Journal of the Japanese Society of Snow and Ice

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A flat box (2cm. wide, 6cm. long, 5mm. high) filled with snow was placed on the table of a microscope in a cold chamber. The upper and lower side of the box was each covered doubly with glass plates. A vertical hole was made in the snow just underneath the objective lens of the microscope. A net made of single silk fibers was stretched in the hole and several snow crystals were put upon it. An electric heater placed at one end of the box produced a horizontal temperature gradient in the snow. Tiny crystals of ice grew on those sides of the crystals put on the net which faced the heater and the other sides of them receded.
Water vapour must have diffused from the hotter part of the snow to the colder part. It is noted by the above mentioned facts, however, that the water vapour did not diffuse a long way along the narrow canals in the snow but diffused only a short way between the adjacent snow particles placed on the line of the temperature gradient, that is, the water vapour produced at the hotter part was not transferred directly to the colder part but was transferred from one snow particle to the adjacent one by a from hand to hand way.
Above mentioned tiny crystals which could be called depth hoar did not appear when the temperature gradient was small. The crystals on the net simply bulked on the sides which faced the heater. The water vapour must have been transferred from particle to particle also in this case. The absence of depth hoar is, therefore, no evidence for the absence of diffusion of water vapour. The snow cover is commonly colder in the upper part of it and water vapour is continually diffusing upwards. The depth hoar is rarely found in the snow cover, because the temperature gradient in it is usually small.
Snow crystal changes its form slowly, as well known, by itself without melting. It was found that the presence of diffusion of water vapour due to temperature gradient exceedingly promoted the change of form of snow crystal.

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