A new approach is described for calculating the mass (m) and terminal velocity (V t) of ice particles from airborne and balloon-borne imaging probe data as well as its applications for remote sensing and modeling studies. Unlike past studies that derived these parameters from the maximum (projected) dimension (D) and habit alone, the ''two-parameter approach'' uses D and the particle's projected cross-sectional area (A). Expressions were developed that relate the area ratio (A r ; the projected area of an ice particle normalized by the area of a circle with diameter D) to its effective density (e) and to V t. Habit-dependent, power-law relationships between e and A r were developed using analytic representations of the geometry of various types of planar and spatial ice crystals. Relationships were also derived from new or reanalyzed data for single ice particles and aggregates observed in clouds and at the ground. The mass relationships were evaluated by comparing calculations to direct measurements of ice water content (IWC). The calculations were from Particle Measuring Systems (PMS) 2D-C and 2D-P probes of particle size distributions in ice cloud layers on 3 days during an Atmospheric Radiation Measurement (ARM) field campaign in Oklahoma; the direct measurements were from counterflow virtual impactor (CVI) observations in ice cloud layers during the field campaign. Agreement was generally to within 20%, whereas using previous massdimension relationship approaches usually produced larger differences. Comparison of ground-based measurements of radar reflectivity with calculations from collocated balloon-borne ice crystal measurements also showed that the new method accurately captured the vertical reflectivity structure. Improvements in the accuracy of the estimates from the earlier mass-dimension relationships were achieved by converting them to the new form. A new, more accurate mass-dimension relationship for spatial, cirrus-type crystals was deduced from the comparison. The relationship between V t and A r was derived from a combination of theory and observations. A new expression accounting for the drag coefficients of large aggregates was developed from observational data. Explicit relationships for calculating V t as a function of D for aggregates with a variety of component crystals were developed.

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Measurementsof falling velocity of individual ice crystals produced by dry ice seeding were made in a cold room. The falling velocity was measured by stroboscopic photographs of falling ice crystals and each crystal was replicated to observe the shape and size. The relationship between the falling velocity and the dimension of crystals was obtained for the plane and the columnar type of crystals. Moreover, the measured falling velocity was generally explained by the computation method which use the drag coefficient obtained from the model experiment.

The falling velocity of individual snow crystals was ovserved by means of stroboscopic photographs. At the same time, the shape, dimension and mass of those snow crystals were also measured from the photomicrographs of them. The plane type crystals were classified into six groups, namely hexagonal plates, crystals with sectorlike branches, crystals with broad branches, dendritic crystals, stellar crystals and thick plates. Furthermore, the columnar crystals were also observed. Consequently, the relationship between the falling velocity and the dimension of particles was obtained for their several major types of snow crystals, and it was found that the relationship was well explained by the computation method which use the drag coefficient determined by the model experiment.