Geoscience Reference
In-Depth Information
Figure 24.5
The föhn or chinook effect,
illustrated (a) by lapse rates
and (b) in cross-section,
showing comparative
temperatures at the same
altitude on windward and
leeward slopes.
(a)
600
700
Cooling
in cloud
at SALR
800
Warming on
descent
at DALR
900
Condensation
level
Cooling
on ascent
at DALR
1000
-10
0
10
20
30
Temperature, °C
(b)
-5°
3
+8°
0°
Snow
+13°
2
3°
+18°
Orographic
rain
6°
1
+22°
12°
0
air ponded up on windward slopes and draining through
passes or other topographic lows. Cold air is also trapped
beneath inversion layers in snow-bound mountains and,
in winter, may trigger violent downslope
windstorms
.
They are common east of the Rocky Mountains. Cold
outflows are also widespread outside these type areas and
generally relate to synoptic pressure systems. They are a
major source of polar air outbreaks south of the European
Alps and include the
mistral
of the lower Rhône valley.
Cold outflows or
katabats
form one element of diurnal
mountain circulation winds
(
Figure 24.6
).
Daytime
heating of confined valley air, especially on sunlit slopes,
induces convective
anabatic
upflow and inflow, coupling
valley and surrounding lowlands with corresponding
upper outflow. Evening cooling commences on upper
slopes and reverses the circulation with surface katabatic
outflow. This is a widespread small-scale phenomenon but
is also, in effect, the system developed over 2 M km
2
in
the Tibetan plateau.
Mountain topoclimate and microclimate
Mountain winds connect the macroclimate with the
immediate slope boundary layers where, in some cases, it
also has its origins. The latter may be divided into a
topoclimate
zone up to 250 m thick, determined chiefly
by slope geometry, and, at its base, a
microclimate
zone
up to 15 m thick, modified by vegetation and slope
material properties. Topoclimate embraces the effect of
rugged topography in stimulating a mosaic of radiation,
temperature, moisture, cloud and wind variations across
