Measurements of the deposition coefficient for ice, and its application to cirrus seeding

Two techniques have been used to measure the deposition coefficient β for water vapour molecules onto ice formed from distilled water, from 0·01M HCl and from 0·1M HCl. One involved measurement of the evaporation rates of ice spheres at laboratory pressure. the other involved measurement of the equilibrium temperature achieved by a shell of ice surrounding a resistor within which electrical power was being dissipated at a known rate; this experiment was conducted at low pressures (< 10⁻²torr). Measurements of β were made over the temperature range −37 to −9°C. It was found that for pure ice β increased from 10⁻³ at −37°C to 5 × 10⁻³ at −9°C. Similar temperature dependency was found for the ice specimens formed from HCl solutions, but the values of β were lower at all temperatures. This reduction was greater for the more contaminated ice. The measured values of β were employed in calculations of the distance which cirrus crystals will fall through undersaturated air below cloud base. Various humidity structures and temperature distributions were considered. For a particularly well‐documented case it was calculated that crystals of dimensions 80 and 300 μm will survive a fall of about 0·5 and 2·3 km respectively; these predictions are consistent with observation. These two values of survival depth are greater than the Maxwellian values (β = 1) by factors of about 3 and 1·2, respectively. It is concluded that in most situations the persistence of a stream of cirrus crystals is not profoundly affected by these new values of β but that in conditions conducive to the formation of middle‐level cloud the survival depths may be substantially increased.