Geoscience Reference
In-Depth Information
3.2.2. Convection
Convection is the transfer of energy, gases, and par-
ticles by the mass movement of air, predominantly
in the vertical direction. It differs from conduction
in that during conduction, energy is transferred from
one molecule to another, whereas during convection,
energy is transferred as the molecules themselves move.
Twoimportant types of convection are forced and
free.
Forced convection is an upward or downward ver-
tical movement of air caused by mechanical means.
Forced convection occurs, for example, when (1) hori-
zontal near-surface winds converge (diverge), forcing
air to rise (sink); (2) horizontal winds encounter a
topographic barrier, forcing air to rise; or (3) winds
blow over objects protruding from the ground, creating
swirling motions of air, or eddies ,which mix air ver-
tically and horizontally. Objects of different size cre-
ate eddies of different size. Turbulence is the effect
of groups of eddies of different size. Turbulence from
wind-generated eddies is mechanical turbulence .
Free convection ( thermal turbulence )isapre-
dominantly vertical motion produced by buoyancy,
which occurs when the sun heats different areas of
the ground differentially. Differential heating occurs
because clouds or hills block the sun in some areas but
not in others, or different surfaces lie at different angles
relative to the sun. Over a warm, sunlit surface, con-
duction transfers energy from the ground to molecules
of air adjacent to the ground. The warmed air above
the ground rises buoyantly, producing a thermal .Cool
air from nearby is drawn down to replace the rising air.
Near the surface, the cool air heats by conduction and
then rises, feeding the thermal. Free convection occurs
most readily over land when the sky is cloud free and
the winds are light.
Table 3.1. Thermal conductivities of four media
Thermal conductivity (
)
at 298.15 K (J m −1 s −1 K −1 )
Substance
Dry air at constant pressure
0.0256
Liquid water
0.6
Clay
0.920
Dry sand
0.298
substances. It shows that liquid water, clay, and dry
sand are more conductive than is dry air. Thus, energy
passes through air more slowly than it passes through
other materials of the same thickness, given the same
temperature gradient. Clay is more conductive and dry
sand is less conductive than is liquid water.
The flux of energy due to conduction (W m 2 )
can be approximated with the conductive heat flux
equation ,
H c =−
T
(3.2)
z
where
T (K) is the change in temperature over a
distance
z (m). At the ground, molecules of soil
and water transfer energy by conduction to overly-
ing molecules of air. Because the temperature gradient
(
z ) between the surface and a thin (e.g., 1-mm)
layer of air just above the surface is large, the conductive
heat flux at the ground is also large. Above the ground,
temperature gradients are smaller and conductive heat
fluxes through the air are smaller than they are at the
ground.
T
/
Example 3.3
Compare the conductive heat flux through a thin
(1-mm) layer of air touching the surface if T
3.2.3. Advection
Advection is the horizontal movement of energy, gases,
and particles by the wind. Like convection, advection
results in the mass movement of molecules.
=
298 K and
12 K with that through the
background troposphere, where T
T
=−
=
273 K and
T
/
z
=−
6.5 K km −1 .
Solution
For air,
3.2.4. Radiation
Radiation ,first defined in Section 2.2, is the transfer
of energy by electromagnetic waves or photons, which
do not require a medium, such as air, for their trans-
mission. Thus, radiative energy transfer can occur even
when no atmosphere exists, such as above the moon's
surface. Conduction cannot occur above the moon's
0.0256 J m −1 s −1 K −1 ;thus,the con-
ductive heat flux at the surface is H c =
=
307Wm −2 .
In the background troposphere, H c =
10 −4
Wm −2 ,which is much smaller than is the value at
the surface. Thus, heat conduction through the
air is important only adjacent to the ground.
1.5
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