Environmental Engineering Reference
In-Depth Information
Figure 1
Mass balance and related flow characteristics of (a) a
valley glacier; (b) an ice sheet and (c) an ice shelf. Mass transfers
and basal velocity are proportional to the size of the arrows.
Source: After Sugden and John (1976).
Thermal energy balance complements mass balance to complete our initial glacier
profile. Radiative and sensible heat fluxes are the principal energy sources. These are
augmented by latent heat - at a rate of about 2·5 ∞ 10
6
J kg
−1
of water - released by
condensation and direct deposition at the surface, and water freezing at depth. Energy is
used for sublimation, ice melt - at a rate of 3·33 × 10
5
J kg
−1
water equivalent - or is
conducted into cold ice, raising its temperature without melting. The relative importance
of each energy source and sink is linked closely with glacier type, and one or two obvious
but important observations can be made (Figure 2). Cold glaciers receive most heat via
short-wave radiation flux and least from warm, moist air advection. This dependence,
enhanced by high albedo and the ability of glacial anticyclones to fend off milder winds,
explains polar ice sheet development in areas of lowest global radiation receipt.
Conversely, temperate glaciers are found in areas of higher radiation flux balanced by
orographic effects on moist airstreams. This difference is reinforced by one final
distinction. Temperate glaciers are
isothermal
, i.e. at pressure-melting point throughout
their depth
and therefore
warm-based
. Cold glaciers are
polythermal
, with basal temperatures
below pressure-melting point, and, hence, are
cold-based
.
