208 
window panes of houses. These ice flowers are of two 
kinds: window frost and window ice. The former is a 
product of the sublimation or freezing of minute super- 
cooled droplets; the latter results from the freezing of a 
water film. Window ice is likely to be formed in a kitchen 
or bathroom, where the moisture is abundant and the 
indoor temperature is just below freezing. The excessive 
water vapor condenses on the pane as a thin water 
film, and the mosaic of ice crystals obtained by the 
freezing of this thin film gives window ice. 
Window frost is quite different from window ice in 
SEY 
t 
Fie. 1—Detached hoar crystals and Fic. 
sheet frost (0.73). 
2.—Growth of hoar 
(X90). 
CLOUD PHYSICS 
numerous infinitesimal droplets condense on thesurface, 
and the glass plate takes on a blurred appearance. 
After a little while, many germs! of hoar crystals 
appear at diverse points and begin to grow. As soon as 
growth starts, the blurred surface around the crystal 
begins to clear, that is, the minute droplets in that 
region evaporate. The vapor pressure of supercooled 
water is higher than that of ice at the same temperature, 
so the water is evaporated from the droplets and con- 
denses on the hoar crystal, resulting in the growth of the 
latter at the expense of the former. The process is 
crystals Fic. 3.—Relaying of freezing action in 
sheet frost (210). 
Fie. 4.—Hoar erystal formed on a glass 
plate covered by an alcohol film (X30). 
its appearance. The most familiar type is shown in 
Fig. 1. It must be again classified into two kinds: the 
detached hoar crystal H and the sheet frost S in Fig. 1. 
The hoar crystal is a product of sublimation, and is 
formed by a mechanism similar to that which produces 
the snow crystals. The sheet frost, which always ex- 
tends over some area of the glass pane, is a two-di- 
mensional assemblage of minute ice granules that are 
formed by the freezing of supercooled droplets scattered 
over the pane. 
The mechanism responsible for the formation of hoar 
crystals can readily be studied by observing the initial 
stage of artificial hoar formation. Experiments have 
been performed in a cold chamber laboratory. Several 
minutes after exposing the plate to supersaturated air, 
Fig. 5—Three stages of a hoar crystal developed on a glass plate covered 
with paraffin film. 
shown remarkably well by slow-motion pictures, one 
frame of which is reproduced in Fig. 2. 
When water droplets are frozen into ice granules 
without evaporating, they form sheet frost. Supercooled 
water itself does not freeze easily, but once it touches 
ice, it becomes solid almost instantaneously. Thus, if 
one of the series of droplets freezes for some reason or 
other, the action is relayed to all the rest of the droplets, 
which also freeze. This relaying action can be seen 
under a microscope of high magnification. From one 
frozen droplet, a thin streamer of ice extends, and the 
moment it touches the next droplet, the one thus 
1. The term ‘‘germ”’ is used in this article to designate the 
primitive ice crystal. 
a | 
