Geoscience Reference
In-Depth Information
Forest
January
April
July
October
Year
Deciduous broad-leaf
0·1
3·4
0·0
3·2
0·8
-0·8
0·5
1·1
0·3
2·2
Needle tree (conifer)
0·7
4·8
0·7
4·8
0·8
6·5
1·0
9·5
0·9
6·8
Japanese cedar
0·2
1·6
0·1
-1·1
0·4
1·5
0·2
0·5
0·2
0·8
Source: After Yoshino (1975).
Note: Positive values indicate that inside the forest is more humid.
more plants grow on the woodland floor. Similarly, pine
trees give a less dense canopy than do spruce; the dark,
unvegetated floor of plantations of Sitka spruce contrast
with the much lighter conditions in pine woodland.
In addition, the nature of the understorey is important.
An open canopy allows the development of one or more
layers of understorey plants, and these, too, intercept both
incoming and outgoing radiation. The extreme example
is shown by the tropical rain forest. Although radiation
inputs are high, the successive layers of trees, bushes and
shrubs intercept so much radiation that only small
amounts reach the forest floor.
interception of moisture by vegetation reduces the
amount of water available at the forest floor, so the net
effect on humidity levels is small.
As daytime temperatures are cooler than those outside,
the relative humidity of the air should also be greater even
if the forest atmosphere contained the same absolute
amount of water vapour. Experiments suggest values
about 5 per cent above those outside, though the precise
differences depend upon the type of woodland as well as
URBAN CLIMATES
Winds in woodland
Patterns of wind in woodland are similar to those in
grassland, although the zone of modified flow extends to
a much greater height. Above the canopy wind speed
normally increases (
Figure 8.5
),
but as the canopy is
approached, velocity falls rapidly. Lowest wind speeds are
often found within the leafy canopy, and where the under-
growth is also dense, velocities may remain low. In most
cases, however, the main trunk zone is more open, so there
is less interference with air flow and wind speeds increase
again. Near the ground, friction and the effect of low-
growing plants cause velocity to fall to lowest values.
Complex patterns of flow often develop in the forest, with
local funnelling and deflection of the wind. We can often
see the results of these flow patterns in the distribution of
dead leaves on the woodland floor. Sheltered areas trap
deep layers of leaves, which, by decay, will add nutrients
to the soil, while more exposed zones are swept clear by
the wind.
The climate modifications found in woodland are small
compared with what happens when cities are built. Instead
of a mixture of soil and vegetation, Earth is covered with
a mosaic of concrete, glass, brick, bitumen and stone
surfaces reaching to heights of several hundred metres.
Amongst this, grass and water surfaces and trees may be
scattered to variegate the 'concrete jungle'. The building
materials have vastly different physical properties from soil
and plants. For example, the warmth of concrete and
brick on a summer's evening is due to their high heat
capacity. This means that the large quantities of heat
added to the material while the sun is shining are slowly
released during the night, adding warmth to the urban
atmosphere. In this way city temperatures are kept
relatively high. We notice the effect most in the evening
when we travel from the cool of the countryside to the
heat of the city (
Figure 8.7
).
It is an effect called the
urban
heat island
. Early blooming of flowers and decreased
snowfall and frost are both indicators of this effect.
Moisture in woodland
In general, vapour pressure is slightly higher in a forest or
in woodland than outside it. This is mainly due to the large
area covered by the leaves in a forest, which transpire
moisture into the atmosphere, from where it is not easily
dispersed because of the lighter winds. In some forests
there may be few actively transpiring leaves near the
ground so modifying the pattern. On the other hand, the
Urban heat island
We can illustrate the different responses of the city and
rural areas by comparing their heat budgets as shown in
Figure 8.8
.
It is the change of the heat budget by the urban
surface which helps to produce the distinctive urban
climate, so let us look in more detail at the way changes
are produced. By day, both rural and urban surfaces
