn’t remove it. UV also scatters through the sky and bounces off the ground.

How to use the UV index

Look up the forecast for where you’ll actually be.
Note the peak and the hours when the index is 3 or higher.
Plan your protection, and any exposed stretches of trail, around those hours.
If you’re heading up a mountain or onto snow, allow for more UV than the forecast shows.

Frequently asked questions

What is a good UV index to be outside?

A UV index of 0 to 2 is low, and most people can be outside without extra protection. From 3 up, protection is recommended.

Can the UV index go above 11?

Yes. The scale has no upper limit. Readings well above 11 occur in the tropics and at high altitude, where the sun is high and the air is thin.

Does the UV index include UVA?

The index is weighted for sunburn risk, which is dominated by UVB, but the weighting extends into the UVA range. Because UVA also damages skin over the long term, broad-spectrum protection is still advised.

Is the UV index the same all day?

No. It follows the sun: near zero at dawn and dusk, highest around solar noon. Use the hourly chart on our home page to see when it crosses 3 where you are.

Sources and further reading

Check today’s UV for your place

This guide is general information about sun safety and isn’t medical advice. Talk to a doctor or dermatologist about your own skin.