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2.5
Energy curve for black body at 6000 K
Solar irradiation curve outside atmosphere
Solar irradiation curve at sea level
O
3
2.0
O
2
H
2
O
1.5
O
2
H
2
O
H
2
O
H
2
O
1.0
H
2
O
UV
O
3
0.5
H
2
O
H
2
O
CO
2
Visible
Infrared
0.0
0
1000
2000
3000
Wavelength (nm)
Figure 2.2
The solar spectrum, showing the main absorption bands associated with gases in
the atmosphere. The wavelengths used in photosynthesis are between 400 nm and 700 nm.
From (USAF,
1960
); courtesy MacMillan.
C
1
I
l
ð
l
Þ¼
ð
2
:
1
Þ
l
5
ð
e
C
2
=
l
T
K
1
Þ
where T
K
is the Sun's Kelvin temperature of 6000K, l is wavelength, and the
constants are C
1
¼
10
2
m K. On its passage
through the atmosphere, this distribution is modified by gaseous absorption, notably
by O
2
,O
3
,H
2
O and CO
2
whose absorption bands are clearly apparent in
Fig. 2.2
as
large 'bites' out of the spectrum which reaches the sea's surface. The spectral balance
is also changed a little through scattering and absorption by the clouds.
On arrival at the sea surface, photons can be reflected or will pass into the sea to be
either absorbed (a) or scattered back (b) into the atmosphere (e.g.
Fig. 2.1
). For overcast
conditions in the atmosphere, only
10
16
Wm
2
and C
2
¼
3.74
1.44
6% of the energy is returned by reflection.
For clear skies a similarly low fraction of energy is returned except at times of low
solar elevations (
∼
20
) when surface reflection increases markedly.
Allowing for subsurface scattering, the total fraction of returned energy, termed
the albedo A, is typically
<
0.08 and rarely exceeds 0.2 so that the great majority of
the incident solar energy, Q
s
(1
<
A), goes into the ocean. Almost all of it is converted
to heat, though an important component in the visible band between 400 and 700 nm,
termed Photosynthetically Available Radiation (PAR), is available for photosynthesis.
At wavelength l the downward vertical flux of energy E
d
(l) decays with depth due
to absorption and scattering, according to:
dE
d
dz
¼
K
d
E
d
ð
2
:
2
Þ
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