UV Index and Time in the Sun Guide

What the UV index measures

The WHO's Global Solar UV Index: A Practical Guide defines the index as a measure of the intensity of UV radiation at the Earth's surface that matters for the human skin. It weights each wavelength by how much it reddens skin, using the CIE erythema action spectrum, and it is defined for a flat, horizontal surface. One index unit is 25 milliwatts of sunburn-weighted UV per square meter: the EPA's recipe for the forecast ends by dividing the total by 25 and rounding to a whole number.

The number in a forecast is the day's peak. The WHO's questions and answers say the maximum daily level occurs in the four-hour period around solar noon, which falls between noon and 2 p.m. depending on place and daylight saving time. So an index of 7 on your phone at 9 a.m. is a forecast for later, not a reading for now. That is why the tool above draws the whole day: the amber curve is the clear-sky index and the shaded bands behind it are the WHO's.

The WHO bands and what to do in each

The bands and their messages are from the WHO guide's tables 1 and 4 and figure 2, and the colors from its UV index sun protection table. The WHO says that even for very sensitive fair-skinned people the risk of short-term and long-term damage below an index of 3 is limited, and under normal circumstances no protective measures are needed. Above 3, protection is necessary, and the message is reinforced at 8 and above. The EPA's scale follows the WHO's and adds a rule you can use without a forecast: if your shadow is shorter than you are, you are getting a lot of UV, so seek shade.

How long can I stay in the sun?

The honest answer from the people who wrote the index is: do not count on a number. The WHO guide has a section headed "Reporting burn times is not recommended". Its reason is that people read a burn time as a safe amount of unprotected sun, and cumulative exposure matters for skin cancer, eye damage and the immune system even when skin never goes red. Cancer Council Australia puts it as: at an index of 3 or above, just 15 minutes of unprotected exposure is enough to start to cause damage to your skin.

The tool still shows a "skin starts to redden in about" figure for skin types I to IV, because that is the question people ask and the physics is simple, and it sits beside the warning on purpose. It is guidance, not a safe limit. The figure is the time for unprotected skin to receive its minimal erythemal dose (MED), the smallest UV dose that reddens it, at the index you have set. The MEDs are the Fitzpatrick figures as tabulated in Czerwińska and Krzyścin (2022): 200, 250, 300 and 450 joules per square meter for types I to IV. The time is the dose divided by the UV power, which is the index times 0.025 watts per square meter. The sources used here give no MED for types V and VI, so the tool gives none.

Worked example: skin type II at an index of 8 is 250 ÷ (8 × 0.025) = 1,250 seconds, about 20 minutes. The table is the same sum at four indexes, rounded down to the minute.

Two cautions on the table. It assumes the index stays put and the skin faces the sun flat-on, and real skin is neither. And a darker skin type is not a green light: the WHO guide notes that more than 90% of non-melanoma skin cancers occur in skin types I and II, which is why its messages are aimed at fair skin that burns, but that skin cancers do occur in darker skin and are often found later, and that the effects on the eye and the immune system do not depend on skin type. The skin-type descriptions for I to IV are from the same Czerwińska and Krzyścin table; the WHO guide's table 2 gives the classification, with types V and VI as naturally brown and naturally black skin. The NHS says to take extra care if you have pale, white or light brown skin, freckles or red or fair hair, light-colored eyes, many moles, or tend to burn rather than tan.

What changes the UV where you stand

The WHO guide lists what moves the index: the height of the sun (so time of day and time of year), latitude (higher nearer the equator), cloud, altitude, ozone and what the ground reflects. The tool uses the first, the cloud and the ground, and leaves altitude and ozone out. The WHO says UV rises by 10% to 12% with every 1,000 meters (3,281 ft) of altitude; the EPA's forecast model uses about 6% per kilometer. The two agencies differ, so a mountain garden's number is the one to take from a local forecast.

Cloud

The EPA's forecast model passes virtually 100% of UV under clear skies, 89% under scattered clouds, 73% under broken clouds and 31% under overcast skies, and the tool uses those. The WHO is blunter about the risk: up to 80% of UV can penetrate light cloud, haze can even add to exposure, and a cloudy day does not mean you cannot burn. Cancer Council Australia says thick cloud is a good filter but UV passes through thin cloud, and that temperature has nothing to do with it. A cool, bright, partly cloudy day in spring is the classic case.

Sand, water and snow

The sand and sea-foam factors treat the reflected share as simply added to what comes from the sky, which is a simplification. The NHS adds that reflected sunlight from snow, sand, concrete and water is particularly hard on the eyes, and that water washes sunscreen off while its cooling effect can make you think you are not getting burned.

How much does shade help?

A lot, but never completely, and the studies that measured it agree on that. Shade blocks the direct beam. It does not block UV scattered by the sky or reflected from the ground, which is why the figures below are never close to zero.

The tool uses 34% for an umbrella and 35% for a sail and a tree, the middle of the ranges above. They are round numbers for a rough guide, not a rating of any one product: a small umbrella, a low sun or a light-colored patio around you all push the real figure up, and a big dense canopy pushes it down.

Three more findings keep that in proportion. On a Tuscan beach, the covers of three gazebos let through 3%, 1% and 5% of UV and an umbrella cover 0.2%, yet under them the UV index still reached about 1 (gazebo) and close to 2 (umbrella) on a sunny summer day, because of the diffuse UV that arrives from the open sky. In a randomized trial in Texas, 81 people of skin types I to III spent 3½ midday hours at a lake beach with either an umbrella or SPF 100 sunscreen; the umbrella group had higher sunburn scores (0.75 against 0.05), and the authors concluded an umbrella alone may not give enough protection for extended exposure. And Cancer Council Australia says shade reduces UV but it can still reach you by reflection, so use it with the other measures.

If you are choosing shade for the garden, shade sail, awning or pergola compares them, how to install a shade sail covers fitting one, how to choose a patio umbrella covers umbrellas and shade for hot climates covers places where the sun is strongest. To see where a shadow will fall through the day, how the sun moves across your garden and the shade sail planner help; the fabric comparer sets out the cloth.

UPF and shade fabric

UPF, the ultraviolet protection factor, says how much UV a fabric lets through to your skin. The Skin Cancer Foundation gives the example that UPF 50 blocks 98% and lets 2% (1/50th) through, rates UPF 30 to 49 as very good protection and 50+ as excellent. In Australia and New Zealand, UPF ratings for clothing follow AS/NZS 4399:2020, Sun protective clothing: Evaluation and classification, a labeling scheme built on an objective, reproducible test of the material. The scope note of the 2017 edition says the protection offered by synthetic shade cloth, sunscreens and sunglasses is the subject of other standards, so a UPF figure on a shirt and one on a roll of shade cloth are not necessarily the same test.

What UPF 50+ does not tell you is how much UV reaches you under the cloth. It describes the fabric. The sides of a sail are open, the sky around it is bright and the ground reflects, which is how a 0.2% cloth can still leave an index of 2 beneath it. Cleaning outdoor fabric and the types of awning are the practical side of keeping the cloth doing its job.

The protection steps

The Australian campaign that taught the world a rhyme began as Slip, Slop, Slap in 1981 and was updated in 2007 to Slip, Slop, Slap, Seek, Slide, to add shade and wraparound sunglasses. The WHO's basic messages are the same ones: limit midday exposure, seek shade, wear protective clothing, a broad-brimmed hat and wraparound sunglasses, use broad-spectrum sunscreen, avoid tanning beds and protect babies and children. It adds that if protection is needed it should include all of the measures, not one, and that sunscreen should never be used to prolong the time you spend in the sun.

• Sunscreen. The numbers differ between health bodies: SPF 15 or higher from the WHO and the EPA, SPF 30 or higher, broad-spectrum and water-resistant from the American Academy of Dermatology, at least 30 with 4-star UVA protection from the NHS, and SPF 50 or higher from Cancer Council Australia. All say to apply generously and reapply about every 2 hours and after swimming or sweating. The NHS says an adult needs around 6 to 8 teaspoons for the whole body and Cancer Council about 7 (35 mL); the EPA says about an ounce.
• Hat. SunSmart says a wide brim of at least 7.5 cm (3 in) for adults, 6 cm (2.4 in) for children over 8 and 5 cm (2 in) for toddlers; caps and visors leave ears and neck exposed.
• Eyes. Cancer Council says sunglasses and a broad-brimmed hat together can cut the UV reaching the eyes by up to 98%.
• Hours. The NHS says shade matters most from 11 a.m. to 3 p.m. in the UK from March to October, and the EPA says UV is strongest from 10 a.m. to 4 p.m. in the US. The NHS says to check the index and consider protection at 3 or higher.
• Babies. The NHS says keep babies under 6 months out of direct sunlight; the WHO says children under 1 must never stay in direct sun.

How the estimate works, and where it is rough

"Estimate for here and now" works out the sun's height at your place and time with NOAA's solar equations (the module is checked against the US Naval Observatory's sunrise, sunset and sun-position figures), then turns height into a clear-sky UV index with the relation Allaart, van Weele, Fortuin and Kelder (2004) fitted to measurements: the index is about 12.5 × cos(zenith)^2.42, where the zenith angle is the sun's angle from straight overhead. With the sun directly overhead that is 12.5; at a sun 33.7° above the horizon it is 3, which is where the tool starts and stops the "UV 3 or more" span. The paper expresses the clear-sky index as a function of the sun's zenith angle and total ozone; its full form also multiplies by (ozone/300)^-1.23, and the tool sets that factor to 1, so it leaves ozone out. The index is not capped at 11: a high sun over high ground can read 12 to 16, and the slider and the estimate both go that far.

That leaves it rough, and you should treat it that way. The tool has no ozone, altitude or haze in it. The WHO says ozone absorbs some of the UV that would otherwise reach the ground and varies over the year and even across the day, so leaving it out can push the estimate either way. The authors of a 2022 study that used the Allaart model with measured ozone compared it with a Brewer spectrophotometer at Belsk, Poland over 2010 to 2015: the two correlated at 0.99, and the model ran 10.5% low on average, which they put down to missing aerosol data. We have not tested the ozone-free version here against a UV meter. When your weather service gives an index for the day, use that: the US National Weather Service computes it from forecast ozone, cloud and ground elevation, as the EPA explains.

Worked examples

These are the tool's own output for 2026, at solar noon unless stated, for skin type II. The solstice noons are very high or extreme before any shade, and it is the shade, not the weather, that pulls them back.

The March London row is worth a look. A clear day at the equinox gives an index near 4, and the window is under four hours, which squares with the NHS's advice to think about protection from March even though most people do not feel the sun is strong. The December New York row is below 3 all day, which is the case the WHO calls limited risk.

This guide and the tool are general information, not medical advice, and not a safe time in the sun. See a doctor about any mole or spot that is new or changing.
Sources: WHO, Global Solar UV Index: A Practical Guide (2002) and its sun protection table; WHO, the UV index Q&A; EPA: UV Index Scale, Calculating the UV Index, Sun Safety Tips; NHS, sunscreen and sun safety; Cancer Council Australia: UV Index, Be SunSmart, Slip, Slop, Slap, Seek, Slide; SunSmart: protect your skin, shade comparison check; American Academy of Dermatology; Skin Cancer Foundation; Standards Australia, AS/NZS 4399:2020; Allaart et al. 2004; Czerwińska and Krzyścin 2022; Utrillas et al. 2010; Denver and Melbourne shade trial 2022; Brisbane street trees 2024; Tuscany beach shade 2022; Ou-Yang et al. 2017.