The surface of the sea is losing heat by evaporation and by long-wave radiation exchange with the sky, both of these rates being of the order of 10-2 W cm-2 in clear weather. Heat lost in this way must be provided by conduction upward from the water beneath and downward from the air above. Short-wave radiation need not be considered since it is not absorbed at the surface. It seems possible therefore that on days of light wind the "skin" temperature of the sea might be appreciably less than the temperature of the layers beneath. Now sea surface temperatures are usually measured by means of a dip bucket and it is clear that water entering the bucket is derived in varying amounts from various depths below the surface, so that the temperature of the 'mixture will not generally be equal to the skin temperature of the sea. In view of these considerations an experiment was carried out with the object of determining the skin temperature by measuring the long-wave radiation emitted from the sea and comparing it with the dip bucket temperature.