Lecture 2:
Radiation/Heat in
the atmosphere
TEMPERATURE…
is a measure of the internal heat
energy of a substance. The molecules
that make up all matter are in constant
motion. By “internal heat energy”, we
really mean this random molecular
motion. Molecular motion is therefore
the reason any substance has a
temperature. The more the molecules
that make up a substance move, the
the higher its temperature.
Temperature Scales
HEAT TRANSFER…
can be accomplished through four means:
(1) Conduction: fast-moving molecules of substance 1
collide with neighboring molecules of substance 2,
which are moving more slowly. This forces the
molecules of substance 2 to speed up. Substance
2 becomes hotter as a result of its physical contact
with substance 1. This form of heat transfer often
occurs between the atmosphere and the earth’s
surface and is also known as sensible heat flux.
HEAT TRANSFER…
can be accomplished through four means:
(2) Phase changes: A liquid evaporates into an
overlying gas, a process which requires energy and
therefore removes heat from the liquid. This also
occurs often between the atmosphere and earth’s
surface and is known as latent heat flux. The
dryness of the desert surface means it can’t cool
through latent heat flux and therefore must cool
almost exclusively through sensible heat flux. The
inefficient ventilation of the desert surface is the
reason the deserts are so hot.
HEAT TRANSFER…
can be accomplished through four means:
(3) Convection: Typically occurs when a liquid or gas
is heated from below. The heated portion becomes
lighter and rises, being replaced by heavier, and
cooler liquid or gas. This redistribution of heat
occurs in both the atmosphere and the ocean.
HEAT TRANSFER…
can be accomplished through four means:
(4) Radiation: The radiation emanating from
substance 1 encounters substance 2, which
absorbs the radiation. The absorbed radiation
heats substance 2.
RADIATION (Electromagnetic waves)
is an wave that moves through space at a constant
speed: ~300,000,000 m/s (photons)
This wave is analogous to the ripples on a pond that
propagate when the pond’s surface is disturbed by a
rock. The difference is that instead of waves of water
propagating through space, radiation involves waves
of an electromagnetic field.
Radiation comes in many forms...
•sunlight •microwaves •heat from a fire •radio waves
•ultraviolet rays •x-rays •gamma rays
The various forms of radiation are distinguished by their wavelength,
the distance between successive crests of the wave.
(a) has a long wavelength
(b) has a short wavelength
The longer the wavelength, the less energetic, so that (a) is less
energetic than (b).
Wavelength, Frequency,
Wavenumber
Frequency ~
ν = cycles per sec (cps, 1 GHz = 109 cps)
Wavelength λ = c / ~
ν
(1 cm = 30 GHz)
Wavenumber ν = 1/ λ
Velocity of light ~ 3 x 108 m/sec
The various forms of radiation are organized according to
their wavelengths (and hence energy levels), creating the
electromagnetic spectrum.
1GHz =109 Hz
Visible light
1 nm =10-3 μm
Energy Units
Energy
Joule (J)
Flux (Power)
J/sec (Watt, W)
Flux Density
J/sec/m2 (W/m2)
Intensity (Radiance) J/sec/m2/sterad (pencil of radiation)
Steradian (solid angle) = A/D2 = area/distance2
All objects constantly emit radiation according to
their temperature and wavelength. Objects that
emit with 100% efficiency are called blackbodies,
and have a distribution of wavelengths of emitted
radiation given by the Planck function (Planck
law), which has a characteristic shape:
This curve is for an
object with a
temperature of about
5800 K, the
approximate
temperature of the
sun’s surface.
the distribution’s
peak wavelength...
…is inversely proportional to
the temperature of the object
(=2898/T, μm). The hotter
the object, the shorter the
typical emission wavelengths
The total energy emitted by the object is
the area under the curve...
…and is proportional to the
fourth power of the object’s
temperature (=σT4). This
relationship is known as the
Stefan-Boltzmann law. So
the energy emitted increases
very quickly as the object’s
temperature increases.
The wavelength distributions of the radiation
emitted by the sun and the Earth are very
different, because the sun is so much hotter than
the Earth.
The Planck functions
for temperatures
characteristic of the
sun and the Earth.
The peak wavelength
of the sun’s
distribution is at
about 0.5 μm (green
light), while the peak
wavelength for the
earth’s distribution is
at about 10 μm
(infrared radiation).
Because there is little overlap in wavelength between the
radiation emitted by the Earth and the radiation emitted
by the Sun, almost all light in the Earth’s atmosphere with
a wavelength less than 5 μm is solar in origin, and is
known as solar
(shortwave) radiation.
At the same time, almost
all light in the earth’s
atmosphere with a
wavelength greater than
5 μm comes from the
Earth-atmosphere and is
called terrestrial
(longwave) radiation.
The total flux of energy transferred from one object to
another varies according to the distance between the
two objects. This relationship is known as the
inverse-square law.
Flux density is
proportional to 1/d2
We expect the
solar energy a
planet receives to
decrease as the
distance from the
sun increases.
The radiation flux can
also vary because of
the angle between the
surface intercepting
the radiation and the
direction of the
radiation’s
propagation. The
more oblique the
angle, the less energy
is absorbed. This is
one reason the poles
receive less energy
than the equator.
The earth also reflects
solar radiation. The
reflectivity or albedo
of the earth is about
0.3, meaning that
about 30% of the
incoming solar flux is
reflected back to
space. Certain
regions are typically
much more reflective
than others (clouds,
ice/snow, and desert).
The incoming radiation from the Sun (upper left) is of different wavelengths and is partially absorbed by atmospheric
molecules. The portion not absorbed by the atmosphere is dominantly visible radiation (lower left) that warms Earth's surface,
and the warmed Earth emits heat radiation (lower right). Some of the latter is absorbed in the troposphere by water and carbon
dioxide, thus retaining the heat there. Other gases, mostly emitted from human activities, absorb some of the transmitted
radiation in the lower stratosphere. The remainder escapes to space. All absorbing gases are termed greenhouse gases and
render some 33°C warmer than it would otherwise be without them.
The annual mean global energy balance for the Earth-atmosphere system. Latent heat is that heat supplied to the atmosphere upon
condensation of water vapor. The numbers are per-centages of the energy from the incoming solar radiation.
Fig. 8.10 The heat balance of the earth and the atmosphere system. The solar (SOL) constant used is 1366 W m–2 so that the
incoming solar flux for climatological energy balance is 342 W m–2 (round off the decimal point), while the global albedo is
taken to be 30%. The “atmosphere” referred to in the graph contains molecules, aerosols, and clouds. The atmospheric
thermal infrared (IR) flux is emitted both upward and downward. The upward IR flux from the surface is computed by using a
climatological surface temperature of 288 K. At the top of the atmosphere, the energy is balanced by radiative flux exchange.
At the surface, however, upward sensible and latent heat fluxes must be introduced to maintain energy balance. Absorption of
the solar flux is obtained from the divergence of net solar fluxes at the top and the surface. The width of the shaded area with
an arrow is approximately proportional to the flux value.