FY 1976

An evaluation of cloud factors for estimating insolation over the ocean

Reed, R.K.

NOAA Tech. Memo. ERL PMEL-8, NTIS: PB-264174/4, 20 pp (1976)

Observations at three coastal sites are used to derive a cloud factor for the computation of insolation at sea. The factor is 1–0.62C + 0.0019, where C is cloud cover in tenths and is noon solar altitude. This relation is also valid for 125 days of recent measurements over the eastern North Pacific Ocean. The relation above can be used to estimate monthly oceanic insolation with a random error less than ±10%. Most previous cloud factors are inappropriate for a number of reasons: (1) relations derived at inland terrestrial locations are not valid at sea; (2) data have been grouped without regard to season (solar altitude); and (3) erroneous clear-sky formulas have been used. Lumb's (1964) formulas are valid, but their use places high demands on the quality of cloud observations. If cloud cover is estimated from satellite photographs, the amount should be increased by about 0.2 to give agreement with visual estimates on which the above cloud factor is based.