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A preliminary investigation is made into the global variation in irradiance under the Clear, the Intermediate (partly cloudy) and the Overcast sky conditions (Nakamura et. al., 1986, 1987). A formula for global irradiance is proposed as a function of solar altitude and geographical altitude (height above the sea level) under each of the conditions based on measured data. These formulae can be used to estimate the cumulative global e over a year for any location given the daily sky conditions.