A water drop 1 mm in diameter, placed on a micro slide, was super-cooled inside a temperature-controlled chamber located under a high-power microscope. The freezing of the drop was observed by the use of cinephoto­micrographic techniques. The chamber and drop were cooled. At the moment the freezing started, a thin ice shell formed immediately on the surface of the water drop and its temperature, monitored by a small thermocouple, jumped to 0°C. Then a large number of small water droplets, ranging from < 1 to 20 um were ejected from the surface of the freezing drop. These droplets fell onto the microslide. The small droplets were ejected continuously for an average of 50 seconds, with the exact length of the ejection period dependent on environmental conditions. During this period the small droplets on the slide near the freezing drop grew more rapidly than the ones farther away. The concentration of small droplets on the micro-slide also increased rapidly, and the interior temperature of the freezing drop remained about 0°C.

After freezing was completed, the growth of the small droplets terminated and the temperature of the frozen drop decreased rapidly to the environmental temperature. Then all of the small droplets began to diminish in size. Those farthest from the frozen drop gradually disappeared. Some of the small droplets froze by contact with spicules from the frozen drop . Similar experiments were also performed with a water drop suspended on a fine fiber. After freezing started, a stream of small water droplets was ejected continuously from the surface of the freezing drop during the entire freezing period. This phenomenon terminated after completion of freezing.

Electric measurements were made in three ways: (i) by placing the probe of a sensitive electrometer into the stream of ejected small droplets; (ii) by placing the probe into the freezing water drop; and (iii) by observing the deflection of the stream of ejected small droplets in an electric field. The results indicated that the ejected small droplets carried net positive charge and that net negative charge was left on the residual frozen drop.

Experimental evidence has shown that positively charged droplets were generated by the freezing of a super-cooled water drop. The possible mechanisms for their generation are:

1) An increase of interior pressure. Unfrozen water in the form of small droplets was excreted from numerous pores which appeared to be weak spots found in cracks on the ice surface. Small spicules formed later at these same weak spots.

2) Condensation of water vapor, sublimated from the ice surface of the freezing drop. The temperature of the freezing drop was higher than that of the surrounding environment.

3) Formation of small droplets by the bursting of air bubbles, which were observed under the ice surface on the drop and in the interior of spicules. The air bubbles were formed as a result of the decrease of solubility of air in water when the temperature of the drop increased upon freezing. Observations of sequences of photo­micrographs of the drop freezing and of the changes in temperature during the freezing period (Fig. 1) revealed that the vapor pressure gradient re­versed direction twice during the freezing period. These reversals occurred when the freezing started and when the freezing was completed, and the water vapor molecules moved outward from the surface of the freezing drop during the entire freezing period A definite radial temperature gradient was maintained within the drop during the freezing period, with the colder region at the surface and the warmer region at the interior. A concentration of positive charge was found in the outer layer of the freezing drop (1) when the small droplets were ejected from the surface; these droplets carried net positive charge with them, while negative charge was left on the residual frozen drop.(ref: THERMOLELECTRIC EFFECT)

It is suggested that this newly observed phenomenon of the ejection of droplets is an important process which occurs under natural conditions in thunderclouds near the freezing level and where the water drops, carried from the base of the thundercloud by updrafts, are freezing. Moreover, it is possible that these ejected small droplets will also freeze after being carried upward to the higher and colder region of the thundercloud. It is widely accepted that the charge generation and separation processes in a thundercloud are closely associated with the development of precipitation and the main charge centers that appear above the freezing level. It is natural to associate their generation with the ice phase. Previously attention has been given to the fragmentation of a freezing water  drop (1, 2) in the forms of shattering, splintering, or bursting, which has occurred occasionally during the freezing period. The observation presented here suggests that the ejection of small droplets by the freezing of a super-cooled water drop may be important in study of thundercloud dynamics and in the generation of thunderstorm electricity.