Have you ever wondered why you have been sunburnt on a cloudy day?
The total dose of UV radiation reaching the earth’s surface and hence, the potential damage to human skin and tissues, varies, depending on many factors.
The sun’s elevation in the sky depends on the time of the day and year. The shorter the distance that photons (making up the total of UV radiation) need to travel though the earth's atmosphere, the greater the intensity of UV radiation. The altitude of a location also effects UV radiation levels as the higher a location is above sea level, the shorter the distance UV radiation travels.
The thinning of the ozone layer located above Antarctica has had a considerable impact on the ability of the atmosphere to absorb UVB, a significant contributor to the increased incidence of skin cancer and other damage to human tissues which has been observed in populations bordering the ozone hole.
Clouds act on UV primarily by scattering radiation which can both reduce and enhance the UV radiation levels depending on the type of cloud cover.
Some clouds absorb infrared radiation and as a result of the diminished heat sensation, people are given a false sense of security and often change their behaviour on cloudy days, unaware that they are exposing themselves to this potential danger.
UV radiation is also reflected from surfaces such as sand, snow and water. These surfaces can increase the UV radiation at ground level and increase the amount of skin damage incurred from UV radiation exposure.
So, when all these factors are considered, it is important to recognise that the net potential UV risk is a result of these associated variables, depending an Individual’s circumstances. The only sure way to significantly reduce the risk of skin damage is with vigilant protection from UV radiation and light, known as photoprotection.
Image and video credits:
- ESA/Hubble (M. Kornmesser & L. L. Christensen)
- Ozone layer video courtesy of NASA
- NASA Goddard Space Flight Center
- Skin cancer and ultraviolet-B radiation under the Antarctic ozone hole: southern Chile, 1987-2000, Jaime F. Abarca, Casiccia - 2002
- Global solar UV index: A practical guide, World Health Organization - 2002
- Sources and measurement of ultraviolet radiation, Diffey - 2002