The Capacitance of Snowflakes

Cloud physics has tackled the problem of ice crystal growth by vapor deposition through the application of the 'electrostatic anology'. To estimate the growth of ice crystals a 'capacitance' needs to be specified that can range from D/2 for spheres to D/6 for a needle with a 10:1 aspect ratio (where D is the maximum dimension of the ice crystal). While values for capacitance of well defined geometrical shapes are well known, the important case of the capacitance of ice aggregates or snowflakes is not.

We will present aircraft observations from an anvil of a convective system obtained during the CRYSTAL-FACE campaign that fulfill the necessary criteria for estimating the mean capacitance of aggregate ice crystals. i) Accurate humidity measurements, ii) repeated observations of the same ice particle distribution as it evolves. The first point is satisfied with the use of a tunable diode hygrometer, while the second point is fulfilled by the aircraft performing a Lagrangian spiral descent that descends at a rate comparable to the mass weighted fall speed of the ice aggregates.

The results of comparing the measured change in ice water content with that predicted through the use of the electrostatic anology indicate that the capacitance of snowflakes is ~0.23D.