Geoscience Reference
In-Depth Information
occurs if the direction of a photon of radiation is
changed by interaction with atmospheric gases and
aerosols. Two types of scattering are distinguished.
For gas molecules smaller than the radiation wave-
length (λ),
Rayleigh scattering
occurs in all directions
(i.e. it is
isotropic
) and is proportional to (1/λ
no temperature change. Whenever water is converted
into water vapour by evaporation (or boiling), heat is
required. This is referred to as the latent heat of vapor-
ization (
L
). At 0°C,
L
is 2.50
10
6
J kg
-1
of water. More
generally,
4
). As a
result, the scattering of blue light (λ ~ 0.4 µm) is an
order of magnitude (i.e.
10) greater than that of red
light (λ ~ 0.7 µm), thus creating the daytime blue sky.
However, when water droplets or aerosol particles,
with similar sizes (0.1-0.5 µm radius) to the radiation
wavelength are present, most of the light is scattered
forward. This
Mie scattering
gives the greyish
appearance of polluted atmospheres.
Within a cloud, or between low clouds and
a snow-covered surface, radiation undergoes mul-
tiple scattering. In the latter case, the 'white-out'
conditions typical of polar regions in summer (and
mid-latitude snowstorms) are experienced, when
surface features and the horizon become indistin-
guishable.
2
Conduction
: By this mechanism, heat passes through
a substance from a warmer to a colder part through
the transfer of adjacent molecular vibrations. Air
is a poor conductor so this type of heat transfer is
negligible in the atmosphere, but it is important in
the ground. The thermal conductivity increases
as the water content of a given soil increases and
is greater in a frozen soil than in an unfrozen one.
3
Convection
: This occurs in fluids (including gases)
that are able to circulate internally and distribute
heated parts of the mass. It is the chief means of
atmospheric heat transfer due to the low viscosity
of air and its almost continual motion.
Forced
convection
(mechanical turbulence) occurs when
eddies form in airflow over uneven surfaces. In
the presence of surface heating,
free
(thermal)
convection
develops.
L
(10
6
J
kg
-1
) = (2.5 - 0.00235
T
)
where
T
is in °C. When water condenses in the
atmosphere (see Chapter 4D), the same amount of latent
heat is given off as is used for evaporation
at the same
temperature
. Similarly, for melting ice at 0°C, the latent
heat of fusion is required, which is 0.335
10
6
J kg
-1
.
If ice evaporates without melting, the latent heat of this
sublimation process is 2.83
10
6
J kg
-1
at 0°C (i.e. the
sum of the latent heats of melting and vaporization). In
all of these phase changes of water there is an energy
transfer. We discuss other aspects of these processes in
Chapter 4.
2 Effect of the atmosphere
Solar radiation is virtually all in the short-wavelength
range, less than 4 µm (see Figure 3.1). About 18 per cent
of the incoming energy is absorbed directly by ozone
and water vapour. Ozone absorption is concentrated
in three solar spectral bands (0.20-0.31, 0.31-0.35
and 0.45-0.85 µm), while water vapour absorbs to a
lesser degree in several bands between 0.9 and 2.1 µm
(see Figure 3.1). Solar wavelengths shorter than 0.285
µm scarcely penetrate below 20 km altitude, whereas
those >0.295 µm reach the surface. Thus the 3 mm
(equivalent) column of stratospheric ozone attenuates
ultraviolet radiation almost entirely, except for a partial
window around 0.20 µm, where radiation reaches the
lower stratosphere. About 30 per cent of incoming solar
radiation is immediately reflected back into space from
the atmosphere, clouds and the earth's surface, leaving
approximately 70 per cent to heat the earth and its
atmosphere. The surface absorbs almost half of the
incoming energy available at the top of the atmosphere
and re-radiates it outward as long (infra-red) waves
of greater than 3 µm (see Figure 3.1). Much of this re-
radiated long-wave energy is then absorbed by the water
vapour, carbon dioxide and ozone in the atmosphere,
the rest escaping through atmospheric
windows
back
into outer space, principally between 8 and 13 µm (see
Figure 3.1). This retention of energy by the atmosphere
is vital to most life forms, since otherwise the average
Convection transfers energy in two forms. The first
is the
sensible heat
content of the air (called enthalpy by
physicists), which is transferred directly by the rising
and mixing of warmed air. It is defined as
c
p
T
, where
T
is the temperature and
c
p
(= 1004 J kg
-1
K
-1
) is the
specific heat at constant pressure (the heat absorbed
by unit mass for unit temperature increase). Sensible
heat is also transferred by conduction. The second
form of energy transfer by convection is indirect,
involving
latent heat
. Here, there is a phase change but
