Sunny and cloudy sky conditions were classified based on a
half-day scale. Sunny mornings and afternoons were identified as
the change of kt with b and the relationship between kt and sinb.
The steps were: First, the mornings or afternoons were selected
if kt changed smoothly with b. Second, the values of kt were plotted
against sinb. The mornings and afternoons were excluded if the
relations of kt to sinb falling away from the major patterns
(Gu et al., 1999).