In estimates of horizontal radiation, its values are determined by correlations among the length of day, noon altitude of the sun, and cloud amount. During winter from 1971 to 1974 in Korea, its correlations are calculated by empirical method are Qo=0.0124 HcTd at clear day and Qs=0.0124 HcTd (1-0.0C41.3) at cloudy day. here Qo, Qs, Hc, Td, C are horizontal radiation at clear day, horizontal radiation at cloudy day, noon altitude of the sun, length of day, and cloud amount. In applications of this equation at 10 stations in Korea, errors are less than 10% when cloud amount are less than 7, and 1O~20% when cloud amount are more than 7. But, according to Laevastu equation in same case, errors are 50% closely.