Geoscience Reference
In-Depth Information
˚c˚c
40
40
30
30
Manaus
20
20
Valentia
10
10
0
0
Toronto
-10
-10
Figure 3.17
Mean annual temperature
regimes in various climates. Manaus, Brazil
(equatorial), Valentia, Ireland (temperate
maritime) and Toronto, Canada (temperate
continental).
-20
-20
-30
-30
J
F
M
A
M
J
J
A
S
O
N
D
chapter). Plate B shows a false-colour satellite thermal
image of the western North Atlantic showing the
relatively warm, meandering Gulf Stream. Maps of sea-
surface temperatures are now routinely constructed
from such images.
Figure 3.20 illustrates the effect of aspect and slope
angle on theoretical maximum solar radiation receipts
at two locations in the northern hemisphere. The general
effect of latitude on insolation amounts is clearly shown,
but it is also apparent that increasing latitude causes
a relatively greater radiation loss for north-facing
slopes, as distinct from south-facing ones. The radiation
intensity on a sloping surface (
I
s
) is
6 Effect of elevation and aspect
When we come down to the local scale, differences
in the elevation of the land and its
aspect
(that is,
the direction that the surface faces) strongly control the
amount of solar radiation received.
High elevations that have a much smaller mass of air
above them (see Figure 2.13) receive considerably more
direct solar radiation under clear skies than do locations
near sea-level due to the concentration of water vapour
in the lower troposphere (Figure 3.19). On average in
middle latitudes the intensity of incident solar radiation
increases by 5 to 15 per cent for each 1000 m increase
in elevation in the lower troposphere. The difference
between sites at 200 and 3000 m in the Alps, for
instance, can amount to 70 W m
-2
on cloudless summer
days. However, there is also a correspondingly greater
net loss of terrestrial radiation at higher elevations
because the low density of the overlying air results in
a smaller fraction of the outgoing radiation being
absorbed. The overall effect is invariably complicated
by the greater cloudiness associated with most mountain
ranges, and it is therefore impossible to generalize from
the limited data available.
I
s
=
I
o
cos
i
where
i
= the angle between the solar beam and a beam
normal to the sloping surface. Relief may also affect the
quantity of insolation and the duration of direct sunlight
when a mountain barrier screens the sun from valley
floors and sides at certain times of day. In many Alpine
valleys, settlement and cultivation are noticeably
concentrated on southward-facing slopes (the adret or
sunny side), whereas northward slopes (ubac or shaded
side) remain forested.
7 Variation of free-air temperature
with height
Chapter 2C described the gross characteristics of the
vertical temperature profile in the atmosphere. We will
now examine the vertical temperature gradient in the
lower troposphere.
Vertical temperature gradients are determined in part
by energy transfers and in part by vertical motion of the
