The results of two studies that reported procedures for estimating insulation at sea are summarized. The insulation under clear skies may be computed reliably with a formula derived from the Smithsonian Meteorological Tables, using a transmission coefficient of 0.7, or with a formula derived by Lumb. For estimating the reduction of insolation by clouds, the factor 1−0.62C+0.0019α is suggested, where C is cloud amount in tenths and α is noon solar attitude. Random errors of estimate within 95% confidence limits are less than 10% for mean monthly data and are about ±20% for weekly periods. Abstract The results of two studies that reported procedures for estimating insulation at sea are summarized. The insulation under clear skies may be computed reliably with a formula derived from the Smithsonian Meteorological Tables, using a transmission coefficient of 0.7, or with a formula derived by Lumb. For estimating the reduction of insolation by clouds, the factor 1−0.62C+0.0019α is suggested, where C is cloud amount in tenths and α is noon solar attitude. Random errors of estimate within 95% confidence limits are less than 10% for mean monthly data and are about ±20% for weekly periods.