The UV Index Explained

Medically Reviewed by Zilpah Sheikh, MD on February 12, 2024
Written by Evan Starkman, Kendall K. Morgan
7 min read

The ultraviolet (UV) index is a number on a scale from 1-11+ that tells you how intense the UV rays from the sun are predicted to be at a particular time and place. It's useful because, while you might be tempted to soak up the sun on a nice, bright day, those warm rays come with health risks. That’s because the sun gives off UV radiation.

Too much UV light can burn your skin in the short term and raise your risk for skin cancer in the long term. UV index is a color-coded scale that gives you an idea of your UV ray exposure.

It works like this:

  • Low exposure (green): 1-2
  • Moderate exposure (yellow): 3-5
  • High exposure (orange): 6-7
  • Very high exposure (red): 8-10
  • Extreme exposure (violet): 11+

The index only predicts levels of UV radiation – not how hot it’ll be outside. It gives you a forecast of the UV level at noon, when the sun tends to be highest in the sky. That said, the UV level rises and falls as the day goes on.

You can look up your UV index forecast by ZIP code or city and state through the Environmental Protection Agency’s website. When you check the UV index, you'll also get recommendations about how to protect yourself from the sun.

The National Weather Service and the Environmental Protection Agency (EPA) developed it in 1994. The agencies created the index as a way for you to plan ahead and protect your skin from the sun. The National Weather Service calculates the UV index each day.

The National Weather Service uses a computer model to figure out what the UV index is. The model calculates how much UV will reach the ground based on the amount of ozone forecasted in the Earth's stratosphere. It also considers how cloudy it is and the elevation at different places.

Ozone is a gas. You can't see it or smell it. But it's made of three oxygen molecules. You may see this written as O3. Ozone is found all throughout the atmosphere. But most of it is in the stratosphere. The stratosphere starts 6-10 miles above the Earth and goes up about 31 miles. 

Ozone acts as a protective shield from UV radiation. Changes in the amount of ozone from seasons, weather, or depletion mean that different amounts of UV will get through at different times.

Calculating the UV index depends on measures of ozone from satellites operated by the National Oceanic and Atmospheric Administration (NOAA). The UV model also considers the angle of the sunlight based on latitude, day of the year, and time.

The calculation also considers UV strength, which depends on wavelengths. Shorter wavelengths are more damaging than longer ones. The calculation then adjusts for elevation. That's because UV gets more intense the higher you are above sea level. 

Clouds also absorb UV. When the skies are blue, almost all UV will come through. Most of it will still come through when there are scattered or broken clouds. If the skies are completely overcast, 31% will get through.

Once all of this is considered, the number is divided by 25 and rounded to the nearest whole number to get a result ranging from 0 (darkness or very weak sunlight and UV) to the mid-teens (very strong sun and UV).

 

If you want to have a way to check the UV index in the palm of your hand, you can download a variety of smartphone apps. Some of them may be more accurate than others. So check out where they are getting their information. 

A good one to try comes right from the EPA. It's called EPA's SunWise UV Index. You can enter your ZIP code or use your location and find out how safe it is to stay outside without sun protection right away. The app also offers daily maps and information about UV index. It's available in English and Spanish.

Many factors can influence the UV index. They include:

Time of day. UV radiation peaks during the middle of the day. It eases up in the early morning and late afternoon.

Cloud cover. If there’s heavy cover, it can block most UV radiation. If the clouds are thin or broken, most UV rays get through. Puffy clouds during fair weather deflect sun rays and can up the amount of UV radiation that reaches the ground.

Land cover. Things like trees can cut the amount of UV radiation you get.

Ozone. This gas found in the upper atmosphere absorbs UV radiation. The more ozone, the fewer sun rays reach the ground. Ozone found on ground level is the main ingredient in smog and can harm your health. Ozone levels vary each day and throughout the year.

Altitude. UV radiation goes up about 2% for every 1,000-foot increase in elevation because of thinner mountain air.

Seasons. UV radiation peaks during spring and summer (April through August). It drops in fall and is lowest during winter.

Properties at the Earth's surface. Things on Earth’s surface can reflect or scatter UV rays. The EPA says snow might reflect up to 80% of UV, sand may reflect 15%, and water 10%.

Latitude. This measures the distance north or south of the equator. UV radiation is strongest at the equator and drops toward the North and South poles.

The UV index will be highest where the sun is direct, the skies are clear, and the elevation is high. The Atacama Desert in Chile is noted as a spot where the highest UV on the planet may occur. That's because it's got high altitude. It's usually cloudless and has low ozone levels, too. One study measured peak levels there at 20. That's high considering that UV index usually maxes out in the mid-teens.

UV generally should be highest in the tropics where there's lots of sun and low ozone. High altitudes in the Southern Hemisphere mean more UV, especially when the sun is right overhead.

If you have a medium to dark complexion, you’re less sensitive to UV exposure in general. Darker skin has more of a pigment called melanin, which helps block UV rays. But the sun can still damage your skin and raise your chances for skin cancer. So it’s still important to keep an eye on the UV index and wear sunscreen and protective clothes.

If you have lighter skin, you’re more likely to get sunburn and skin cancer. Check the UV index and be extra careful in the sun if you have:

  • Pale skin
  • Blond, red, or light brown hair
  • Freckles or moles
  • A history of getting sunburns
  • Skin that burns easily
  • Gotten treatment for skin cancer
  • A family member who’s had skin cancer

 

You'll burn faster the higher the UV index is. But there's no way to tell you how long it would take you to burn. Your chances of burning depend on steps you take to protect your skin, including using sun protection and limiting your time outside. Some people burn more easily than others, too. You might burn very fast while someone else won't.

Shadow rule

The shadow rule is a way to gauge UV rays without looking it up or checking your smartphone app. You can tell by looking at your shadow. If your shadow is shorter than you are, the sun's rays are strong. This usually happens around midday. You're most likely to burn during this part of the day.

When the UV index is 1, you don't need to worry too much about sun protection. You should be able to spend lots of time outside without taking many precautions. But if you have pale, sensitive skin, it's possible you could still get burned on a day with a UV index of 1 or 2, especially if you stay out for hours without any sun protection.

The amount of UV you’re exposed to depends on more than the strength of the sun’s rays. It also matters how long your skin is exposed to the sun and whether you’re wearing protective clothes and sunscreen.

 

There isn’t a best UV index number for tanning, because there’s no safe or healthy way to get a tan. Getting too much UV can lead to sunburns and premature aging. It can also lead to skin cancer, including melanoma, the most serious type.

UV index is a way to plan for how much sun protection you'll need at certain places and times. It depends on lots of factors, including weather, ozone, seasons, elevation, and more. While it's a good thing to check, it's always a good idea to take steps to protect your skin from the sun, especially if you are prone to burning.

  • Is UV still high in the shade?

Shade can help you to limit your UV exposure. But you can still get burned in the shade when UV bounces off of surfaces, such as sand, water, or concrete. Using a beach umbrella can help you get less sun, but it may not help as much as you'd expect.

  • What does UV index 13 mean?

A UV index of 13 is extremely high. Anything over 11 is considered extreme UV exposure. If you're outside with a UV index this high, you'll need to protect yourself. If you have to be outside, wear a shirt, hat, and sunscreen. Look for shade, but remember that UV can reflect off surfaces. Even in the shade, you may still do damage to your skin.