This paper discusses findings in regard to cloud distributions over the equatorial Pacific and the development of a method, compatible with actual cloud distributions, for using certain existing formulas to compute incoming solar radiation from meteorological data.
U. S. Air Force Uniform Summaries of Surface Weather Observations for 10 central and western equatorial Pacific sites indicate that the very large cirriform cloud contribution to total sky cover, noted in the July 1957–June 1958 Canton Island weather data, is a general characteristic of the cloud distribution over a large part of the western equatorial Pacific. Since most formulas for computing incoming solar radiation depend on a mean total cloud cover entry, a large bias toward thin cirriform cloud could lead to sizeable underestimations of incoming solar radiation and cause significant errors in heat budget calculations for this region.
In order to determine a suitable cloud cover term, monthly mean total sky cover, low cloud cover and total opaque sky cover values were obtained from the July 1957-June 1958 Canton Island weather data and tested in six commonly used formulas for computing incoming solar radiation. On comparing computations with recorded values, the mean total sky cover entry gave large underestimations of incoming solar radiation with all formulas. Test results suggested the use of a monthly mean sky cover term obtained by using mean low cloud cover values for the daylight hours of fair weather days and mean total opaque sky cover for the daylight hours of days dominated by disturbance weather. The Kimball, Black, Budyko and Berliand formulas gave very satisfactory results when using the suggested term. Computations from the latter three formulas were within 10% of recorded values for all months. The use of this approach with Black's formula is particularly appropriate, since the computations are then based on the incoming solar radiation on a horizontal surface at the top of the atmosphere, rather than on the radiation received at the earth's surface under clear sky conditions.