I believe that typically the sensor would be mounted horizontally (subsequently the leveling bubble). The reason is because physical quantities like water evaporation rates (lake water, for example), evapo-transpiration rates (from vegetation), melting rates of snow and ice, sublimation rates of snow and ice, etc., depend in part on the solar power (measured in W/m
2) that is incident on the surface of the ground. The solar rays are more direct in the summer than in the winter; they are more direct near the equator than near the poles; and they are more direct in the afternoon than in morning or evening, for examples.
Incidence Direct Indirect 12 rays/m
2 7 rays/m
2|||||||||||| \ \ \ \ \ \ \__________ __________
1 m
2 1 m
2As another example, unlike a sunflower that orients itself to face the sun throughout the day to maximize its absorption of solar power, the surface of the ground (or of a lake) does not orient itself to directly face the sun. Thus, it is usually more natural to measure the solar power incident on a horizontal plane and consequently, to level the sensor horizontally.
Isn't the sensor shaped like a fish-eye lens? I believe the sensor also measures diffuse light, according to NOAA, "for the entire hemisphere" (meaning the entire sky above). If so, then mounting the sensor non-horizontally would not measure all diffuse light from the sky (since part of the sensor would be viewing the ground).
But hey, whatever floats your bubble! Regards,
Kevin...
Reference:
http://www1.ncdc.noaa.gov/pub/data/uscrn/documentation/site/sensors/solarradiation/Descriptions/summarycurrentsolarradiationsensor.doc