UV Radiation

Ultraviolet radiation (UVR) is the portion of solar radiation with wavelengths of 200-400 nm (nanometers). Radiation of these short wavelengths vibrates at very high frequency and is powerful enough to damage living cells and tissues.

The most energetic UVR, UV-C (200-280 nannometers) is lethal, but nearly all is absorbed by ozone and oxygen in the atmosphere. Both UV-B (280-320 nm) and UV-A (320-400 nm) radiation reach earth’s surface and are damaging to living organisms.

Although nine percent of the sun’s radiation is UVR, much less reaches Earth’s surface, because ozone (O3) in the stratosphere strongly absorbs it. For that reason, the stratospheric ozone layer is often called the ‘ozone shield’. The thickness of the stratospheric ozone layer varies seasonally and latitudinally, owing to differences in the amount of sunlight in a region. Water vapor and aerosols also absorb UVR; little of it reaches the sea surface or ground on heavily cloudy days or where air is polluted.

Man-made chemicals---particularly chloroflourocarbons (CFCs) used as refrigerants, fire suppressants, and propellants---have destroyed portions of the ozone shield during recent decades, allowing more UVR to reach Earth’s surface. These gases can be released into the atmosphere both deliberately or accidentally, in the mistaken belief that they are inert. When UVR comes in contact with a CFC molecule, the cyclical reaction can repeatedly destroy ozone molecules.

Areas of ozone thinning occur over high latitude areas, principally in spring and summer. Ozone thinning is greatest over polar areas because seasonal circular air currents trap CFCs for months at a time and high altitude ice crystals accelerate their ability to deplete ozone.

A one percent decrease in overhead ozone allows one percent more UV-B light at 310 nm and three percent more at 305 nm to reach Earth’s surface. 

How Was It Measured?
UV Radiation was measured as the number of times between 2000 and 2004, in each 1-degree cell, that the monthly average exceeded the climatological mean +1 standard deviation within the entire dataset (1996-2004). These values were summed across the 12 months to provide a single value, ranging from 0-19.