Geoscience Reference
In-Depth Information
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 reach hurricane force, with
gusts exceeding 45 m s
-1
(100 mph). Down-slope storms
of this type have caused millions of dollars of property
damage in Boulder, Colorado, and the immediate vicin-
ity. These windstorms develop when a stable layer close
to the mountain-crest level prevents air to windward
from crossing over the mountains. Extreme amplifica-
tion of a lee wave (see Figure 6.13) drags air from above
the summit level (4000 m) down to the plains (1700 m)
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.
DISCUSSION TOPICS
Compare the wind direction and speed reported at
a station near you with the geostrophic wind velocity
determined from the MSL pressure map for the
same time (data sources are listed in Appendix 4).
Why would there be no 'weather' if the winds were
strictly geostrophic?
What are the causes of mass divergence (conver-
gence) and what roles do they play in weather
processes?
In what situations do local wind conditions differ
markedly from those expected for a given large-scale
pressure gradient?
FURTHER READING
Books
Barry, R. G. (1992)
Mountain Weather and Climate
,
Routledge, London, 402pp. [Chapter on circulation
systems related to orographic effects.]
Oke, T. R. (1978)
Boundary Layer Climates
, Methuen,
London, 372pp. [Prime text on surface climate processes
in natural and human-modified environments.]
Scorer, R. S. (1958)
Natural Aerodynamics
, Pergamon
Press, Oxford, 312pp.
Simpson, J. E. (1994)
Sea Breeze and Local Wind
,
Cambridge University Press, Cambridge, 234pp. [A
well-illustrated descriptive account of the sea breeze
and its effects; see chapter on local orographic winds.]
Troen, I. and Petersen, E. L. (1989)
European Wind
Atlas
, Commission of the Economic Community, Risø
National Laboratory, Roskilde, Denmark, 656pp.
SUMMARY
Air motion is described by its horizontal and vertical
components; the latter are much smaller than the
horizontal velocities. Horizontal motions compensate
for vertical imbalances between gravitational acceler-
ation and the vertical pressure gradient.
The horizontal pressure gradient, the earth's
rotational effect (Coriolis force), and the curvature
of the isobars (centripetal acceleration) determine
horizontal wind velocity. All three factors are accounted
for in the gradient wind equation, but this can be
approximated in large-scale flow by the geostrophic
wind relationship. Below 1500 m, the wind speed and
direction are affected by surface friction.
Air ascends (descends) in association with surface
convergence (divergence) of air. Air motion is also
subject to relative vertical vorticity as a result of curva-
ture of the streamlines and/or lateral shear; this,
together with the earth's rotational effect, makes up
the absolute vertical vorticity.
Local winds occur as a result of diurnally varying
thermal differences setting up local pressure gradients
(mountain-valley winds and land-sea breezes) or due
to the effect of a topographic barrier on airflow crossing
it (examples are the leeside föhn and bora winds).
Articles
Banta, R.M. (1995) Sea breezes: shallow and deep on the
California coast.
Mon. Wea. Rev.
, 123(12), 3614-22.
Beran, W. D. (1967) Large amplitude lee waves and
chinook winds.
J. Appl. Met
. 6, 865-77.
Brinkmann, W. A. R. (1971) What is a foehn?
Weather
26,
230-9.
Brinkmann, W. A. R. (1974) Strong downslope winds at
Boulder, Colorado.
Monthly Weather Review
102,
592-602.
Buettner, K. J. and Thyer, N. (1965) Valley winds in the
Mount Rainer area.
Archiv. Met. Geophys. Biokl
. B 14,
125-47.
