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Cloudiness

Cloudiness observations on board ships are estimated visually. In very low light conditions, observers often miss high level clouds. This tends to bias night time cloudiness toward a smaller fraction of cloud cover (Sverdrup 1933; Riehl 1947; Schneider et al. 1989). It is possible to reduce this bias by rejecting cloudiness observations taken during dark sky conditions. These conditions can be determined by the altitude of the sun and the phase and altitude of the moon.

Another bias in cloudiness results from sky obscured observations. A sky obscured observation means that clouds are not visible due to smoke, blowing dust, fog, snow, or some other phenomenon. This condition represents over 6%of the observations in the period 1952-81 (Hahn et al. 1992). Sky obscured observations are coded separately and cannot be assigned one cloudiness value, as not all sky obscuring phenomena are associated with clouds. But if none of these observations are included, heavy fog- or snow-associated cloudiness would be eliminated and the cloudiness would be biased toward a lower value. To eliminate this bias, the present weather code can be checked for each sky obscured observation (see section 6.2).


Fri Oct 20 12:28:33 EDT 1995