Geoscience Reference
In-Depth Information
8
local climates
surface remains cool and the snow fails to melt. Where the
snow is dirty it absorbs more radiation, heats up and is
more likely to melt. Vegetation, too, may protect the snow
from the heat of the sun, while, even late in spring, snow
may be preserved in shaded hollows or on hill slopes
Let us look at the causes of these differences in more
detail. We will start by considering the simplest possible
conditions of a horizontal, bare soil surface.
Living as we do in the lowest few metres of the atmos-
phere, we should have a special interest in the climate of
this zone. Unfortunately it turns out to be a very diverse
and complicated zone. Climatic differences equivalent
to a change in latitude of several degrees can occur in a
matter of a few metres. These are examples of micro-
climatic conditions at the surface.
When the sun is shining, for instance, the ground
may become too hot to walk on barefoot, as on a dry,
sunny beach in midsummer. At the ground surface the
temperature may exceed 65
Microclimate over bare soil
C, while at head height it may
Many different properties of the soil influence conditions
in the thin layer of atmosphere above it. Soils vary in
colour. Darker soils, such as those rich in organic matter,
absorb radiant energy more efficiently than do light-
coloured soils. Moisture in the soil is also important. Wet
soils are normally dark, but water has a large heat capacity;
that is to say, it requires a great deal of energy to raise its
temperature. Being wet, it is likely to experience evapora-
tion that requires energy. A moist soil, therefore, warms
up more slowly than a dry one (
Figure 8.1
).
A complication with heat transfer into soils is that air
is a poor conductor of heat. If there is a large amount of
air between the soil particles, heat transfer into the soil is
slow. This means that on a hot, sunny day the heat is
trapped in the upper layers, so the surface layers warm up
more rapidly. Because of this, dry sandy soils can get very
hot when the sun shines. Water conducts heat more easily
than air, so soils which contain some moisture are able to
transmit warmth away from the surface more easily than
dry soils. However, if the soil contains a lot of water, the
be only 30
C, and in the shade, where most temperature
observations are made, it may be as low as 20
C. Similar
variations can be found in wind speed, humidity, and even
precipitation catch at the ground. So, we may ask, what
is it about this layer near the ground which produces
such major gradients - gradients that are not repeated
anywhere in the free atmosphere?
The main reason for such variability is that we are
dealing with the main exchange or activity zone between
the ground surface and the atmosphere. Energy is reaching
this zone both from the sun and, to a much lesser extent,
from the atmosphere. It is absorbed and then returned to
the atmosphere in a different form, or is stored in the
ground as heat. This absorption process is very sensitive
to the nature of the surface. Conditions such as surface
colour, wetness, vegetation, topography and aspect all
affect the interaction between the ground and the atmos-
phere. We can sometimes see the effects clearly in snowy
weather. Clean snow reflects solar radiation and so the
