Geoscience Reference
In-Depth Information
Table 5.1.
Thermal conductivity K of selected natural materials
Material
Remarks
Thermal Conductivity (Wm
−1
k
−1
)
Sandy Soil, 40% pore space
Dry
0.03
Saturated
2.20
Peat Soil, 80% pore space
Dry
0.06
Saturated
0.50
Snow
Fresh
0.06
Old
0.42
Ice*
0°C, pure
2.24
Water*
4°C, still
0.57
*properties depend on temperature and salinity
Source
: Compiled by Oke (
1987
).
5.3
The Downward Solar Radiation Flux
5.3.1
The Clear-Sky Flux
The total downward flux of solar radiation R
SS
represents a combination of direct
beam and diffuse beam components which together are often termed global radi-
ation. Diffuse radiation is largely isotropic (i.e., the flux is roughly the same no
matter what direction it is coming from), although the intensity is higher under the
portion of the celestial dome nearest the sun. The fundamental control on the global
radiation flux reaching the surface is the TOA (or extraterrestrial) flux. The TOA
flux is zonally symmetric. At the North Pole, the TOA flux is zero from the autum-
nal to spring equinoxes. By contrast, although the solar zenith angle (the angle
between zenith and the sun) at the Pole at local noon on the summer solstice is still
a large 66.5°, the attendant twenty-four-hour daylight yields a daily mean TOA flux
of 522 W m
−2
, compared to a value of only 383 W m
−2
at the equator. Because of
the combined effects of day length and solar zenith angle, the monthly mean TOA
flux for northern high latitudes actually increases with latitude during May through
August, decreasing with latitude in other months.
Because there is an intervening atmosphere, the solar flux at the surface is smaller
than the TOA flux. In the absence of cloud cover, the solar radiation received at
the surface depends on latitudinal variations in the solar zenith angle and elevation
(which determine atmospheric path length), and associated path-length dependen-
cies of non-cloud atmospheric scattering and absorption. Solar radiation is attenu-
ated primarily through absorption by water vapor in several bands between 0.9 and
2.1 µm, absorption by ozone in three bands (0.20-0.31, 0.31-0.35 and 0.45-0.85
µm) and absorption and scattering by aerosols. Ozone absorption is nearly com-
plete at wavelengths shorter that 0.285 µm (in the ultraviolet region). As outlined
in
Chapter 4
, most ozone absorption occurs in the stratosphere. The surface albedo
also has a minor effect on the clear-sky solar radiation reaching the surface by
