The frequency and amounts of contamination are site dependent. ARM suggests cleaning all exposed optical surfaces every day, preferably early in the morning. Domes should also be wiped clean after precipitation events. When the indicating paper in the desiccator plug turns pink, the desiccant needs to be recharged or replaced. Obstructions to the sky view can artificially lower the readings by shading the instrument.
Some obstructions can artificially increase the readings by reflecting direct UV onto the sensor.
Some examples of objects to avoid when positioning an instrument include poles, roof vents, elevator shafts, trees, light-colored buildings or buildings with glass windows. Basically anything that extends significantly above the height of the instrument sensor. Special care should be taken to stay away from objects that may shade the direct sunlight at any time during the year. A solar calculator can be used to calculate the extreme positions of the sun around June 22 and December 22. On these dates, the sun rises and sets farthest north and south, respectively, of any time of year.