Environmental Engineering Reference
In-Depth Information
continues to cool through longwave radiation emission and may eventually reach very
low temperatures. This microclimatological effect can be very pronounced on clear, calm
nights which allow radiation cooling to continue at a high rate.
One result of this process is the formation of frost hollows. Farmers always take care
that frost-sensitive crops are not grown where cold air is likely to accumulate and give
ground or even air frosts. It is for this reason that, in frost-susceptible areas, fruit orchards
are cultivated on valley slopes, allowing the cold air to drain through the trees without
accumulating. A classic example of a frost hollow was found in the Austrian Alps. A
limestone sinkhole with a steep back wall facing north-east allowed cold air to become
stagnant. Figure 8.14 shows temperatures at different levels on one particular night.
Towards west-south-west the sinkhole is intersected by a col which allows the stagnant
cold air to remain in the lowest 50 m of the hollow. Temperatures as low as −51° C have
been recorded when the ground was snow-covered. Even coastal Antarctica is usually
much warmer than that! The frequent occurrence of frost has affected vegetation, so
Plate 8.2 The effect of trees on night-time temperatures. The
area beneath the tree is free of frost.
Photo: Peter Smithson.
that few trees grow near the base, to give an inverted vegetation gradient.
LOCAL WINDS
VALLEY-BREEZE SYSTEMS
If the katabatic winds, described above, are not prevented from flowing they begin to
form an organized system of cold air drainage downslope and down-valley. Speeds are
low, perhaps 1 m per second or less, and the movement tends to pulsate with intermittent
surges - like that which can be seen in water running down a sloping road surface.
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