Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 147

allow the surrounding water to freeze. Droplets formed around the condensation nuc-
lei are too small or too salty to freeze directly at this temperature. Most storm clouds,
therefore, are a three-phase mixture of water vapor, supercooled liquid droplets, and
small ice crystals. The affinity of ice surfaces for attracting water vapor is slightly great-
er than that of the supercooled liquid surface (stated another way, saturation vapor
pressure is lower over ice than over liquid water at the same temperature). Therefore,
water vapor molecules have a tendency to deposit more rapidly on small ice seed crys-
tals (hence drying the air near them), whereas water vapor tends to evaporate from su-
percooled droplets (thus moistening the air near them). The net result is a vapor flow
from the supercooled droplets to the ice crystals, causing shrinkage of the former and
growth of the latter (Fig. 4.1). Thus, it can be seen that snow crystals grow molecule
by molecule (analogous to bricks placed one by one in a complex building project), and
this helps explain why snowflakes can be so delicate and varied. This mechanism is re-
ferred to as the Wegener-Bergeron-Findeisen process, named after persons involved in
the development of the theory (Knight 1967; Hobbs 1974; Pruppacher and Klett 1987;
Nelson and Baker 1996; Wood et al. 2001; Barry and Gan 2011).
Snow and ice crystals grow in some variation of the hexagonal (six-sided) crystal
system (Fig. 4.2). Once formed, ice crystals and snowflakes are subject to continual
change. They may grow through deposition and accretion or diminish through sublima-
tion and melting, and they may be fragmented and recombined in numerous ways. The
variations on the basic hexagonal pattern display almost infinite variety. We are taught
from childhood on that every snowflake is different! In absolute terms this is true, but
most often snow crystals falling from homogeneous cloud conditions resemble one an-
other closely in basic shape. Snow crystals are generally small and simple when first
formed in the cold, dry air of high altitudes. As they fall, snow crystals can become
larger and more complex when they encounter warmer or more moisture-laden atmo-
spheric layers, often becoming large enough to earn the name snowflakes. Thus, snow
received at the summits of mountains is often quite different from that received on the
mid-elevation slopes; in fact, it may melt to rain by the time it reaches the valley bot-
toms. Most rainfall outside the tropics begins as snowfall at high altitudes (Knight 1967;
Hobbs 1974, Barry and Gan 2011).