The second most important atmospheric variable affecting surface UV radiation after cloud cover is total column ozone. Ozone strongly absorbs radiation in the UV and accounts for the sharp cutoff of the solar spectrum at about 290 nm. The most common measurement made by Brewer instruments is that of total column ozone, which is the amount of ozone in a column of the atmosphere compressed to standard temperature (STP = 273°K and pressure = 1,013.25 mb). Column ozone is measured in Dobson Units (DU = 10 cm ozone at STP). Total ozone measurements are routinely reported to the World Ozone and UV Data Centre (WOUDC) in Toronto. Direct sun measurements are the preferred measurement type because they are the most accurately defined. Measurements are also made using zenith sky radiance, global irradiance, and direct irradiance from the moon.
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