Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 108

optic setting (McGinnis 2000; Barry 2008). Tropical mountains tend to have precipita-
tion maxima at lower elevations, with the maximum zone rising with decreasing annual
totals. In middle latitudes, the general trend is for precipitation to increase with eleva-
tion, often to the highest observation station (Mass 2008). The existence of such a zone
has been challenged, as calculations of the amount of precipitation necessary to main-
tain active glaciers in high mountains and observations of relatively heavy runoff from
small alpine watersheds seem to call for more precipitation in certain mountain areas
than climatic station data would indicate (Smith et al. 2009; Daly et al. 2010).
Currently, the question is moot, the problem being one of measurement. There are
very few weather stations in high mountains, and even where measurements are avail-
able, their reliability is questionable (Scherrer et al. 2010). As one author says, “Pre-
cipitation in mountain areas is as nearly unmeasureable as any physical phenomenon”
(Anderson 1972: 347). This is particularly true at high altitudes with strong winds. Not
surprisingly, many studies have shown that wind greatly affects the amount of water
collected in a rain gauge (Fig. 3.20; Zhihua and Li 2007). Considerable effort has been
made to alleviate this problem by the use of shields on gauges, location in protected
sites, use of horizontal or inclined gauges, the use of radar techniques, and statistical
corrections (Peck 1972; Zhihua and Li 2007).
Measuring snow is even more difficult, since the wind not only drives falling snow
but redistributes it on the ground (Hiemstra et al. 2002). Correction factors have been
developed for certain types of gauges (Kyriakidis et al. 2001). There are also problems
in storage and melting of snow for water equivalency, as well as losses due to evapor-
ation. The major problem, however, is accurate monitoring of snowfall. Small clearings
are used in conifer forests and, above timberline, snow fences are increasingly used to
enclose and shield the gauges. This still does not guarantee accurate measurements,
but shielded gauges (whether for rain or snow) record greater amounts of precipita-
tion than unshielded gauges in the same location. For example, the University of Color-
ado has operated a series of weather stations in the Front Range of the Rocky Moun-
tains since 1952. The measured precipitation amounts from the two highest sites above
treeline increased abruptly in 1964 when snow fences were erected around the record-
ing gauges. Before the gauges were shielded, the average annual amount was 655 mm
(25.8 in.); it jumped to 1,021 mm (40.2 in.) after the snow fence was installed. The data
now show an absolute increase in precipitation with increasing elevation (Table 3.4).
More reliable instrumentation in the Alps has led to similar results, at least up to an
elevation of 3,000 m (10,000 ft) (Schmidli et al. 2002).