Geoscience Reference
In-Depth Information
June than in December. Finally, the NH summer is
about a week longer than is the NH winter because
the Earth takes longer to pass between equinoxes from
March to September than from September to March
(Figure 12.1).
From the sun's point of view, the Earth appears as a
circular disk (rather than a sphere) absorbing the sun's
radiation. Thus, the quantity of incoming solar radiation
received by the Earth is the solar constant multiplied by
the cross-sectional area of the Earth,
Sun
Axis of
rotation
Tropic of
Cancer
Equator
23.5
o
23.5
o
Vernal equinox, Mar. 20
Autumnal equinox, Sept. 23
Sun
R
e
2
(m
2
), where
R
e
=
10
6
m(6,378 km) is the
Earth's radius
.
Not all incoming solar radiation is absorbed by the
Earth. Some is reflected by snow, sea ice, deserts, and
other light-colored ground surfaces, as well as clouds.
The fraction of incident energy reflected by a surface
is the
albedo
or
reflectivity
of the surface. The albedo
varies for different-colored surfaces and changes with
wavelength. Table 12.1 gives mean albedos in the visi-
ble spectrum for several surface types, showing that the
albedo of the Earth and atmosphere together (
plane-
tary albedo
)isabout 30 percent. More than two-thirds
of the Earth's surface is covered with water, which has
an albedo of 5 to 20 percent (with a typical value of
8 percent), depending largely on the angle of the sun
relative to the surface. Soils and forests also have low
albedos. Much of the Earth-atmosphere reflectivity is
due to clouds and snow, which have high albedos.
Taking into account the cross-sectional area of the
Earth and the Earth's albedo (
A
e
,fraction), the total
6.378
×
Tropic of
Capricorn
Earth
Sun
Figure 12.2.
Positions of the sun relative to the Earth
during solstices and equinoxes. Of the four times
shown, the Earth-sun distance is greatest at the
Northern Hemisphere summer solstice.
December and June corresponds to a difference of 6.9
percent in solar radiation reaching the Earth between
these months. In other words, 6.9 percent more radiation
falls on the Earth in December than in June.
Despite the excess radiation reaching the top of the
Earth's atmosphere in December, the Northern Hemi-
sphere (NH) winter still starts in December because the
Southern Hemisphere (SH) is tilted toward the sun in
December, as shown in Figure 12.1. It also shows that
the axis of rotation of the Earth is currently tilted all year
by
23.5 degrees from a line perpendicular to the plane
of the Earth's orbit around the sun. This angle is called
the
obliquity
of the Earth's axis of rotation. As a result
of the Earth's obliquity, the sun's direct rays hit their far-
thest point south, 23.5
◦
Slatitude, on December 22 (
NH
winter solstice
,
SH summer solstice
) and their farthest
point north, 23.5
◦
Nlatitude, on June 22 (
NH summer
solstice
,
SH winter solstice
)(Figures 12.1 and 12.2).
The latitude at 23.5
◦
Siscalled the
Tropic of Capricorn
and that at 23.5
◦
Nisthe
Tropic of Cancer
.Winter and
summer solstices are the shortest and longest days of the
year, respectively. On March 20 (
vernal equinox
) and
September 23 (
autumnal equinox
), the sun is directly
overthe Equator and the length of day equals that of
night.
For three reasons, temperatures in the NH are warmer
in June than in December, even though the Earth is
closer to the sun in December (Figure 12.1). First,
the sun's rays are directly over the NH in June. In
December, they are directly over the SH; thus, they
are slanted, making them diffuse and less intense over
the NH (Figure 12.2). Second, NH days are longer in
∼
Table 12.1.
Solar albedos and thermal-infrared
emissivities for several surface types
Albedo
Emissivity
Type
(fraction)
(fraction)
Earth and atmosphere
0.3
0.90-0.98
Liquid water
0.05-0.2
0.92-0.96
Fresh snow
0.7-0.9
0.82-0.995
Old snow
0.35-0.65
0.82
Thick clouds
0.3-0.9
0.25-1.0
Thin clouds
0.2-0.7
0.1-0.9
Sea ice
0.25-0.65
0.96
Soil
0.05-0.2
0.9-0.98
Grass
0.16-0.26
0.9-0.95
Desert
0.20-0.40
0.84-0.91
Forest
0.10-0.25
0.95-0.97
Concrete
0.1-0.35
0.71-0.9
Sources:
Estimates
from
Sellers
(1965);
Oke
(1978);
Liou
(1992); and Hartmann (1994).
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