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UV index, sunscreen amount & reapply timer

WHO UV index scale · 7-day hourly forecast

UV index forecast

Look up any town to see how strong the sun's UV will be, hour by hour for the next seven days, and when the World Health Organization advises sun protection.

    No place chosen yet. Search for a town or use your location to load its real forecast. Until then this page shows no UV numbers at all.

    The WHO UV index scale

    • 0–2

      LowNo protection required. You can safely stay outside.

    • 3–5

      ModerateProtection required. Seek shade during midday hours, slip on a shirt, slop on sunscreen and slap on a hat.

    • 6–7

      HighProtection required. Seek shade during midday hours, slip on a shirt, slop on sunscreen and slap on a hat.

    • 8–10

      Very highExtra protection. Avoid being outside during midday hours, make sure you seek shade; shirt, sunscreen and hat are a must.

    • 11+

      ExtremeExtra protection. Avoid being outside during midday hours, make sure you seek shade; shirt, sunscreen and hat are a must.

    Bands, colours and messages from the WHO Global Solar UV Index guide. The forecast you load is coloured the same way.

    How to read the UV forecast

    1. Choose a place. Type a town, city or postcode and press Find, then pick the right match. Add a country after a comma (“Perth, Australia”) to skip the namesakes. “Use my location” asks your browser for your position instead. The last place is remembered on this device.
    2. Pick a day. The forecast covers seven days, in the chosen place's own local time.
    3. Read the peak. The big number is the day's highest UV index with the forecast cloud cover. Following WHO, it is rounded to a whole number and coloured by band. The clear-sky figure is what the UV would reach if the sky cleared, so on a changeable day expect something between the two.
    4. Check the hours. Each bar is the average UV over one hour; the line is the clear-sky value. The “UV 3+” time is when WHO advises sun protection, and from 8 upwards it asks for extra protection.
    5. Glance at the sundial. It is centred on solar noon, when the sun is highest, with each hour's UV painted around the rim. WHO's rule of thumb: if your shadow is shorter than you are, UV is likely to be high.

    Example: if the bars first reach 3 in the hour from 10:00 to 11:00, peak at 8 between 13:00 and 14:00 and fall back below 3 after 17:00, the day's peak is Very high. The tool then shows sun protection advised from 10:00 to 17:00 and WHO's message for that band: avoid the midday sun, seek shade, and wear a shirt, a hat and sunscreen.

    The numbers are Open-Meteo's model forecast, not a measurement, and they describe a flat, open spot. Shade, altitude and bright ground such as snow, sand or water change what reaches your skin, as explained below.

    What changes the UV where you stand

    • Cloud: WHO notes that over 90% of UV can get through light cloud, so a grey day is not automatically a low-UV day. That is why the clear-sky value is worth a look.
    • Altitude: UV rises by roughly 10% for every 1,000 m you climb.
    • Ground: clean snow reflects up to 80% of sunburning UV and sand up to 25%, adding to what comes from the sky.
    • Shade: can cut UV by 50% or more, but scattered UV from the open sky still reaches you. The British Association of Dermatologists says a beach umbrella offers only about 3 to 5 times more protection than no shade at all.
    • Time of day: WHO estimates that about 60% of the day's UV arrives between 10 am and 2 pm. That assumes clocks close to sun time: where clocks run ahead of the sun, as in much of Europe in summer, the strong hours come later on the clock. The sundial shows where solar noon falls.

    Skin tone and the UV index

    The WHO scale is pitched at people with fair skin, who burn most easily. The British Association of Dermatologists recommends sun protection from a UV index of 3 for people with white skin. For people with black or brown skin it sets the point at 11 or more, except for those with a condition that makes skin sensitive to light (such as vitiligo or lupus) or a high risk of skin cancer, who need protection at lower levels too. Anyone worried about darker patches can protect just those areas.

    Some medicines and perfumed products can make skin burn more easily too; WHO suggests asking a pharmacist. The BAD advises keeping babies under 6 months out of direct sun altogether.

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    Questions about the UV index

    What exactly does the UV index measure?

    It measures how strongly the sun's ultraviolet light reddens skin, at ground level on a flat surface. WHO, the World Meteorological Organization, UNEP and ICNIRP weight the sunlight by the standard skin-reddening action spectrum, so each step of 1 on the index stands for 25 milliwatts per square metre of sunburning UV. The scale starts at 0 and has no upper limit.

    Why does the forecast show two values?

    The main value includes the cloud cover Open-Meteo forecasts; the clear-sky value assumes no cloud at all. Cloud forecasts are uncertain, and WHO suggests giving a range when cloud is variable, so on a mixed day plan for the higher figure during the brightest spells.

    Do I need sun protection when it is cloudy or cool?

    Possibly. UV is not felt as heat, and WHO notes that over 90% of UV can pass through light cloud. Go by the UV index rather than the temperature: WHO advises protection whenever it is 3 or higher.

    Is UV 3 the right threshold for every skin tone?

    WHO's scale is aimed at fair skin, which burns most easily. The British Association of Dermatologists advises protection from UV 3 for people with white skin and from 11 for people with black or brown skin, except for people with light-sensitive skin conditions or a high skin cancer risk, who need protection at lower levels too.

    How accurate is the forecast?

    It is a weather-model forecast, so treat it like any other: good for planning, less certain several days ahead and when clouds come and go. It also describes open ground. Altitude, snow, sand, water and shade change your own exposure, and the next few hours are best judged by looking at the sky as well.

    What is sent when I search or use my location?

    Only what Open-Meteo needs: the place name you type goes to its geocoding service, and the forecast request carries the place's coordinates rounded to two decimals (about 1 km). With “Use my location”, your browser asks for permission first, and only the rounded coordinates leave your device. Nothing goes to Rose Quartz Glow.

    Sources

    • World Health Organization, WMO, UNEP and ICNIRP. Global Solar UV Index: A Practical Guide. WHO, 2002. who.int (the scale, band colours, protection messages, rounding, cloud, altitude, reflection, shade and shadow rule).
    • British Association of Dermatologists, The sunscreen fact sheet (UV thresholds by skin tone, beach umbrella shade, babies).
    • Open-Meteo, Weather Forecast API and Geocoding API documentation (hourly UV is the mean of the preceding hour; clear-sky UV assumes no cloud).
    • NOAA Global Monitoring Laboratory, Solar Calculator equations, used for solar noon, sun height and shadow length.

    UV forecasts: Weather data by Open-Meteo.com, licensed under CC BY 4.0; values are rounded for display.