Environmental Engineering Reference
In-Depth Information
Plate 25.4
Fast-moving lee wave clouds above mountains
fringing Frobisher Bay, Baffin Island, in the east Canadian
Arctic, seen from 11 km altitude.
Photo: Ken Addison.
trapped beneath inversion layers in snow-bound mountains and, in winter, may trigger
violent downslope
wind
-
storms
. 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 25.11). 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.
