Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 149

istics (Bentley and Johnson 1931). During the 1930s to 1950s, a patient scientist from
Japan spent a great deal of time studying the seemingly infinite varieties, trying to make
some physical sense of snow crystal form. Ukichiro Nakaya (1954) grew snow crys-
tals indoors in a cold chamber where temperature and humidity could be carefully con-
trolled. He grew snow crystals from small “ice seeds” frozen onto a strand of rabbit hair
and noted the form results for varying temperatures and amounts of supersaturation.
Nakaya's original results are shown in Figure 4.5 and are summarized in Table 4.1.
The crystal form changes in a consistent manner depending on cloud temperature
and degree of supersaturation. It is most typical for one type of crystal to fall from a
given cloud, rather than having a mix of types all falling at once. The bottom line is that
if you can identify the basic form of the snow crystal at the ground, you can tell what
the conditions are in the clouds above. Nakaya poetically referred to this connection
between crystal form and cloud conditions as “letters from the sky.”
The principal forms of snow crystals falling from the atmosphere are generally
grouped into eight to ten main types. The newer International Commission on Snow and
Ice (ICSI) classification scheme has nine different types (Fierz et al., 2009) and includes
rime, which is formed on terrestrial surfaces when supercooled water is deposited dir-
ectly on object. The major atmospheric forms are shown in Figure 4.6. This and older
classification schemes are applicable only to falling snow or snow that has been on the
ground a short period of time (a few hours to days depending on temperature), which is
referred to as new snow.