Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 84

would only receive the sun early in the morning and late in the evening. During the winter sol-
stice only north-facing slopes with gradients of less than 15° would receive any sun at all. (After
Geiger 1965: 374.)
East-and west-facing slopes are also affected differently by solar radiation. Soil and
vegetation surfaces are often moist in the morning, owing to the formation of dew or
frost. On east-facing slopes, the sun's energy has to evaporate this moisture before the
slope can heat appreciably. By the time the sun reaches the west-facing slope, however,
the moisture has already evaporated, so the sun's energy more effectively heats the
slope. The driest and warmest slopes, therefore, face southwest rather than south.
Cloud cover, which varies latitudinally, seasonally, and according to time of day, can
greatly influence the amount of solar energy received on slopes. During storms, the en-
tire mountain may be wrapped in clouds; even during relatively clear weather, moun-
tains may still experience local clouds. In winter, stratus clouds and fog are character-
istic on intermediate slopes and valleys, but these frequently burn off by midday. In sum-
mer, mornings are typically clear but convection clouds (cumulus) build by mid-after-
noon from thermal heating. Consequently, convection clouds result in east-facing slopes
receiving greater sunlight while stratus clouds, as described above, allow more sun on
west-facing slopes. As clouds move over mountains, build, and dissipate, they have a
marked effect upon the amount and character of radiation received.
FIGURE 3.8 Topo- and microclimatic influences of slope and aspect on vegetation types. The northern
hemisphere example is given where more solar receipt on south-facing slopes warms temperat-
ures to where forest is replaced by grass. North-facing slopes are shaded and cooler with more
soil moisture retention and thicker forests. On a larger scale, forests move down valleys following
moisture and cooler temperatures created by cold air drainage. (After Kruckeberg 1991.)