Geoscience Reference
In-Depth Information
effect of the atmosphere on energy transfer. Heat
energy can be transferred by three mechanisms:
but it is important in the ground. The thermal
conductivity increases as the water content of
a given soil increases and is greater in frozen
than in unfrozen soil.
1
Radiation
: Electromagnetic waves transfer
energy (both heat and light) between two
bodies, without the necessary aid of an inter-
vening material medium, at a speed of 300
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
(mechan-
ical turbulence) occurs when eddies form in
airflow over uneven surfaces. In the presence
of surface heating,
free
(thermal)
convection
develops.
×
10
6
m s
-1
(i.e., the speed of light). This is
so with solar energy through space, whereas
the earth's atmosphere allows the passage of
radiation only at certain wavelengths and
restricts that at others.
Radiation entering the atmosphere may be
absorbed in certain wavelengths by atmos-
pheric gases but, as shown in
Figure 3.1
, most
shortwave radiation is transmitted without
absorption. Scattering 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 (
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
(= 1004J 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 no
temperature change. Whenever water is converted
into water vapor by evaporation (or boiling), heat
is required. This is referred to as the latent heat of
vaporization (
L
). At 0°C,
L
is 2.50 × 10
6
J kg
-1
of
water. More generally,
L
(10
6
J
kg
-1
) = (2.5 - 0.00235
T
)
),
Rayleigh scattering
occurs in all
directions (i.e., it is
isotropic
) and is propor-
tional to (1/
λ
λ
4
). As a result, the scattering of
blue light (
λ
~ 0.4
μ
m) is an order of magnitude
(i.e.,
×
10) greater than that of red light (
λ
~
0.7
m), thus creating the daytime blue sky.
However, when water droplets or aerosol
particles, with similar sizes (0.1-0.5
μ
m 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
multiple 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 indistinguishable.
μ
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
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,
