When do I get the most UV light?

UV radiation is strongest when the sun is highest. Is this down to the angle of the sun or UV attenuation? When does my face get the most UV?

Mike Follows, Sutton Coldfield, West Midlands, UK

When the sun is directly overhead, which only ever occurs in the tropics, sunlight has its shortest journey through the atmosphere. When the sun is lower in the sky, more ultraviolet light is attenuated because the sunlight has a longer journey through the air. That means your body will get most UV at midday, when the sun is highest in the sky. Of course, it depends where you are on the planet, as UV increases when you are closer to the equator.

Climbing a mountain will also reduce the distance sunlight has to travel through the atmosphere before hitting you: UV radiation levels increase by about 10 per cent for every 1000 metres of ascent. In addition, UV exposure rises when you are on a highly reflective surface, like water, snow or even dry sand.

About 20 years ago, I was taken by surprise when I discovered that my lower legs were sunburned after I had been wandering on the snow around Jungfraujoch in Switzerland, which is at an altitude of almost 3500 metres. I was there for only a couple of hours and didn’t apply sunscreen to my legs because I was wearing trousers. However, sunlight had reflected off the snow and up inside my trousers, burning my unprotected skin.

Olivier Sorg

Geneva, Switzerland

The UV radiation spectrum is divided into three bands: UVA (320 to 400 nanometres), which is close to visible light, UVB (280-320 nm) and UVC (100-280 nm). The most energetic band, UVC, is absorbed by ozone in the atmosphere, and more UVB is absorbed by the atmosphere than UVA. Therefore, when the sun is low in the sky, the proportion of UVB falls compared with UVA. Because UVB can cause more biological damage than UVA, the effects of UV radiation on human skin are stronger when the sun is high in the sky.

When the sun is directly overhead, known as the zenith, its light doesn’t hit the face of a person walking below. A walker’s face is likely to get the most impactful UV radiation when the sun is between 40 degrees and 60 degrees above the horizon. Of course, if a person is lying on their back, their face will get the most UV when the sun is highest in the sky.

Chris Daniel

Colwyn Bay, Conwy, UK

The strength of ultraviolet radiation at ground level varies through the day due to attenuation that is largely dependant on the sun’s angle in the sky. One recent study found that the greatest amount of UV radiation reaches the eye when the sun is at an elevation of 40 degrees. In the UK, the sun generally climbs higher than this only in the summer months.

This suggests that the face, much of it being a nearly vertical surface, receives the most radiation at this angle. The parts of the face that are angled slightly upwards, such as the nose, will be more susceptible to burning when the sun is higher in the sky, when irradiance will also be greater. But caution is needed even then as the eyes and face will still be subject to UV radiation reflected from the ground and backscattered from elsewhere in the sky.

Jonathan Wallace

Newcastle upon Tyne, UK

All light waves are scattered as they interact with molecules and particles in the atmosphere, a process known as Rayleigh scattering. Different wavelengths are affected to a different extent, with short wavelengths (blues and UV) affected the most.

This means that as sunlight passes through the atmosphere, blue light is filtered out more than other wavelengths. The more atmosphere the light passes through, the greater the effect. At sunset and sunrise, when sunlight strikes Earth at a low angle compared with the observer and therefore passes through a large amount of atmosphere on its route to the surface, the result tends to be red and yellow skies.

This effect is enhanced when there are more particles in the air, which is why volcanic explosions can lead to spectacular sunsets.

To answer this question – or ask a new one – email lastword@newscientist.com.

Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.

New Scientist Ltd retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.

You can also submit answers by post to: The Last Word, New Scientist, 25 Bedford Street, London WC2E 9ES.

Terms and conditions apply.