Environmental Engineering Reference
In-Depth Information
Figure 8.14
Temperature distribution in the Gstettneralm
sinkhole near Lunz, Austria, 21 January 1930.
Source: After Schmidt (1930).
The downslope flows eventually combine into a downvalley flow, known as a mountain
wind, as it emerges on to the lowlands.
By day this cold air drainage does not occur, except where snow and ice surfaces
maintain cooling. Instead it is replaced by upslope winds. These are produced by heating
on the slope, which causes the warm air to rise upslope as an anabatic wind. Cool air
from the valley floor flows in to replace this warm air and a valley breeze is generated
(Figure 8.15). These valley breeze systems could not last long if the continuity of the
flow was not maintained. This is usually found as a counter-wind at higher levels. If the
pressure gradient wind is strong it increases local mixing so that major temperature
differences are prevented. No cold air is available to sink downslope or warm air to rise
upslope, so the formation of the breeze is stopped. Like so many microclimatological
phenomena, valley and mountain breezes require clear skies and light winds for their
operation.
SEA BREEZES
The driving force of the valley and mountain breezes is a temperature gradient.
Temperature contrasts develop between slopes and valley floors, between uplands and
lowlands, so that the nature and strength of the wind depends upon the precise form of
the gradient. This thermal control of winds occurs at all scales, from the general
circulation of the atmosphere (Chapter 6) down to the smallest eddy of heat rising from
the ground. We have already referred to one wind system which forms at the local scale,
but an even more widespread thermally driven wind at this scale is the sea breeze.
Sea breezes are formed by the different responses to heating of water and land. If we
have a bright, sunny morning with little wind, the ground surface warms rapidly as it
absorbs short-wave radiation. Most of this heat is retained at the surface, although some
will be transferred through the soil. As a result, the temperature of the ground surface
increases and some of the heat warms the air above. When the sun sets, the surface starts
