UV Index Explained: Scale, Colours and What Affects It

What the UV index measures

The UV index (UVI) is a simple number for a complicated thing. The World Health Organization describes it as a measure of the level of UV radiation. The higher the number, the greater the potential for skin and eye damage, and the less time it takes for harm to occur.

A number helps because UV does not follow temperature or brightness. Cancer Council Australia points out that UV can stay strong on cool or cloudy days, so it recommends checking the UV index rather than judging by the weather.

The number is calculated, not measured on your street. The US EPA explains that a model combines the angle of the sunlight with forecast ozone to estimate UV strength across the UV wavelengths. Each wavelength is weighted by how strongly it affects skin, the weighted values are added up, the total is adjusted for elevation and cloud, and the result is divided by 25 and rounded (US EPA).

The UV scale

The scale starts at 0 and has no fixed top: values of 11 and above are called extreme. Cancer Council Australia publishes the five bands below (source).

The coloured dots are UV GO’s own colours, matched to the closest of its three verdict colours. Green covers 0 to 2, yellow covers 3 to 7, and red covers 8 and above. Other bodies group the numbers differently. The WHO’s public question-and-answer page uses three bands: 0 to 2, 3 to 7, and 8 and above. The US EPA scale groups them the same way, and for 8 and above it adds a rule of thumb: if your shadow is shorter than you, seek shade (US EPA).

The number worth remembering is 3. The WHO says sun protection measures should be used when the UV index reaches 3 or above, and Cancer Council Australia says sun protection times are issued when the index is forecast to reach 3 or above, because at that level UV can damage skin and lead to skin cancer.

Why UV peaks around midday

UV follows the sun’s height in the sky. The higher the sun, the more ultraviolet radiation reaches the ground, and the WHO notes that levels vary with the time of day and the time of year.

The daily UV index you see in a forecast is the day’s maximum. The WHO says that maximum occurs during the four-hour period around solar noon, and that solar noon falls between noon and 2 p.m. depending on location and whether daylight saving time applies. Cancer Council Australia describes peak UV hours as usually between 10 am and 4 pm.

For planning, this means the middle of the day is where the number is highest and the early morning and late afternoon are where it drops. UV GO compares the hourly forecast inside your free time, balancing UV with rain, heat, cold and wind.

Season, latitude and altitude

Three things set the baseline before the weather has a say. All three come from the WHO fact sheet.

• Season. The sun is higher in summer, so UV is higher.
• Latitude. The closer you are to the equator, the higher the UV.
• Altitude. The air is thinner higher up, so less UV is absorbed on the way down.

The EPA puts a number on altitude: UV intensity increases about 6% per kilometre of elevation above sea level. By that rule, a ridge at 2 km would see about 12% more UV than the same sky at sea level. The WHO adds one more factor, ozone: the less ozone in the atmosphere, the more UV reaches the surface.

Clouds, water, sand and snow

Clouds help, but less than most people think. The WHO says UV levels are highest under cloudless skies and can still be high with cloud cover. The EPA gives the share of UV that gets through in its model: virtually 100% under clear skies, 89% with scattered clouds, 73% with broken clouds and 31% when fully overcast (US EPA).

So thin cloud barely changes the number, and even a grey overcast sky lets through about a third of UV in the EPA model. That is the reason a cloudy afternoon can still burn.

The ground matters as well. The WHO lists water, sand and fresh snow as surfaces that increase UV levels. The sources used here do not give percentages for reflection, so treat it as a reason to be more careful at the beach or on a ski slope, not as a figure to calculate with.

How UV GO uses the UV index

UV GO reads the hourly UV forecast for your location from Open-Meteo, rounds each hour’s value to a whole number (5.5 rounds up to 6), and gives the hour points. These thresholds are UV GO’s own rules and the current values are listed in How it works:

• UV 0 to 2 adds 0 points.
• UV 3 to 5 adds 2 points.
• UV 6 to 7 adds 4 points.
• UV 8 and above adds 6 points.

If you choose “Long in the sun”, the UV points are multiplied by 1.5, because a longer stay means more exposure.

Rain, heat, cold and wind add points too. The best time is the window inside your free time with the lowest average score, and lower is better. The average is rounded to the nearest whole number, with .5 rounded up: 0 to 2 shows green, 3 to 5 shows yellow and 6 or more shows red. The app compares exact averages before rounding. Best means the lowest score among eligible windows, even when all of them are red. Missing factors add no points, so a low score can reflect incomplete data.

The UV index also drives the short reason under your result, such as “UV 5, use sunscreen”, and two items on the what-to-take list. Sunscreen appears when the highest rounded UV across the window’s forecast hours is 3 or above, which matches the WHO threshold above. Hat and sunglasses appear at UV 6 or above, which is a UV GO choice that lines up with the start of the High band.

Windows start on the hour and use forecast hours marked as daytime. A 30-minute outing uses the same one-hour forecast as a 1-hour outing, while displaying a 30-minute result. All times are local to the selected place.

UV GO is a planning aid. It tells you when the numbers look best inside your free time and what to bring, and it does not know your skin, your medication or your plans. For that, see the risks and how to protect yourself.