Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 82

haska 1970). UV has been cited for a number of harmful effects, ranging from the re-
tardation of growth in tundra plants (Körner 2003) to cancer and eye damage in humans
(Parisi and Downs 2004; Lichte et al. 2010). UV is mainly responsible for the deep tans
of mountain dwellers and the painful sunburns of neophytes who expose too much of
their skin (Lichte et al. 2010). The wavelengths responsible for sunburn occur primarily
between 280 and 320 um, while those responsible for darkening the skin occur between
300 and 400 um. Wavelengths less than 320 um are known to cause skin cancer and
weaken the immune system.
EFFECT OF SLOPES ON SOLAR RADIATION
The play of the sun on the mountain landscape is like a symphony. As the hours, days,
and seasons follow one another, the sun bursts upon some slopes with all the strength of
fortissimo while the shadows lengthen and fade into pianissimo on others. The melody
is continuous and ever changing, with as many scores as there are mountain regions,
but the theme remains the same. It is a study of slope angle and orientation.
The closer to perpendicular the sun's rays strike a surface, the greater their intensity.
The longer the sun shines on a surface, the greater the heating that takes place (Ander-
son 1998). In mountains, every slope has a different potential for receiving solar ra-
diation, which can be measured using the following variables: latitude, time of year
(height of sun), time of day, elevation, slope angle, and slope orientation (Huo and Bailey
1992). The basic characteristics of solar radiation on slopes are illustrated in Figure 3.7.
This very useful diagram shows the situation for one latitude at four times of the year,
at four slope orientations. It does not include the effects of clouds, diffuse sky radiation,
the receptiveness of different slopes to the sun's rays, or shadow effects.
Most mountain slopes receive fewer hours of sunshine than a level surface, although
slopes facing the sun may receive more energy than a level surface (especially at higher
latitudes). In the tropics, level surfaces usually receive a higher solar intensity than
slopes because the sun is always high in the sky. Whatever the duration and intensity
of sunlight, the effects are generally clearly evident in the local ecology (Fig. 3.8). In
the northern hemisphere, south-facing slopes are warmer and drier than north-facing
slopes and, under humid conditions, are more favorable for life. Timberlines go higher
on south-facing slopes, and the number and diversity of plants and animals are greater
(Germino and Smith 2000). Humans take advantage of the sunny slopes. In the east-
west valleys of the Alps, most settlements are located on south-facing slopes. In spring,
north-facing slopes may still be deep in snow while south-facing slopes are clear. As a
result, north-facing slopes have traditionally been left in forest while south-facing slopes
are used for high pastures (Fig. 3.9).