Geography Reference
In-Depth Information
to the mouth of the Rio Grande, although it is highly variable from year to year. The frost
line in tropical mountains is much more sharply delineated. In Quito, at 2,850 m (9,350
ft), frost is practically unknown. The vegetation consists of tropical evergreen plants
which blossom continuously; farmers plant and harvest crops throughout the year. By an
elevation of 3,500 m (11,500 ft), however, frost becomes a limiting factor (Troll 1968).
At an elevation of 4,700 m (15,400 ft) on El Misti in southern Peru, it freezes and thaws
almost every day.
The fundamental relationships between these disparate freeze-thaw regimes are
demonstrated in Figure 3.15. Each site has a similar average annual temperature of
−8°C to −2°C (18°F to 28°F), but the daily and annual ranges are markedly different.
Yakutsk, Siberia, experiences strong seasonality, with a frost-free summer period of 126
days, but in winter the temperatures remain below freezing for 197 days. Alternating
freezing and thawing take place for 42 days in the spring and fall. At Sonnblick in the
Austrian Alps, the winter season is much longer (276 days), with a very short summer
during which freezing and thawing can occur at any time. El Misti, however, is domin-
ated by an almost daily freeze-thaw regime. This type of weather has been character-
ized as “perpetual spring”: The sun melts the night frost every morning, and the days
are quite pleasant. The 12-hour day adds to the impression of spring. As discussed in
Chapter 7 and 8, these different systems provide greatly contrasting frameworks for the
survival of plants and animals, as well as for the development of landscapes.
Humidity and Evaporation
Water vapor constitutes less than 5 percent of the atmosphere but is by far the single
most important component with regard to weather and climate. It is highly variable in
space and time. Water vapor provides energy for storms, and its abundance is an in-
dex of the potential of the air for yielding precipitation. It absorbs infrared energy from
the sun and reduces the amount of short-wave energy reaching the Earth; serves as a
buffer from temperature extremes; and is important biologically, since it controls the
rate of chemical reactions and the drying power of the air. The moisture content of the
atmosphere decreases rapidly with increasing altitude. At 2,000 m (6,600 ft) it is only
about 50 percent of that at sea level, at 5,000 m (16,400 ft) it is less than 25 percent,
and at 8,000 m (26,200 ft) less than 1 percent of that at sea level (Table 3.2). Within this
framework, however, the presence of moisture is highly variable. This is true on a tem-
poral basis, between winter and summer, day and night, or within a matter of minutes
when the saturated air of a passing cloud shrouds a mountain peak (McCutchan and
Fox 1986; Huntington et al. 1998; Xie et al. 2010). It is also true on a spatial basis,
between high and low latitudes, marine and continental locations, the windward and
leeward sides of a mountain range, or north- and south-facing slopes. The general up-
ward decrease in water-vapor content, and the variations that occur, are illustrated by
the east and west sides of the tropical Andes (Fig. 3.16). The contrast in absolute hu-
midity between these two environments is immediately apparent, although the differen-
ce decreases with elevation and probably disappears altogether above the mountains.
Imata, Salcedo, and Arequipa on the west have only about half the water-vapor content
of stations on the east (Cerro do Pasco, Pachachaca, Huancayo, Bambamarca). Values
similar to those at Arequipa occur at elevations 2,000 m (6,600 ft) higher on the east
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