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12 detecting ice nucleation. Figure 1 summarizes the effectivity measurements of some past efforts. The threshold of nucleation (the ..--.----__N...-..-"__'._'_~.~_".,_......"...,,_ '. '>' ....,c,.".._"~.-..._,...._,.. ...._~.-_.,_. warmest temperature at which ice production can be detected) varies from ,~. ~_._..."......._.-..-.'---"- r-' -60e to -90e and the effectivity at cold temperatures (-200C) spans three orders of magnitude. Smith and Heffernan (1954) used a amall cloud chamber, 76 liters in volume. The walls of the chamber were cooled. No mechanism for COOling the air within the chamber other than oonduction of heat from the air to the walls was provided. Fog was maintained by introduotion of steam at the bottom of the chamber. The fog volume was reported to be 10 liters. Temperature variations within this system were probably quite large in the verticle. Aerosol samples were introduoed at the top of the chamber. Only a small proportion of aerosol particles at the bottom of the chamber were subjected to the oorrect temperature and water conditions of the experiment. Warburton and Heffernan (1964) round the threshold to be 20e warmer and efteotivity at -200C two orders of magnitude greater than reported in Smith and Hefternan (1954) (Fig. 1). Warburton and Hefternan (1964), also used a mixing cloud chamber. The amaller size, 10 liters, allowed faster heat conduction trom the air to the walls. Temperature and relative humidity was uniform in the lower 75' of the chamber (Warner, 1957). Fog was produoed by inserting a semipermeable membrane containing water at a temperature or 20oe. This membrane had to be changed at five minute intervals. The warm water vapor in a sub-zero environment produced large supersaturations that reoocured every five minutes as each membrane wasreplaoed with a fresh one. Mossop (1968) showed, by varying the humiditier temperature, that greater I I I I I I I I I I I I I I I I I I I