The Environment's Role in Sun Exposure
Submitted by
staff on Thu, 08/20/2009 at 4:45pm.
Outdoor tanners are at the mercy of many uncontrollable elements. Here are a few things to consider if tanning outdoors:
Solar Elevation (height of the sun in the sky)
The intensity of outdoor ultraviolet light (the sun), and especially UVB, depends on the height of the sun in the sky. This will vary depending on the season of the year, the time of day, and the latitude at which you live. UV intensities are highest during the summer months in the 4-hour period around noon (or 13:00 if daylight saving is in effect).
UVB intensity varies more with the time of the day compared to the intensity of UVA. As a rule of thumb, "when your shadow is shorter than your own height" you may receive half or more of UVB during the 4 hours around solar noon on a clear summer day. In summer at noontime, UVB is two to three times more intense in equatorial areas than in northern Europe. At about 600 latitude the total UVB exposure during the months of January and February can be less than one clear day's exposure around midsummer.
Latitude and Altitude
The UV intensity at the earth's surface is related to the angle at which the UV rays pass through the atmosphere. In the tropics (close to 00 latitude, or near the equator) solar UV is more intense because it has less distance to travel through the atmosphere to the earth's surface.
UV intensities increase with altitude. This is because the amount of atmosphere available to absorb UV is reduced, so more and shorter wavelength UV is able to reach higher altitude areas. At high altitudes, skiers can be exposed to higher intensities of UV, especially because snow is an excellent reflector.
Atmospheric Scattering
Solar UV is composed of direct and scattered radiation. The sky looks blue because the blue rays from sunlight are highly scattered by the atmosphere. UV is scattered even more than blue light; this can cause an increase in a person's UV exposure.
Clouds and Haze
UV intensities are highest under cloudless skies. Clouds generally reduce UV intensity, but light or thin clouds have little effect and under certain conditions may even enhance the UV intensity. Hazy days generally have higher amounts of water vapor; UV in the atmosphere increases and can result in a higher personal UV exposure. Thus, even though haze or cloud cover can cause one to feel cooler, the UV exposure can still be high.
Ground reflection
The reflective properties of the ground have an influence on UV exposure. Most natural surfaces such as grass, soil and water reflect less than 10% of incident UV. However, fresh snow reflects 80% of UV. During spring in higher altitudes under clear skies, reflection from snow can increase UV exposure levels to those encountered during the summer. Sand also reflects (10-25%) and can significantly increase UV exposure at the beach.
Reflected UV is a key source of UV exposure to the eye. Acute effects, such as snow-blindness while skiing or photokeratitis at the beach, can result from UV reflected from snow or sand respectively.
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