How Solarpunk works out UVA and UVB

Solarpunk forecasts both kinds of ultraviolet light, UVA & UVB, that reach the Earth's surface for your place, hour by hour with an 8-day forecast.

The differences between UVA, UVB & UVC

UVA (315–400 nm) is the longest wavelength of UV light. It is termed "soft" UV (with its upper frequencies used in "blacklights") and is not absorbed by the ozone layer so reaches the Earth's surface in the largest amounts.

It is the wavelength closest to visible light and lacks the intense energy to cause immediate sunburn, but is still capable of causing long-term, deep-tissue damage and premature skin ageing over time.

UVB (280–315 nm) is medium-wave UV. Most of it is absorbed by the Earth's ozone layer, though 5-10% reaches the ground. High altitude areas around the equator can see 10-20% let through.

UVB is responsible for sun burns which is why the UV Index (see below) is primarily built around it.

UVC (100–280 nm) is short-wave UV and has the most energy. The upper range (200–280 nm) is known as "germicidal UV" because it scrambles the DNA/RNA of viruses and bacteria to neutralize them. It is completely absorbed by the atmosphere and ozone layer and does not reach the Earth's surface.

UVB: the Global Solar Ultraviolet Index (UVI)

Our UVB figure is the Ultraviolet Index (UVI), the World Health Organization's (WHO) measure of the sun's power to burn skin, which you will find reported by most weather forecasters.

The UVI's weighting is strong in UVB but falls away through UVA, so this index mostly reflects UVB.

Solarpunk's UVI is from atmospheric forecast models, including Copernicus's Atmosphere Monitoring Service, which compute it from the ozone column, the height of the sun, altitude, aerosols and cloud.

It isn't imported from a consumer weather app, although we do compare it with other published forecasts where their terms allow, and with ultraviolet measured at ground stations, to catch the days when the UV Index is high enough to wear sunscreen (moderate and above) but a forecast might read low.

Solarpunk combines the models' figures with a high-resolution cloud forecast for your location, and shows the result rounded to a whole number on the WHO's five levels:

• 0 to 2: Low
• 3 to 5: Moderate
• 6 to 7: High
• 8 to 10: Very high
• 11 and over: Extreme

UVA: Solarpunk's forecast

UVA is 95 per cent of the ultraviolet reaching the ground, and the UV Index was never built to show it.

The ozone layer absorbs most UVB and leaves UVA largely alone.

A low sun's long path through the air scatters short wavelengths far more than long, so UVB collapses towards dusk and through the winter while UVA fades gently.

Cloud dims ultraviolet 15-45% less than it dims visible light, and lets through a larger share of UVA than UVB.

So UVA is present for longer in the day, and through the winter, than the UV Index suggests.

No weather service forecasts UVA, so Solarpunk built its own model. For each hour of the forecast it calculates UVA, in watts per square metre, from the sun's radiation reaching the ground, how much of that light the sky is scattering, and the height of the sun.

The model was calibrated against NASA's hourly satellite record of UVA (the POWER project, 315 to 400 nanometres), then verified against fourteen measurement stations on five continents.

Solarpunk's UVA scale

UVA is given in watts per square metre and the scale was built from NASA's satellite measurements of UVA:

• Low: UV Index 0 to 2; UVA under 8 W/m²
• Moderate: UV Index 3 to 5; UVA 8 to 18 W/m²
• High: UV Index 6 to 7; UVA 18 to 46 W/m²
• Very high: UV Index 8 to 10; UVA 46 to 60 W/m²
• Extreme: UV Index 11 and over; UVA 60 W/m² and over

The UV Index is weighted for sunburn and the UVA ratings on a sunscreen (the UVA circle, UVA stars, PA grades) measure how well a product protects, not how much UVA is in the sunlight.

The top levels of the scale follow the satellite readings, but UVA works differently from UVB and UVA's harm is a dose that builds cumulatively over days.

In studies showing the impact of repeated small doses of UVA:

• Small daily doses over eight weekdays produced early tissue injury where one dose of more than twice the total did nothing (Lavker and colleagues, 1995).
• Small doses twice a week for 24 weeks reduced the skin's elastic tissue, with no recovery three months after they stopped (Lowe and colleagues, 1995).
• Repeated low doses switched on the enzyme that breaks down collagen more strongly each day, where the same dose once did not (Wang and colleagues, 2014).

Our UVA scale's lower levels reflect how quickly a day's dose is building and takes into account the cumulative impact of repeated small doses.

Ongoing calibration

Solarpunk checks its forecasts against the measured record as each new period is published, and recalibrates where necessary. The model is updated regularly with new sources added to the model.

UVA is checked against NASA's POWER project while the UV Index is checked against ground stations in Britain, Europe, Australia and the United States:

• the UK Health Security Agency's solar monitoring network
• Reading University's spectroradiometer
• the World Ozone and Ultraviolet Radiation Data Centre's stations
• the ARPANSA network, Australia's radiation protection agency
• the USDA's network in the United States
• the German Federal Office for Radiation Protection's network

Sources and credits

Weather data by Open-Meteo.com, licensed CC BY 4.0 and Met Office licensed CC BY-SA 4.0. Contains modified Copernicus Atmosphere Monitoring Service information 2026. The UVA model is calibrated against data obtained from the National Aeronautics and Space Administration (NASA) Langley Research Center's POWER Project, funded through the NASA Earth Science Division's Applied Sciences Program.

Ground measurements of ultraviolet: UKHSA's solar monitoring network and Reading University's spectroradiometer, through UK-AIR (Defra), public sector information licensed under the Open Government Licence v3.0; UV observations courtesy of ARPANSA; the USDA UV-B Monitoring and Research Program, Colorado State University; the World Ozone and Ultraviolet Radiation Data Centre, operated by Environment and Climate Change Canada for WMO's Global Atmosphere Watch, and the agencies contributing to it; the German UV monitoring network of the Bundesamt für Strahlenschutz and its partners.

Using the Solarpunk UVA scale for your app or service

Please get in touch if you would be interested in including our UVA forecast in your service.

References

• Calbó, J., Pagès, D., González, J.-A. (2005). Empirical studies of cloud effects on UV radiation: A review. Reviews of Geophysics 43(2), RG2002. doi.org/10.1029/2004RG000155
• ISO 17166:1999 / CIE S 007/E-1998. Erythema reference action spectrum and standard erythema dose. International Organization for Standardization and the International Commission on Illumination.
• Lavker, R. M., Veres, D. A., Irwin, C. J., Kaidbey, K. H. (1995). Quantitative assessment of cumulative damage from repetitive exposures to suberythemogenic doses of UVA in human skin. Photochemistry and Photobiology 62(2), 348 to 352. doi.org/10.1111/j.1751-1097.1995.tb05280.x
• Lowe, N. J., Meyers, D. P., Wieder, J. M., Luftman, D., Borget, T., Lehman, M. D., Johnson, A. W., Scott, I. R. (1995). Low doses of repetitive ultraviolet A induce morphologic changes in human skin. Journal of Investigative Dermatology 105(6), 739 to 743. doi.org/10.1111/1523-1747.ep12325517
• NASA Langley Research Center. The POWER Project (Prediction Of Worldwide Energy Resources). power.larc.nasa.gov
• Wang, F., Smith, N. R., Tran, B. A. P., Kang, S., Voorhees, J. J., Fisher, G. J. (2014). Dermal damage promoted by repeated low-level UV-A1 exposure despite tanning response in human skin. JAMA Dermatology 150(4), 401 to 406. doi.org/10.1001/jamadermatol.2013.8417
• World Health Organization (2016). Radiation: Ultraviolet (UV) radiation. Questions and answers, 9 March 2016. who.int
• World Health Organization, World Meteorological Organization, United Nations Environment Programme, International Commission on Non-Ionizing Radiation Protection (2002). Global Solar UV Index: A Practical Guide. WHO/SDE/OEH/02.2, ISBN 9241590076. who.int