Geoscience Reference
In-Depth Information
Anti-valley
wind
div.
div.
Ridge
wind
Ridge level
Valley wind
Plain
Distal
end
Valley
Proximal
end
(A)
(B)
Figure 6.10
Valley winds in an ideal V-shaped valley. A: Section across the valley. The valley wind and
anti-valley wind are directed at right angles to the plane of the paper. The arrows show the slope and ridge
wind in the plane of the paper, the latter diverging (div.) into the anti-valley wind system. B: Section running
along the center of the valley and out on to the adjacent plain, illustrating the valley wind (below) and the
anti-valley wind (above).
Source: After Buettner and Thyer (1965).
before sunrise at the time of maximum diurnal
cooling. As with the valley wind, an upper return
current, in this case up-valley, also overlies the
mountain wind.
Katabatic drainage is usually cited as the cause
of frost pockets in hilly and mountainous areas. It
is argued that greater radiational cooling on the
slopes, especially if they are snow-covered, leads
to a gravity flow of cold, dense air into the valley
bottoms. Observations in California and else-
where, however, suggest that the valley air remains
colder than the slope air from the onset of
nocturnal cooling, so that the air moving down-
slope slides over the denser air in the valley
bottom. Moderate drainage winds will also act to
raise the valley temperatures through turbulent
mixing. Cold air pockets in valley bottoms and
hollows probably result from the cessation of
turbulent heat transfer to the surface in sheltered
locations rather than by cold air drainage, which
is often not present.
2
Land and sea breezes
Another thermally induced wind regime is the
land and sea breezes (see
Figure 6.11
). The vertical
expansion of the air column that occurs during
daytime heating over the more rapidly heated land
(see Chapter 3B.5) tilts the isobaric surfaces
downward at the coast, causing onshore winds at
the surface and a compensating offshore move-
ment aloft. Typical land-sea pressure differences
are of the order of 2mb. At night, the air over the
sea is warmer and the situation is reversed,
although this reversal is also the effect of down-
slope winds blowing off the land.
Figure 6.12
shows that sea breezes can have a decisive effect
on temperature and humidity on the coast of
California. A basic offshore gradient flow is
perturbed during the day by a westerly sea breeze.
Initially, the temperature difference between the
sea and the coastal mountains of central California
sets up a shallow sea breeze, which by midday is
