β Scribed by T. W. Choularton; J. Latham
No coin nor oath required. For personal study only.
Two techniques have been used to measure the deposition coefficient Ξ² for water vapour molecules onto ice formed from distilled water, from 0Β·01M HCl and from 0Β·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^β2^torr). Measurements of Ξ² were made over the temperature range β37 to β9Β°C.
It was found that for pure ice Ξ² increased from 10^β3^ at β37Β°C to 5 Γ 10^β3^ at β9Β°C. Similar temperature dependency was found for the ice specimens formed from HCl solutions, but the values of Ξ² were lower at all temperatures. This reduction was greater for the more contaminated ice.
The measured values of Ξ² 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 wellβdocumented case it was calculated that crystals of dimensions 80 and 300 ΞΌm will survive a fall of about 0Β·5 and 2Β·3 km respectively; these predictions are consistent with observation. These two values of survival depth are greater than the Maxwellian values (Ξ² = 1) by factors of about 3 and 1Β·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 Ξ² but that in conditions conducive to the formation of middleβlevel cloud the survival depths may be substantially increased.
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