Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 97

FIGURE 3.15 Freeze-thaw regimes at different latitudes and altitudes. Frost-free days indicate the
number of days when freezing did not occur, ice days are those when the temperature was con-
tinually below freezing, and frost alternation days are the days when both freezing and thawing
occurred. Note that the greatest number of the latter occur in tropical mountains. (Adapted from
Troll 1958: 12-13.)
The lower absolute humidity and the tendency toward aridity at higher altitudes sug-
gest greater evaporation rates with elevation. However, this may not be true, since
the few studies of alpine evaporation have conflicting results (see Barry 2008). Several
studies do indicate increased evaporation with elevation (Matthes 1934; Henning and
Henning 1981; Sturman and Tapper 1996). Two years of water balance data from a
high-elevation (2,800-3,400 m) lake in California's Sierra Nevada show that evaporation
accounts for 19-32 percent of the ablation (Kattelman and Elder 1991). Snowfall con-
tributed 95 percent of the precipitation, and 80 percent of the evaporative (sublimation)
losses came from snowcover. However, other studies have shown that evaporation does
not exceed 10 percent of the total ablation (Hock 2005). Whichever of these tendencies
is accepted as being the more general, it should be noted that these particular alpine
areas are exceptionally, if not uniquely, dry environments, with high solar intensities,
strong winds, and persistent subfreezing temperatures (Terjung et al. 1969). Most in-
vestigations on snowfields and glaciers in other regions have tended to show that evap-
oration is relatively unimportant in total ablation and may actually inhibit ablation, ow-
ing to the heat it extracts (Hock 2005).
Evaporation and the factors that control it in a natural environment are exceedingly
complex (Penman 1963; Calder 1990). The rate depends upon temperature, solar in-