Cloudiness

To reduce the bias caused by the underestimation of cloudiness in low light levels, we reject all observations taken under dark skies. A cloudiness observation is considered acceptable only if one of the following bright sky conditions apply (Hahn et al. 1992):

or

where and are the altitudes of the sun and moon, respectively, with negative altitudes being below the horizon. The phase of the moon can vary between 0 (new moon) and 1 (full moon). Equations to determine the altitude of the sun and altitude and phase of the moon can be found in Appendix B.

We also use the treatment of the sky obscured observation after Hahn et al. (1992). Some of the phenomena which obscure the sky, such as fog, are associated with overcast skies. If the Present Weather code indicates that the sky is obscured due to rain, snow, fog, or any other cloud-induced phenomenon, we consider the fractional cloudiness for the observation to be 1 and include it as a valid observation.