Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 92

Ambient temperatures are normally measured at a standard instrument-shelter
height of 1.5 m (5 ft). Such measurements generally show a progressive decline in tem-
perature and a lower temperature range with elevation (Table 3.3; Figs. 3.10, 3.12).
Yet there is a vast difference between the temperature at a height of 1.5 m (5 ft) and
that immediately next to the soil surface. Paradoxically, the soil surface in alpine areas
may experience higher temperatures (and therefore a greater temperature range) than
at low elevations, due to the greater intensity of the sun at high elevations (Anderson
1998). At an elevation of 2,070 m (6,800 ft) in the Alps, temperatures up to 80°C (176°F)
were measured on a dark humus surface near timberline on a southwest-facing slope
with a gradient of 35° (Terjung et al. 1969). This is comparable to the maximum tem-
peratures recorded in hot deserts! At the same time, the air temperature at a height of
2 m (6.5 ft) was only 30°C (86°F), a difference of 50°C (90°F). Such high surface tem-
peratures may occur infrequently and only under ideal conditions, but somewhat less
extreme temperatures are characteristic, demonstrating the vast differences that may
exist between the surface and the overlying air (Fig. 3.13). The soil surface in alpine
tundra will almost always be warmer during the day than the air above it (Pomeroy et
al. 2006). It may also become colder at night, although the differences are far less than
during the day. The low growth of most alpine vegetation may be viewed as an adapta-
tion to take advantage of these warmer surface conditions. In fact, several studies have
shown that tundra plants may suffer more from high temperatures than from low tem-
peratures (Körner 2003).
Temperature ranges vary not only with elevation, but latitudinally. The contrast in
daily and annual temperature ranges is one of the most important distinguishing char-
acteristics between tropical and midlatitude or polar climates. The average annual tem-
peratures of high tropical mountains and polar climates are similar. The average an-
nual temperature of El Misti in Peru at 5,850 m (19,193 ft) is −8°C (18°F), compar-
able to many polar stations. The use of this value alone is grossly misleading, given the
vast differences in the temperature regimes. Tropical mountains experience a temper-
ature range between day and night that is relatively greater than other mountain areas,
due to the strongly positive heating effect of the sun in the tropics. On the other hand,
changes in average temperature from month to month, or between winter and sum-
mer, are minimal. This is in great contrast to midlatitude and polar mountains, which
experience lower daily temperature ranges but are increasingly dominated by strong
seasonal gradients. Knowledge of the differences between these temperature regimes
is essential to understanding the nature and significance of the physical and biological
processes at work at each latitude.
Figure 3.14a depicts the temperature characteristics of Irkutsk, Siberia, a subpolar
station with strong continentality. The most striking feature of this temperature regime
is its marked seasonality. The daily range is only 5°C (9°F), while the annual range is
over 60°C (108°F). Thus in winter, from October to May, temperatures are always be-
low freezing, while in summer they are consistently above freezing. The period of stress
for organisms, then, is concentrated into winter. An alpine station at this latitude would
have essentially the same temperature regime, except for a relatively longer period with
negative temperatures and a shorter period with positive temperatures. More poleward
stations would show an even smaller daily temperature range (Troll 1968).