We all know that UV radiation from the sun is the main cause of skin cancer, but what many people don’t know is what impacts the level of UV radiation reaching us each day. In total seven influences on solar radiation have been identified:
Where the sun sits in the sky determines the level of UV radiation reaching us. The reason for this is the amount of atmosphere the UV must penetrate to reach us. The longer the path through the atmosphere, the more UV that is absorbed, and this occurs when the sun is near the horizon, whereas when the sun is directly above us the amount of atmosphere is lower which means that less UV is absorbed and a higher level reaches us.
Ozone is an important part of our atmosphere and high levels of ozone absorb higher levels of UV. Since the discovery of the hole in the ozone over the Antarctic in the early 1980s ozone levels have been observed to decline worldwide, resulting in increased levels of solar UV reaching the earth’s surface.
Having a cover of cloud over the sky can reduce the amount of UV radiation reaching the earth’s surface. This is particularly the case when there are significant levels of cloud cover. However it would be wrong to assume that some level of cloud cover means that the UV level is below a level that requires protection. Sometimes reflection of the sun’s rays of clouds can increase UV levels above the expected values.
Ground surface reflectivity
With solar UV radiation we probably think that the threat comes from above, however there are some surfaces that can increase the ambient levels of UV through reflection. Metal surfaces, concrete and beach sand can all reflect UV back onto a person, increasing the risk of skin damage. Reflected UV is also harmful because the body’s natural defences of the face such as eyebrows, eye sockets and eye lids do not provide protection against the angle of reflected UV.
UV increases at higher altitudes as the atmosphere has less chance to absorb the incoming UV. It has been found that UV increases by up to 4% for every 300m increase in altitude.
Aerosols and pollutants
Small particles suspended in the air can scatter solar UV. Whilst the effect is minor it can still have an impact on levels of UV reaching the earth’s surface.
Direct and diffuse UV
UV radiation has the ability to diffuse, which means that if even if you are standing under shade but you can see large areas of the sky, the scattered UV can still reach you. This explains why shade, even though it is an important part of sun protection, does not provide adequate protection on its own.
The level of UV radiation that reaches us each day is impacted by a number of factors, and this can be the reason why the UV level fluctuates from day to day, or why you get sunburnt when you think that you are safe. Understanding these influences highlights why checking the UV level each day is important before spending time outside to determine when the UV is 3 or above and sun protection is required.