UVA, the UV Index and sunscreen

It's Solarpunk's own scale for UVA. We calculate UVA in watts per square metre for every hour from the forecast's sunlight, calibrated against NASA's satellite measurements of UVA. The levels use the UV Index's numbers, so the words match: Moderate from 3 (8 W/m²), High from 6 (18 W/m²), Very high from 8 (46 W/m²) and Extreme from 11 (60 W/m²).

How Solarpunk works out UVA and UVB

UVA is ultraviolet light from 315 to 400 nanometres, about 95 per cent of the UV that reaches the ground. UVA reaches the deeper layers of the skin and contributes to skin ageing and wrinkling. Small UVA doses, day after day, switch on the enzyme that breaks down collagen and wear away the skin's elastic tissue.

UVB is ultraviolet light from 280 to 315 nanometres, the part of sunlight that burns the outer layers of the skin. Getting sunburnt increases the risk of skin cancer. UVB is highest in the middle of the day.

The World Health Organization's measure of the sun's power to burn skin. It adds up the sun's UV, weighting each wavelength by how strongly it reddens skin, so UVB counts most. UVA is still part of it, but each unit of UVA counts for far less, so although UVA is about 95 per cent of the UV reaching the ground, the UV Index mostly reflects UVB. From 3, WHO says protection is necessary.

Only a little. The UV Index weights each wavelength by how strongly it reddens skin, and UVA counts for far less than UVB. UVA is about 95 per cent of the UV reaching the ground, but the UV Index mostly reflects UVB, so there can be a lot of UVA when the UV Index is low.

UVB is the part of sunlight that burns, and the UV Index mostly measures it. UVA reaches the deeper layers of the skin and contributes to skin ageing. It is there for longer in the day, and through the winter, than the UV Index suggests. Solarpunk shows both.

UVA, UVB and UVC explained

No. They use WHO's words and numbers, but they describe different things. A Moderate UV Index is the level at which WHO says you need protection from burning. Moderate UVA means the day's UVA dose is building at a pace that, day after day, adds up to the changes measured in skin studies.

UVA adds up over days: small doses, day after day, wear away the skin's elastic tissue. And according to Cancer Research UK, UVA and UVB can both damage the skin and cause skin cancer. The UV Index you see on weather forecasts doesn't tell you about UVA.

Yes. Early and late in the day, and in winter, UVA can be Moderate or higher on Solarpunk's scale while the UV Index is Low. At Chiswick in 2025, nearly half the year's UVA arrived while the UV Index was under 3.

Weather forecasts give the UV Index, the World Health Organization's measure of the sun's burning strength, which is mostly UVB.

For each hour, Solarpunk calculates UVA in watts per square metre from the sunlight forecast to reach the ground, how much of that light the sky scatters, and the height of the sun. The model was calibrated against NASA's satellite record of UVA, and we recalibrate it as NASA publishes each new period of the record.

Yes. The forecast's sunlight already allows for cloud, and the model also uses how much of the light the cloud scatters, because scattered light carries a larger share of UVA. Cloud dims UV 15 to 45 per cent less than it dims visible light (Calbó and colleagues, 2005), so thin or broken cloud cuts UVA by less than you might think.

Ozone absorbs a lot of UVB and hardly any UVA, so the ozone layer affects the UV Index but makes very little difference to UVA. Haze, particles and water in the air are already allowed for in the forecast's sunlight, which is what Solarpunk calculates UVA from.

No. It forecasts all of UVA, from 315 to 400 nanometres, the same band as NASA's satellite record. In a study of repeated exposure that split UVA into parts with filters, every part up to 400 nanometres did the same damage to the deeper skin (Lavker and Kaidbey, 1997).

From NASA's satellite measurements of UVA. The top two levels start where the UV Index's do: Very high at the amount of UVA that comes with a UV Index of 8, and Extreme at the amount that comes with 11. Moderate starts at 8 W/m², about what a clear winter midday gives in Britain. No published study sets a level of UVA at which skin needs protecting, so the scale is Solarpunk's own. It shows how quickly the day's dose is building, which matters because of what studies of repeated small doses found.

Yes. Given the sunlight NASA's satellites recorded, the model's yearly UVA is within a median of 1 per cent of NASA's own record at fourteen places on six continents, from Reykjavik to Cape Town and Singapore, and within 4 per cent at every one. We check the UV Index separately, against ground stations in Britain, Europe, Australia and the United States, and keep checking both as new data is published.

Not yet. If you're a researcher and would like to test the forecast independently, please get in touch.

For your face, about a quarter of a teaspoon, and the same again for your neck and ears. For your whole body, about seven teaspoons, a shot glass's worth. Include your neck and chest if they're uncovered: they age in the sun too.

A finger length is a line of sunscreen from the base of a finger to its tip. The whole body is about a shot glass.

Wear SPF 30 or higher to protect against sunburn; we suggest SPF 50. Look for the UVA symbol, 5 UVA Stars or PA++++ to protect against skin ageing. Most people put on about half the amount an SPF is tested with, and at half the amount it keeps off less, as the table shows.

Look for one of three labels. UVA circle: the EU standard, showing UVA protection is at least a third of the SPF. UVA stars: Boots' own UK rating system, from 1 to 5; the NHS advises at least 4. PA: the Japanese standard, from PA+ to PA++++, also widely used by Korean brands. A higher SPF also brings more UVA protection.

It protects no better, but it is often kinder to sensitive skin: dermatologists recommend mineral sunscreens, made with zinc oxide or titanium dioxide, for people with sensitive skin. The catch is a white cast, which is stronger at high SPFs, so a mineral SPF 50 can look quite white. Some use nano-sized particles to reduce it, which may be a concern.

Yes. Check the open jar symbol: the M and the number say how many months it lasts once opened. And make sure it's not past its expiry date.

Yes, you can still burn. Up to 80 per cent of the sun's UV gets through light cloud, the World Health Organization and the Met Office say, and Cancer Research UK says over 90 per cent can. Under thick overcast, measurements usually find 30 to 70 per cent still gets through, depending on the cloud. Cloud also lets through a larger share of UVA than of the light you see, so a dull day still carries UVA. Solarpunk's UVA forecast allows for each hour's cloud.

Yes. UVA, the part of sunlight linked to skin ageing, can get through window glass, so a sunny seat by a window still counts.

The forecast is for open ground. Buildings and trees block the direct sun, but you still get UV scattered from the sky and reflected off bright surfaces. The World Health Organization says shade from trees, umbrellas or canopies doesn't give complete protection.

Fresh snow can reflect up to 80 per cent of the sun's UV, so it reaches you from the sky and from the snow. It can double your exposure, especially high in the mountains.

Yes. UV rises by about 10 to 12 per cent for every 1,000 metres you climb, the World Health Organization says. So at the top of Whistler Blackcomb in Canada, 2,284 metres, it is about a quarter stronger than at sea level. In the Alps, at the top of Chamonix's Aiguille du Midi, 3,842 metres, it is about 40 per cent stronger, and in the Rockies, at the top of Breckenridge, 3,962 metres, about 40 to 50 per cent. Fresh snow adds to that: it can double your exposure.

UV gets stronger the higher you go, about 10 to 12 per cent for every 1,000 metres, the World Health Organization says. How much gets through a plane's window, we can't say.

A way of sorting skin into six types by how it reacts to the sun. Type I always burns and seldom tans; type II usually burns and sometimes tans; type III sometimes burns and usually tans; type IV seldom burns and always tans; type V is naturally brown skin and type VI naturally black skin. It is named after the dermatologist Thomas Fitzpatrick.

Anyone can get sunburnt, and on darker skin it may feel tender or itchy rather than look red. Sunlight can also trigger melasma, patches of darker skin more common in people of colour, or make it worse. For black or dark brown skin, the British Association of Dermatologists says routine sun protection is rarely needed in the UK, unless you have a condition such as melasma.

Darker skin makes more pigment in response to light, so sun can leave dark patches. In a study of 20 people with skin types IV to VI, long-wave UVA darkened the skin at doses of everyday sun, and visible light darkened it more and for longer; neither darkened fair skin. Sunlight can trigger melasma or make it worse, the British Association of Dermatologists says, and can make other dark spots more noticeable. A tinted sunscreen with iron oxides adds protection against visible light.

It may. The British Association of Dermatologists says visible light may also contribute to melasma, and that tinted sunscreens with iron oxides give added protection against it. Solarpunk forecasts ultraviolet, not visible light.

Yes. Solarpunk is a free iPhone app that forecasts UVA hour by hour for the next eight days, alongside the UV Index. It tells you each day whether to wear sunscreen, anywhere from 65.5 degrees south to 65.5 degrees north.

Solarpunk is a UV forecast app. It forecasts UVA, the part of sunlight linked to skin ageing, and shows it beside the UV Index, which measures the part that burns. Its name comes from solarpunk, a movement that asks what the world looks like if we get it right.

Weather apps give the UV Index, which mostly measures UVB, the part of sunlight that burns. Solarpunk is unique in showing UVA beside it, and tells you each day whether to wear sunscreen.

It shows each day's UVA and UVB hour by hour, says whether to wear sunscreen and how much to put on, and can remind you in the morning. It has a Home Screen widget and an Apple Watch app, and you can ask Siri: "Do I need sunscreen today in Solarpunk?"

It's made by Imaginists, whose founder has spent 20 years in the skincare and beauty industry developing suncare products. We made it because UVA is poorly understood, and most people don't put on enough sunscreen to protect against skin ageing and sunburn. Sunburn raises the risk of skin cancer.

The company behind Solarpunk, set up in 2023 by Ed Saper, who has spent 20 years in the skincare and beauty industry developing suncare products.

Solarpunk is free, and always will be. No subscription, no in-app purchases and no adverts.

Not yet. Free for iPhone, worldwide. Android coming soon.

No. It's a UV forecast. For skin concerns, see a doctor or pharmacist.

There's no account to make and no advertising. The app asks for the forecast by a square about 2 km across, never your exact position.

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