Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 89

the effects of mountain mass on vegetation development can be observed in the Oregon
Cascades. Except for Mount McLoughlin in southern Oregon, timberline is highest and
alpine vegetation reaches its best development in the Three Sisters Wilderness area,
where three peaks join to form a relatively large landmass above 1,800 m (6,000 ft)
(Price 1978). On the higher but less massive peaks of Mount Hood and Mount Washing-
ton, a few kilometers to the north, the timberline is 150-300 m (500-1,000 ft) lower and
the alpine vegetation is considerably more impoverished. The development of vegeta-
tion involves more than climate, of course, since plant adaptations and species diversity
are related to the size of the gene pool and other factors (Körner 2003). Nevertheless,
as discussed further in Chapter 7, vegetation is a useful indicator of environmental con-
ditions, and a positive correlation between vegetation development and mountain mass
can be observed in most mountain areas.
FIGURE 3.10 Mean annual temperature with altitude in the Cascade Mountains. Dots represent U.S.
Weather Bureau First Order Stations in Washington State. Temperatures were calculated for peri-
od 1971-2000.
TEMPERATURE INVERSION
Temperature inversions are ubiquitous in landscapes with marked relief, and anyone
who has spent time in or around mountains will have experienced their effects. Inver-
sions are the exception to the general rule of decrease in temperature with elevation.
During a temperature inversion, temperatures are lowest in the valley and increase
upward along the mountain slope. Eventually, however, temperatures decrease again,
so that an intermediate zone, the thermal belt, will experience higher night temperat-
ures than either the valley bottom or the upper slopes. Inversions typically break up by
thermal heating about three hours after sunrise (Whiteman et al. 2004).
Cold air is denser and therefore heavier than warm air. As slopes cool at night, colder
air begins to slide downslope, flowing underneath and displacing warm air in the val-
ley. Temperature inversions are best developed under calm, clear skies, where there is