Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 188

for glacial growth are complex and may vary for different regions. Glaciers are cre-
ated not simply by the lowering of temperature but by the interplay of different climat-
ic factors (Ageta and Higuchi 1984; Bitz and Battisti 1999; Kaser and Osmaston 2002,
Benn and Evans 2010; Barry and Gan 2011). Nevertheless, temperature remains the
crucial factor, and there is evidence that temperatures were several degrees lower dur-
ing the height of the ice ages. Once formed, a glacier responds to and reflects changing
climatic conditions.
A glacier's “state of health” from a climatic point of view can be determined by ana-
lysis of its mass balance (Ahlmann 1948, Paterson 1994). Whether a glacier grows, re-
treats, or maintains itself depends on its mass balance, or budget. This is determined by
total snow accumulation as opposed to what is lost through ablation (i.e., melting, calv-
ing, evaporation, and sublimation). A simple indicator of glacial status on a year-to-year
basis is the location of the annual snowline (as discussed earlier in this chapter), or firn
limit, which represents the maximum extent of summer melting (Fig. 4.31). Since firn is
snow at least one year old, the firn limit is the zone dividing one year's snow from that
of the previous year (or, in some cases, fresh snow from glacial ice). The firn limit on a
glacier is generally quite distinct near the end of the summer, and can be identified by
field examination or from aerial photographs (Fig. 4.32).
FIGURE 4.33 Muir Glacier photographed in 1941. (Photo by W. Field. Courtesy National Snow and Ice
Data Center.)