The amount of ozone in the atmosphere varies, and so the intensity of
UV-irradiance on the earth's surface does. Strong short term variations
of total ozone in the northern hemisphere are observed in the months of
January, February and March. The lowest concentrations of total ozone there
can be found in late fall. Alongside ozone, other components af the earth's
atmosphere, such as aerosol, absorb UV radiation.
- In the months of May, June and July, when solar altitude is at its highest, UV irradiance also reaches its highest rates.
- In the course of one day, UV irradiance is at its highest between 11.00 and 15.00 hours Central European summer time.
- The lower geographical latitude, the higher is solar altitude, i.e. UV irradiance is more intense in countries situated to the south.
- Irradiance also rises with the sea level, by about 15 - 20 % at an increase in altitude by 1000m.
- Reflection can enhance UV irradiance. Snow can intensify it up to twofold, and surfaces of sand and water also reflect, even if to a considerably less extend than snow.
- The impact of cloud cover on UV irradiance is very strong: a closed, thick cloud cover is capable of blocking up to 90 % of radiation, whereas thin cloud layers or fog can act as amplifiers by reflection.