Geoscience Reference
In-Depth Information
the large- and the small-scale patterns of wind
velocity. These are particularly noticeable in the
case of local winds. Under normal conditions,
wind velocities tend to be least about dawn when
there is little vertical thermal mixing and the lower
air is less affected by the velocity of the air aloft (see
Chapter 7A). Conversely, velocities of some local
winds are greatest around 13:00-14:00 hours,
when the air is most subject to terrestrial heating
and vertical motion, thereby enabling coupling
to the upper air movement. Air always moves
more freely away from the surface, because it is not
subject to the retarding effects of friction and
obstruction.
Table 6.2
gives a summary classification of local
winds, each of which is now discussed in detail.
volume ratio of lowland/valley air is typically
about 2 or 3:1 and this difference in heating sets
up a density and pressure differential, which
causes air to flow from the lowland up the axis
of the valley. This valley wind (
Figure 6.10
) is
generally light and requires a weak regional
pressure gradient in order to develop. This flow
along the main valley develops more or less
simultaneously with
anabatic
(upslope) winds,
which result from greater heating of the valley
sides compared with the valley floor. These slope
winds rise above the ridge tops and feed an upper
return current along the line of the valley to
compensate for the valley wind. This feature may
be obscured, however, by the regional airflow.
Speeds reach a maximum around 14:00 hours.
At night, there is a reverse process as denser
cold air at higher elevations drains into depres-
sions and valleys; this is known as a
katabatic
wind.
If the air drains downslope into an open valley, a
'mountain wind' develops more or less simulta-
neously along the axis of the valley. This flows
towards the plain, where it replaces warmer, less
dense air. The maximum velocity occurs just
1
Mountain and valley winds
Terrain features give rise to their own special
meteorological conditions. On warm, sunny days,
the heated air in a valley is laterally constricted
compared with that over an equivalent area of
lowland, and so tends to expand vertically. The
Table 6.2
Classification of local winds
Name
Characteristics
Forcing
Anabatic
Daytime upslope warm flow
Horizontal density gradient towards the slope
Katabatic
Night-time downslope cold flow
Gravity and horizontal density gradient away
from the slope
Mountain wind
Night-time cold down-valley flow
Mountains-to-plains density gradient
Valley wind
Daytime warm up-valley flow
Plains-to-mountains density gradient
Anti-mountain wind
Above the mountain wind in the opposite
Compensation current
direction
Anti-valley wind
Above the valley wind in the opposite
Compensation current
direction
Sea breeze
Day-time flow from the seas to the land
Density gradient from cool sea to heated land
Land breeze
Night-time flow from land to sea
Density gradient from cool land to warmer sea
Foehn (Chinook)
Down lee slope with increasing T and
Blocked flow on windward side; or flow
lower RH
crossing mountains with cloud/precipitation on
windward slope
Bora
Down lee slope with air colder than that
Blocked flow of cold air upwind
it replaces
Barrier wind
Low-level flow parallel to the mountains,
Blocking reduces the flow speed normal to the
directed poleward
barrier reducing the Coriolis force
