Geoscience Reference
In-Depth Information
4
-10°
-12°C
0°
3
10°
2
SALR
D.A.L.R.
0°C
20°
Condensation
level
1
D.A.L.R.
T
a
12°C
Tb
18°C
0
Ta
Tb
Figure 6.15
The föhn effect when an air parcel is forced to cross a mountain range 3km high. T
a
refers
to the temperature at the windward foot of the range and T
b
to that at the leeward foot.
noticeable in the lee of the Welsh mountains, the
Pennines and the Grampians in Great Britain,
where the importance of föhn winds lies mainly in
the dispersal of cloud by the subsiding dry air. This
is an important component of so-called 'rain
shadow' effects.
In some parts of the world, winds descending
on the lee slope of a mountain range are colder
than the air they displace (despite adiabatic
warming through descent). The type example of
such 'fall-winds' is the
bora
of the northern
Adriatic, where cold northeasterly flows cross the
Dinaric Alps, although similar winds occur on the
northern Black Sea coast, in northern Scandinavia,
in Novaya Zemlya and in Japan. These winds occur
when cold continental air masses are forced across
a mountain range by the pressure gradient and,
despite adiabatic warming, displace warmer air.
They are therefore primarily a winter phenomenon.
On the eastern slope of the Rocky Mountains
in Colorado (and in similar continental locations),
winds of either bora or chinook type can occur
depending on the initial airflow characteristics.
Locally, at the foot of the mountains, such winds
may attain hurricane force, with gusts exceeding
45m s
-1
(100mph). Down-slope storms of this
type have caused millions of dollars of property
damage in Boulder, Colorado, and the immediate
vicinity. These wind storms develop when a stable
layer close to the mountain-crest level prevents air
to windward from crossing over the mountains.
Extreme amplification of a lee wave (see
Figure
6.13
) drags air from above the summit level
(4000m) down to the plains (1700m) over a short
distance, leading to high velocities. However, the
flow is not simply 'down-slope'; winds may affect
the mountain slopes but not the foot of the slope,
or vice versa, depending on the location of the
lee wave trough. High winds are caused by the
horizontal acceleration of air towards this local
pressure minimum.
