Learn to read UV Index scale — stay safe under the sun

Learn to read UV Index scale — stay safe under the sun


When you are outdoors, it is important to know not only the air temperature or wind speed, but also the level of ultraviolet radiation from the sun, otherwise known as the UV Index.

Knowing the index is especially important for people doing various sports and outdoor activities: You spend long periods of time under the bright and powerful sun hiking, riding a bike, fishing or surfing, and there is no shade around to hide in, or you just can't do it.

In this article, we will understand what this index is, how to read the UV Index scale and what to do to stay safe.

What is UV Index?

The UV Index, or Ultraviolet Index (UVI), is an index developed and standardized for use around the world by a collaborative effort between the World Health Organization (WHO), the United Nations Environment Programme (UNEP), the World Meteorological Organization (WMO), and several other such agencies.

The full official name of the index is the Global Solar Ultraviolet (UV) Index. Other names people used to say are Sun Index, Solar Radiation Index, UV Ray Index, UV Rating, UV Level, and so on.

As you can already see, the index was made to measure the level of ultraviolet radiation from the sun. Why should we protect ourselves from it? Let's understand it briefly:

  • Ultraviolet radiation is one of several major types of radiation on par with visible light, infrared light, and others. It mainly comes from the sun.
  • Unlike the visible radiation to the human eye, we cannot see ultraviolet rays, or we have to use special devices to do so. Yet we are almost constantly exposed to this radiation.
  • The UV level depends on the Sun elevation (the higher the sun in the sky, the higher the UV) latitude and altitude (the closer to equator, the higher the UV), cloud cover, ozone, ground reflection — these are the main factors.
  • Ultraviolet radiation from the sun is bad for our skin and our general well-being. Don't panic. In small quantities, it is considered even useful. In general, without the sun, there would be no humans and all life on earth. But if you are in direct sunlight all day, you can get sunburn.
  • Movement helps to reduce the effects of radiation, as in the case of active sports, but only partially. Still, long and systematic exposure to the sun can cause serious health problems like premature skin aging, skin cancer, and even DNA changes — ok, we will not scare you anymore.

That's why health experts and meteorologists around the world recommend wearing protection against the bright sun or better just not being under the sun.

They also have begun to make predictions about UV radiation, after the creation of the index — just as they do for air or sea temperature, different wind conditions, atmospheric pressure, and so on. Over time, the UV index became an integral part of weather forecasts around the world.

By the way, if you are wondering how the UV Index is calculated, the formula looks like this:

UV Index scale formula / World Health Organization

How to read the UV Index scale to stay safe under the sun?

The UV Index is a simple scale with numbers from 1 to 11+. Each level of the scale show the risk of harm from unprotected sun exposure. For a better and quicker understanding of the scale, the numbers are also highlighted in color. Well, here's what recommended to do in case of certain index values:

UV Index 1—2 (green) is a low

Wear sunglasses in bright sunny weather, but remember, that over 90% of UV still can penetrate light clouds, if and when they appear. If you get sunburned easily but need to be in the sun, wear long-sleeve clothing and use sunscreen of at least SPF-30.

It's also good to know that dry beach sand reflects up to 15% of UV. The sea foam reflects about 25% of UV, and the clear snow reflects up to 80% of sunburning UV.

At the same time, UV increases by 4% for each 300 m increase in altitude (with every 1000 m — by 10% to 12%), and at half a meter depth in the water UV is still 40% as intense as on the surface.

UV Index 3—5 (yellow) is moderate

Also, wear sunglasses and clothing with long sleeves. Besides, be sure to wear a hat. Use the same sunscreen of at least SPF-30. In the middle of the day, when the sun is at its strongest, stay in the shade most of the time. Shades can reduce UV by 50% or more.

The UV Index scale / World Health Organization

UV Index 6—7 (orange) is high

Minimize the time you spend outside from 10 a.m. to 2 p.m. daily, but better up to 4 pm. 60% of UV is received between this time. Outside the equator and tropics in general, the highest levels of UV occur when the sun is at its maximum elevation, at around midday (solar noon) during the summer. Wear long-sleeved clothing, a hat, and sunglasses, and use sunscreen of at least SPF-30.

UV Index 8—10 (red) is very high

Be sure to wear also long-sleeved clothing and pants instead of shorts. Wear a hat with a wide brim, which creates more shadow around your head and face. Also wear sunglasses and use sunscreen of at least SPF-30, and stay in the shade as much as possible between 10 a.m. and 4 p.m.

If you work outdoors under the sun, it is good to know that indoor workers receive 10% to 20% less yearly UV exposure.

UV Index 11+ (violet) is extreme

Use all the protective equipment and precautions described above. At this level of index, the skin burns in minutes.

The highest UV Index of an incredible 43.3 points (!) was recorded on December 29, 2003, in Licancabur volcano on the border between Bolivia and Chile, in South America. I guess, at that moment in history, this place was quite dangerous to spend all the day hiking.

Important to know:

  • The recommendations you see are for normal, healthy adults with a little bit of a tan. People with lighter skin are more likely to sunburn.
  • People with various serious contraindications are not recommended to be in the sun for a long time.
  • Children and the elderly are also advised not to spend much time in the bright, strong sun without moving.

Here are some more facts and helpful recommendations in the PDF document on the official website of the World Health Organization.

Where to find UV Index in Windy.app?

Ultraviolet Index in Windy.app is the same world standard index used by the WHO: the numbers from 1 to 11+ and the colors from green to purple.

To get the UV index forecast do the following steps:

1. Open your favorite or the nearest spot to your current location — for example, a popular cycling spot Passo dello Stelvio in the Italian Alps. You can do it both in the Search and on the Wind and precipitation map.


Click on the "star" to make this spot a favorite:


2. Choose a Bike weather profile by an icon right to the weather forecast models under the wind rose.


3. Get UV index for today, tomorrow, and up to 10 days. See, what it was in the past 10 days by sliding the screen to the left. If you see a small circle instead of a number, it means that the level of the index is less than 0, even safer.



4. Add UV Index to your custom weather profile from the general list of all 50+ weather parameters you can have in Windy.app.




Text: Ivan Kuznetsov, an outdoor writer from the Dolomites, Italy, and Karelia, Finland. His favorite sports are cycling, hiking and sauna. Read his other articles.

Cover photo: Arni-svanur-danielsson / Unsplash

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