Geoscience Reference
In-Depth Information
Wind speed (m/s)
10
20
30
40
Downdraught
Updraught
9
CIRRUS
25
8
7
20
6
LOW
STABILITY
ALTOCUMULUS
LENTICULARIS
ALTOCUMULUS
LENTICULARIS
5
15
4
10
AMPLITUDE
3
STABLE
ROTOR
CLOUD
ROTOR
CLOUD
2
5
CAP CLOUD
WAVELENGTH
MAIN
ROTOR
1
UNSTABLE
0
0
0
20
-40
-30
-20 -10
Temperature (°C)
0 0
km
Figure 6.13
Lee waves and rotors are produced by airflow across a long mountain range. The first wave
crest usually forms less than one wavelength downwind of the ridge. There is a strong surface wind down
the lee slope. Wave characteristics are determined by the wind speed and temperature relationships
shown schematically on the left of the diagram. The existence of an upper stable layer is particularly
important.
Source: After Ernst (1976).
contrast, there was a 20 percent decrease on the
initial run-up to the hill and a 40 percent decrease
on the lee side, probably due to horizontal
divergence. Knowledge of such local factors is
critical for siting wind energy systems.
A wind of local importance near mountain
areas is the
föhn
, or
chinook
. It is a strong, gusty,
dry and warm wind that develops on the lee side
of a mountain range when stable air is forced to
flow across the barrier by the regional pressure
gradient; the air descending on the lee slope
warms adiabatically. Sometimes, there is a loss of
moisture by precipitation on the windward side of
the mountains (
Figure 6.15
). The air, having
cooled at the saturated adiabatic lapse rate above
the condensation level, subsequently warms at the
greater dry adiabatic lapse rate as it descends on
the lee side. This also reduces both the relative and
the absolute humidity. Other investigations show
that in many instances there is no loss of moisture
over the mountains. In such cases, the föhn effect
is the result of the blocking of air to windward of
the mountains by a summit-level temperature
inversion. This forces air from higher levels to
descend and warm adiabatically. Southerly föhn
winds are common along the northern flanks of
the Alps and the mountains of the Caucasus and
Central Asia in winter and spring, when the
accompanying rapid temperature rise may help to
trigger avalanches on the snow-covered slopes. At
Tashkent in Central Asia, where the mean winter
temperature is around freezing point, tempera-
tures may rise to more than 21
C during a föhn.
In the same way, the chinook is a significant
feature at the eastern foot of the New Zealand
Alps, the Andes in Argentina, and the Rocky
°
