Frozen drops were observed at Fairbanks, Alaska in early winter of 1976. They fell along with dendritic snow crystals and displayed a polycrystalline structure . From the difference in falling velocity between the frozen drops and the snow crystals, it is concluded that the drops were formed in the upper part of a cloud and were frozen during their fall by colli sion with snow crystals which were formed in the lower part of the cloud.

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The method of melting snow with subterranean water has been indispensable for roads and also utilized for roofs in snow-covered areas. Also, recently the methods of electrically melting snow and anti-freeze systems have been applied to pedestrian crossings and buildings. At the same time, the effective operation of melting snow installation has been taken up as an important problem from the viewpoint of water and electric energy saving. But, it is difficult to handle a snow melting installation effectively, so a practical device detecting snow fall automatically and controlling a snow melting installation has been much desired.
The authors produced, by way of trial, a simple snow fall detector applying the principle that infrared rays emitted by a diode are reflected diffusely by fallen snow. The theory of measurement is as follows. As infrared rays are set not to reach the photo-receiving element (photo-diode)owing to reflection by the black snow-catching plate with a heater, the detector does not operate when it is not snowing. On the other hand, while it is snowing, a part of the infrared rays are reflected diffusely by falling snow on the snow catching plate, and reach the photo diode;therefore, the detector notices falling snow and operates the snow melting installation. When it stops snowing, as the fallen snow on the snow catching plate is melted by the heater, infrared rays reflected by the black snow-catching plate cannot reach the photo diode, the detector stops operating and the snow melting installation stops operating. The authors performed a series of experiments on snow fall detection fcom February 13 to 26th, 1980, which showed an accuracy of 80% in detecting properly. Problems which need to be discussed in the future include sensitivity of the detector, setting time of timer and the relation between the above-mentioned items and the weather conditions.

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Uniaxial compression tests of columnar-grained ice which was segregated in soil specimens were conducted. Specimens (5 cm in diameter and 9 cm in height) were prepared by using frost susceptible clay which was sampled from Tokyo. The tests were carried out systematically with respect to the dependence of compressive strength on temperature, direction of loading, strain rate and specimen height.
When the load is applied ta the direction of ice growth, the longitudinal uniaxial compressive strength increases with the lowering of temperature, giving values of 190 kgf/cm² at -10°C and 250 kgf/cm² at -20°C. These values are larger than those of various types of ice, Fujinomori frozen clay and Toyoura frozen sand. When the loading is perpeiadicular to the growth directian, the lateral uniaxial compressive strength is approximately equal to the commercial ice strength, being about 50 kgf/cm² at -20°C. The compressive strength, when the loading is applied to the ice growth direction, has a maximum at a strain rate of between 0.5%/min and 2%/min for -3°C. When the initial specimen temperature was about -0.4°C. the specimen temperature dropped with partial fusion during compression at a faster strain rate than 0.4%/min, and consequently the relation between the longitudinal strength and the specimen temperature agrees with the relation between pressure and freezing point calculated from the Clausius-Clapeyron equation. For a constant strain rate, the longitudinal strength decreases as the specimen height decreases from 15 cm to 1 cm For a height less than 1 cm, on the other hand, it decreases. When the load is applied to the direction of ice growth, the transition from ductile failure to brittle failure occurred at the strain rates of 0.6, 0.2 and 0.04%/min for constant temperatures of -1.7, -3 and -18°C, respectively. In the range of ductile failure, grain boundary cracks and grain inside cracks are distributed regularly in the growth direction and/or perpendicular to it.

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[in Japanese]

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