Time-Resolved Measurements of the Masses and Densities of Individual Frozen Hydrometeors

We present the first direct automated measurements of masses and densities of individual frozen, and semi-frozen hydrometeors, including their dependence on particle size and atmospheric temperature. The measurements were obtained with a new thermal-imaging instrument, the Differential Emissivity Imaging Disdrometer (DEID) that was developed at the University of Utah and being commercialized by Particle Flux Analytics, Inc. The DEID has been shown through laboratory and field experiments to be capable of providing accurate estimates of individual snowflake and bulk snow hydrometeor mass and density. The method exploits the rate of heat transfer during the melting of a hydrometeor on a heated metal plate, as viewed which is a function of the temperature difference between the hotplate surface and the top of the hydrometeor. Measurements obtained in the Wasatch Mountains of Utah of the mass-diameter relationship show general agreement with prior estimates that used more laborious techniques but there are notable new results with regards to density. First, there is a strong dependence of hydrometeor density on temperature. Second, hydrometeor density is generally lower than has been previously assumed, and it decreases with particle size as the inverse square root rather than the inverse of size.