What Is the UV Index?

Don't know how to interpret the local UV index? We'll show you how to do it and describe the functions of the forecast.

Reviewed and approved by el médico Diego Pereira.

Written by Josberth Johan Benitez Colmenares

Last update: 01 May, 2023

The World Health Organization (WHO) warns that the increased incidence of skin cancer worldwide is associated with excessive exposure to UV radiation from the sun. Exposure can also lead to skin burns, and there’s a way to know when the sun’s radiation is safe. It’s known as the UV index, and allows you to plan day-to-day outdoor activities.

The UV Index was designed by the WHO, the United Nations Environment Programme, the World Meteorological Organization, the International Commission on Non-Ionizing Radiation Protection, and the German Federal Office for Radiological Protection. This is known as the Global Solar UV Index (UVI), and is a guide used by authorities around the world.

UV Index characteristics

The UV Index presents a daily forecast of the expected intensity of ultraviolet radiation from the sun. The local authorities supply a daily bulletin, which includes the maximum level expected for that particular day. Usually, a four-hour period around solar noon is taken into account; that is, between 12 pm and 4 pm Note that solar noon varies according to geographic location and time of year.

The purpose of the UV index is to alert us to the times during the day when there’s a greater risk of skin damage and, therefore, when protective measures are most recommended. Most people are used to planning their activities and choosing their clothes according to the daily weather forecast.

The intention isn’t only to encourage the use of sunscreen during these times, but to avoid direct exposure to the sun’s rays. Similarly, we should use clothing that covers the skin’s surface and plan activities at times when ultraviolet radiation is lower. Interpreting the index is very easy, only some basic criteria must be taken into account.

How to interpret the UV index?

The UVI index is a standard guide that has been applied in almost every country in the world. However, some local meteorological agencies may include slight differences in it. These aren’t substantial, so they aren’t that far from the standard model.

Researchers have endorsed its use after more than two decades of application, so it’s a safe and reliable measure to avoid excessive exposure to the sun’s ultraviolet rays.

UVI values develop a spectrum ranging from 1 to 11, which are associated with different recommendations for sun protection. There are several ways to distribute the degree of exposure to UV rays, although the Environmental Protection Agency (EPA) lists five alert levels:

• Low: Forecasts less than 2 on the spectrum are categorized as low risk for UV exposure. Outdoor activities are recommended, although people with very fair complexions or those who are especially sensitive to the sun should use sunscreen.
• Moderate: Includes forecasts that range between 3 and 5 on the spectrum of the index. People should take precautions against them, and the use of sunscreen, sunglasses, and clothing that covers the skin is always recommended. It’s also suggested to minimize the time spent in direct exposure to lightning.
• High: Forecasts that range between 6 and 7 are classified as high exposure to UV rays. Sun exposure should be avoided during midday and the hours before and after. If you plan to go outside, it’s essential to use sunscreen and clothing that covers most of the skin’s surface.
• Very high: When the forecast estimates a range from 8 to 10, the exposure is classified as very high. It’s recommended to avoid outdoor activities that involve direct interaction with the sun between 10 am and 4 pm. Otherwise, you may expose yourself to burns and other associated complications. You must follow the previous protection recommendations.
• Extreme: Finally, when the forecast estimates a spectrum of 11, the exposure risk is classified as extreme. It’s the highest value of the index, so you should avoid outdoor activities as much as possible. Even temporary exposure can result in burns to the surface of the skin.

Often these categories are replaced by colors at the time of supplying the daily bulletin. When this is done, green (low), yellow (moderate), orange (high), red (very high) and violet (extreme) are used. It’s very important that you know this color system, as, for practical reasons, it tends to be used when giving the forecast.

Factors that affect the UV index

Different phenomena can increase or decrease the real scope of the forecast of the day of exposure to ultraviolet rays. Most newsletters include these variables, but you should still be aware. The most important are the following:

• Cloud cover: Some clouds can partially block the amount of UV rays from reaching the earth’s surface. However, fair-weather clouds (very white and fluffy) have the opposite effect. These reflect them with greater intensity, so that the real spectrum of the forecast may be greater.
• Ozone levels in the atmosphere: Similarly, the presence of ozone can increase or decrease the amount of ultraviolet rays that reach the ground. Its composition varies from one day to the next, so you shouldn’t trust the previous day’s forecast.
• Terrestrial cover: Man-made or natural structures can totally or partially dissipate UV rays. Although the shadow of the trees may give the feeling that you’re safe from it, in reality the exposure is still high under them. Some car windows can let both UVA and UVB rays through.
• Other features: For example, the presence of sand or snow. These can reflect or enhance UV rays. White or gray structures can also do the same.

The time of day, latitude, altitude, and season are other factors that come into play. Usually, the UV index of the day is provided by radio, press, and on TV, although there are also applications for mobile phones and tablets that give us the day’s forecast. By checking it online, you can easily plan your outdoor activities.