Solar radiation and UV

Light is a wave, and its wavelength determines what it does. Red light has long waves, violet shorter ones, UV radiation shorter still. The shorter the wave, the more energy each particle of light carries. UV has enough energy to change building blocks in skin cells directly, including the genetic material. Sunburn is the skin's inflammatory reaction to this damage.

Roughly, UV-A and UV-B are distinguished. UV-B carries more energy and is mainly responsible for sunburn. UV-A penetrates deeper into the skin and contributes to skin ageing. Both are regarded as causes of skin cancer. The sun's even shorter-wave UV-C radiation is stopped in the atmosphere, mainly by the ozone layer.

How much UV reaches the ground depends on its path through the air. When the sun is high, the path is short and a lot arrives. When it is low – morning, evening, winter – more is filtered out. At altitude there is less filtering air above you, so radiation rises with every metre of height.

To judge the strength, there is the UV index, a worldwide uniform measure introduced by the WHO together with other organisations. The higher the number, the faster sunburn develops. From about 3, the “moderate” range, protection is recommended; at high values you should spend the midday hours in the shade if possible.

Weather services publish the UV index in their forecasts, and many weather apps show it too. Go by this number, not by how warm it feels. The warmth comes mainly from other light and from warm air – UV can still be high in a fresh wind and cool air.

A simple rule of thumb without technology is the shadow rule: if your shadow is shorter than you are, the sun is high and UV radiation is usually strong.

Protection starts with behaviour: seek shade, avoid the midday hours, wear long sleeves and a hat with a broad brim. Tightly woven fabric blocks more than thin, see-through fabric. A neck flap on the hat protects a spot that often burns.

Sunscreen is an addition for bare skin – face, hands, ears, neck. It only works if enough is applied, and it must be reapplied after sweating, swimming or wiping. Typically forgotten spots are the ears, lips, the underside of the chin and the nostrils, which are irradiated from below on snow.

Reflection greatly increases exposure. Fresh snow throws back a large part of the UV radiation – according to the WHO up to about 80 percent. Water and bright sand reflect less, but noticeably. Anyone out on snow, on the water or on the beach therefore needs particularly good protection.

The eyes can get a “sunburn” too. Snow blindness is a painful inflammation of the cornea. It often shows only hours after exposure: the eyes feel full of sand, water, are sensitive to light, and vision can be greatly reduced. On a trip this can become dangerous because you can no longer see the way safely.

On snow, glaciers and water, sunglasses with UV protection and side shields are therefore compulsory kit – even when it is cloudy. If the glasses are lost, improvised slit goggles made of cardboard, cloth or bark help: less light reaches the eye through narrow slits. Anyone with symptoms should rest in a darkened place and seek medical advice.

Children's skin and eyes are particularly sensitive. Sunburn in childhood is regarded as a risk factor for later skin cancer. For children, shade, clothing and a hat are therefore even more important.

Ordinary window glass blocks most UV-B radiation but lets part of the UV-A through. You therefore hardly get sunburnt behind a car window, but the radiation is not entirely without effect.

The energy of UV light can also be put to use: in solar water disinfection (SODIS) it damages pathogens in clear water. Strict rules of its own apply, though – more in the solar disinfection entry. The same property that hits germs also hits your skin.