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Factors that affect UVR Levels

The amount of UVR received at ground level is depend on many factors the most important of which are ozone levels, sun height, and variations in cloud cover.  Major factors include:

• sun angle
• geographic position
• seasonal effects
• cloud cover
• ozone
• scattering
• reflectivity of the environment
• altitude

Sun angle

When the sun is closer to the horizon, as in the early morning and late afternoon, its UVR passes through a thicker layer of atmosphere, particularly ozone, so that the intensity of the sun’s UVR is decreased at that time.  The higher the sun in the sky the shorter path the UVR has to travel through the atmosphere, so less is absorbed resulting in higher levels of UVR at the surface.

Geographic position

The closer a place is to the equator the higher the incidence of UVR.

Seasonal effects

No matter where we are, the season with the most UVR risk is the one where the sun is closest to overhead.  That is, summer is the most dangerous time with the highest UVR levels.  In winter the sun is at a greater angle in the sky and the UVR has to travel through more atmosphere to reach us.

Cloud cover

Heavy cloud can reduce the intensity of UVR and scattered cloud has a variable effect on UVR levels, which rise and fall as clouds pass in front of the sun.  Light clouds reduce the intensity of ultraviolet radiation by scattering the wavelengths.  A thin stratus cloud cover over the sky may reduce the UVB intensity by 20-30 percent, and thicker cloud cover will absorb even more.  Because clouds also absorb infrared, decreasing the amount of radiant heat, people tend to stay out in the sun for longer.  However, the ultraviolet radiation is still penetrating through the clouds so the risk of sunburn is increased because people think it unnecessary to protect themselves on a cloudy day.

Ozone

Ozone absorbs UVC and UVB radiation.  Atmospheric absorption prevents all UVC and most UVB from reaching the earth’s surface.  The level of ozone in the atmosphere rises and falls naturally from day to day and seasonally.  Publicity about the ozone ‘hole’ has caused concern about possible increases in UVR levels in high latitude areas in the Southern Hemisphere.  The ozone hole does not extend as far north as Australia and New Zealand, but stratospheric winds can occasionally carry ozone-depleted air causing short-term decrease in ozone and a corresponding rise in UVR levels.

Scattering

There is often as much UVR received from the open sky as there is directly from the sun, due to scattering of solar UVR by molecules and particles in the atmosphere.  At times the amount of indirect or scattered UVR reaching us is greater than the direct UVR.  If you can see the blue sky, you are receiving UVR – the more sky you can see the more UVR you are receiving.

Reflectivity of the environment

UVR levels will increase in a highly reflective environment.  White paint, snow, water, light coloured building materials, and even soil can reflect UVR onto our skin.

Altitude

The higher the altitude, the higher the UVR as it passes through less atmosphere and so less is absorbed.  The intensity of UVR increases by approximately 12 percent for every 1000 metres rise in altitude.