How Much UV Reaches My Skin?

Your level of exposure to UV radiation depends upon the following factors:

Factor	How much UV radiation?	Did you know?
Time of day	Greatest when the sun is highest in the sky at midday, between 10 am and 4 pm.	How much UV exposure you're getting can be measured by your shadow. If it's shorter than you, then your exposure is high; if it's taller, your exposure is lower.
Season	Greatest in late spring and early summer: from May to August in the Northern hemisphere and from November to February in the Southern hemisphere.
Altitude	Greater at higher elevations like in the mountains, where the air and cloud cover is thinner.	UV levels increase with altitude at the rate of 2% for every 1000-foot rise in elevation, or 1 UV index number for every 4000 feet in summer.*
Geography	Strongest at the equator and in the tropics, where the sun is highest in the sky.	Weakens as you move towards the earth's poles (both north and south).
Cloud cover	Strongest on cloudless days.	Up to 80% of UV rays can penetrate light clouds, haze, and fog.
Reflecting surface	The whiter the surface, the higher the UV level. Snow reflects the sun like a mirror. Light clothing is more reflective than dark clothing.	Fresh snow reflects up to 85% of the sun's rays, nearly doubling your exposure. Sand and concrete reflect up to 12% Grass and water reflectup to 5%.
Ozone holes	Breaks in the protective ozone layer of the atmosphere significantly increase exposure to UV.	Holes exist over extremely cold areas, especially over the South Pole and the Arctic.
Length of exposure	The longer you are out in the sun or on a tanning bed, the more UV radiation you receive.	This includes any time spent outdoors, including walking, getting the mail, waiting for a train or bus, or playing an outdoor sport.