Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 72

ponentially. Thus, half the weight of the atmosphere occurs below 5,500 m (18,000 ft)
and pressure is halved again in the next 6,000 m (Fig. 3.3).
The ability of air to hold heat is a function of its molecular structure. At higher alti-
tudes, as molecules are farther apart, there are fewer in a given parcel of air to re-
ceive and hold heat. Similarly, water vapor, carbon dioxide, and suspended particulate
matter decrease rapidly with altitude (Tables 3.1, 3.2). These constituents, important in
determining the ability of the air to absorb heat, are all concentrated in the lower at-
mosphere. Water vapor is the chief heat-absorbing constituent. Half of the water vapor
in the air occurs below 1,800 m (6,000 ft), diminishing rapidly above this elevation and
barely detectable above 12,000 m (40,000 ft).
The importance of water vapor as a reservoir of heat can be seen by comparing the
daily temperature ranges of deserts and humid areas. Both may heat up equally during
the day but, because of the relative absence of water vapor to absorb and hold the heat
energy, deserts cool down much more at night. The mountain environment responds in
a fashion similar to that of a desert, but is even more accentuated. As the thin, pure
air of high altitudes does not effectively intercept radiation, that radiation is lost to
space. Mountain temperatures respond almost entirely to radiation fluxes, not to the
temperature of the surrounding air, although some mountains receive considerable heat
from precipitation. The sun's rays heat the high thin air very little. Consequently, al-
though the temperature at 1,800 m (6,000 ft) in the free atmosphere changes very little
between day and night, a mountain peak intercepts and absorbs the sun's rays. The soil
surface may be quite warm, but the envelope of heated air is usually only a few meters
thick and displays a steep temperature gradient.
In theory, every point along a given latitude receives the same amount of sunshine;
in reality, clouds interfere. Cloudiness is controlled by distance from the ocean, direc-
tion of prevailing winds, dominance of pressure systems, and altitude. Precipitation nor-
mally increases with elevation, but only up to a certain point, and is generally heaviest
on middle slopes, where clouds first form and cloud moisture is greatest. Precipitation
then decreases at higher elevations. Thus, lower slopes can be wrapped in clouds while
higher slopes are sunny. In the Alps, for example, the outer ranges receive more precip-
itation and less sunshine than the higher interior ranges. The herders in the Tien Shan
and Pamir Mountains of Central Asia traditionally take their flocks higher in winter than
in summer to take advantage of the lower snowfall and sunnier conditions at higher el-
evations. High mountains have another advantage with respect to possible sunshine: In
effect, they lower the horizon. The sun shines earlier in the morning and later in the
evening on mountain peaks than in lowlands. The same peaks, however, effectively raise
the horizon for adjacent lands, thus delaying sunrise or creating early sunsets.