Solar Energy
and Energy Balance
in the Atmosphere
Mid-chapter summary
• The
temperature of a substance is a measure of the average kinetic
energy (average speed) of its atoms and molecules.
• Evaporation
(the transformation of liquid into vapor) is a cooling
process that can cool the air, whereas condensation (the
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transformation of vapor into liquid) is a warming process that can
warm the air.
Heat is (thermal) energy in the process of being transferred from
one object to another because of the temperature difference
between them.
In conduction, which is the transfer of heat by molecule-to-molecule
contact, heat always flows from warmer to colder regions.
Air is poor conductor of heat.
Convection is an important mechanism of heat transfer, as it
represents the vertical movement of warmer air upward and cooler
air downward.
All objects with a temperature above absolute zero emit radiation.
The higher an object’s temperature, the greater the amount of
radiation, and the shorter the wavelength of maximum emission.
Methods for heat transfer in the atmosphere
• Thermal conduction
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♦ It requires: physical contact, temperature difference
♦ The energy flow is from a body with high T to a body with low T.
♦ It is important only for a thin layer close to the ground.
Convection (advection)
♦ Winds in the atmosphere can transport energy.
♦ Very efficient way of energy transport in the atmosphere.
Radiation
♦ All bodies with T> 0 K emit EM waves.
♦ EM waves can propagate in vacuum.
♦ The EM spectrum of a body is the energy it emits at different
wavelengths
♦ The higher the temperature the more energy is emitted (SB Law)
♦ The wavelength at which the peak of the EM emission of a body
occurs is determined by its temperature (Wien’s Law).
Radiation and Temperature
• All bodies with T>0K emit radiation (electromagnetic energy).
• The
origin of the emission is the transition of the atoms
(molecules) from one energy state to another.
• The
wavelength and the amount of energy emitted by the body
depend on its temperature.
• Higher
T -> larger internal energy -> atoms vibrate faster -> the
radiation has shorter wavelength and higher energy.
The Sun’s electromagnetic spectrum
Temperature and Emitted Energy
Energy
E  T 4
max 
const.
T
E  T 4
The Sun emits ~(6000/288)4~188,000 times more energy than the Earth!!!
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Sun/Earth radiation
Sun
♦ T=6000 K
♦ max=0.5 mm
♦ Maximum in visible
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Earth
♦ T=288 K = 15 C
♦ max=10 mm
♦ Maximum in IR
The Sun emits (6000/288)4~188,000 times more
energy than the Earth!!!
Radiative Equilibrium
Emission
>
Absorption
Cooling
Emission
<
Absorption
Heating
Emission
=
Absorption
Equilibrium
T=constant
The Earth’s Balancing Act:
Absorption, Emission and Equilibrium
• First, suppose the Earth had no atmosphere at all
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♦ Energy from the Sun would be absorbed during daytime
♦ Energy would be radiated away 24/7
Average surface temperature at equilibrium: 255K=-18C=0F
Why aren’t we freezing to death?
The greenhouse effect of the
atmosphere
No Atmosphere
T=255 K
With Atmosphere
T=288 K
Selective Absorbers/Emitters
• Trees: absorb visible energy from Sun, radiate in infrared
• Snow away from trees: reflects visible sunlight, stays cold
• Snow
near trees: reflects visible sunlight, but absorbs the
infrared radiation from nearby trees, heats up and melts.
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Atmospheric Greenhouse effect
The gases in the Earth’s atmosphere are selective absorbers
♦ Greenhouse gases: absorb the infrared radiation from Earth
♦ Also selectively radiate in the infrared, a fraction goes back to Earth
♦ The Earth’s greenhouse effect: H2O 60%; CO2 26 %; rest 14%
H2O, CO2
O2 , O3
H2O, CO2
O3
Fig. 2-11b, p. 41
Fig. 2-11a, p. 41
Fig. 2-11c, p. 41
The greenhouse effect of the
atmosphere
No Atmosphere
T=255 K
With Atmosphere
T=288 K
The lower atmosphere is heated from
below!!!
The Incoming Solar Radiation
• Insolation = Incoming Solar Radiation
• The
solar constant: the energy from the sun at the TOP of the
atmosphere per unit surface per unit time (1367 W/m
• Scattered
light: scattering from molecules, dust particles,
aerosols.
2)
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♦ The scattered (diffuse) light goes in all directions.
♦ Shorter wavelengths are scattered more efficiently by the
atmosphere (the sky is blue, see Chapter 19).
Reflected light: some light is sent backwards.
♦ Albedo: the percentage of the incident light that is reflected.
♦ The albedo is a measure of the reflectivity of the surface
Absorption
Table 2-3, p. 44
The solar energy budget
The Energy Balance
• The
amount of incoming energy should be exactly equal to the
amount of the outgoing energy!!!
♦ The Earth
IN: Solar radiation+Cloud IR emission
OUT: Conduction+Convection+H2O Evaporation+ IR
emission+Reflection of solar light
♦ The atmosphere:
IN: from the ground (convection,conduction,latent
heat,IR emission)+solar radiation
OUT: to the ground (IR emission+reflected light)+to
space(IR radiation)
♦ The Earth + atmosphere:
IN: Solar Radiation
OUT: IR radiation (ground+atmosphere)+Reflected light
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The Earth Energy Balance