This paper describes a device developed for evaluating net vapor exchanges between snow and air that minimizes many of the errors inherent in pan measurements and provides a measurement of snowmelt during the study period. It consists of (1) a cylinder for holding an ‘undisturbed’ snow core, (2) a melt collection chamber, and (3) hydraulic pistons for maintaining the snow core level with the melting snow surface.The change in weight of the snow core with time is a measure of the net vapor exchange between the air and the snow. Results are presented for seven 14‐hour days during May–June 1972 and six 24‐hour days during May–June 1973. The study site is located on sagebrush rangeland on the Reynolds Creek Experimental Watershed in southwest Idaho at an elevation of 2.13km.