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In-Depth Information
considerably more direct solar radiation under
clear skies than locations near sea level due to
the concentration of water vapor in the lower
troposphere (
Figure 3.19
). On the average in mid
latitudes the intensity of incident solar radiation
increases by 5-15 percent for each 1000m increase
in elevation in the lower troposphere. The
difference between sites at 200 and 3000m in the
Alps, for instance, can amount to 70W 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 available data.
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 slopes. The radiation
intensity on a sloping surface (
Is
) is
Is
=
Io
cos
i
40
30
Manaus
20
Valentia
10
0
-10
Toronto
-20
-30
J
FM
A
M
J
J
A
S
O
N
D
Figure 3.17
Mean annual temperature regimes
in various climates: Manaus, Braziil (equatorial),
Valentia, Ireland (temperate maritime) and Toronto,
Canada (temperate continental).
boreal summer to be warmer but its winters to be
colder on the average than the austral equivalents
of the Southern Hemisphere (summer, 22.4
°
C
versus 17.1
C). Heat
storage in the oceans causes them to be warmer in
winter and cooler in summer than land in the
same latitude, although ocean currents give rise
to some local departures from this rule. The
distribution of temperature anomalies for the
latitude in January and July (
Figure 3.18
)
illustrates the significance of continentality and
the influence of the warm currents in the North
Atlantic and the North Pacific in winter.
Sea-surface temperatures can now be esti-
mated by the use of infrared satellite imagery (see
C, this chapter).
Plate 7.4
shows a false-color
satellite thermal image of the western North
Atlantic with the relatively warm, meandering
Gulf Stream. Maps of sea-surface temperatures
are now routinely constructed from such images.
°
C; winter, 8.1
°
C versus 9.7
°
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 the 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.
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 in which 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
7 Variation of free air temperature
with height
Chapter 2C described the gross characteristics of
the vertical temperature profile in the atmosphere.
