Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 75

airflow influence transiting storms, making local forecasts difficult (Lin et al. 2005). The
same funneling effect occurs over mountain passes as winds are deflected around peaks
or ridges on either side of the pass.
TABLE 3.1
Average Density of Suspended Particulate Matter in the Atmosphere with Changing Elevation
BLOCKING AND DISTURBANCE OF THE UPPER AIR
High-pressure areas prevent the passage of storms. Large mountain ranges such as the
Rockies, Southern Alps, Andes, and Himalayas are very efficient at blocking storms,
since they are often the foci of anticyclonic systems (because the mountains are a center
of cold air), and thus storms detour around the mountains (Insel et al. 2010). Moun-
tains also cause other perturbations to upper-air circulation, with consequent effects
on clouds and precipitation (Epifanio and Rotunno 2005; Lin et al. 2005; Neiman et al.
2010; Xu et al. 2010). This occurs on a variety of scales: locally, with the wind immedi-
ately adjacent to the mountains; on an intermediate scale, creating large waves in the
air; and on a global basis, with the larger mountain ranges actually influencing the mo-
tion of planetary waves and the transport momentum of the total circulation (Park et
al. 2010). Disturbance of the air by mountains generally creates a wave pattern much
like that found in the wake of a ship, and may result in the kind of clear-air turbulence
feared by airline pilots, or it may simply produce lee waves with their beautiful lenticu-
lar (standing-wave) clouds, associated with mountains the world over (Scorer 1961).
Mountains also influence the location and intensity of jet streams, which greatly in-
fluence mountain weather (Insel et al. 2010). The jet streams may split to flow around
the mountains, rejoining to the lee of the range, where they often intensify and produce
storms (Neiman et al. 2010). In North America, such storms, known as “Colorado Lows”
or “Alberta Lows,” reach their greatest frequency and intensity in spring, sometimes
causing heavy blizzards on the Great Plains and Prairie provinces (Sato and Kimura
2003). The tornadoes and violent squall lines that form in the American Midwest also