Unit 5: Radiation and the
Heat Balance of Planet
Earth
Source: http://science.howstuffworks.com/environmental/energy/solar-cell5.htm
OBJECTIVES
• Understand the processes by which heat
flows within the Earth system
• Follow the cascade of Solar energy to the
Earth’s surface and the resulting energy
exchanges between surface and
atmosphere
• Link the greenhouse effect to the Earth’s
habitability
• Understand latitudinal differences in net
radiation
• Explore global energetics
The Electromagnetic
Spectrum
Figure 2.6
Solar and Terrestrial Radiation
Emission by the Sun and Earth on a per unit area basis. Both emit radiation over many wave-lengths, but there is
little overlap in their emission curves. Note that the curves are not drawn at the same scale. The terrestrial axis is
exaggerated by a factor of 1,000,000 relative to the solar curve.
Heat Conduction between the Earth’s
surface and the sub-surface
Daily and Seasonal Patterns
Heat is conducted from higher toward lower temperatures. On a daily basis heat is stored in
the soil column during the day and lost at night. Similarly, seasonal average temperature
gradients lead to heat storage and loss in the warm and cold seasons. The daily temperature
cycle does not extend very far into the soil, but the day/night temperature range is large
compared to the seasonal cycle.
Latent and Sensible Heat Transfer
• Conduction
• Convection
• radiation
Sensible and latent forms of convective heat transfer. At left conduction and radiation from
the warm surface move heat into the atmosphere. After that vertical and horizontal
motions produce a corresponding transfer of sensible heat into cooler parts of the
atmosphere. On the right water vapor is carried upward by mixing and to the left by winds.
This results in both a vertical and horizontal latent heat transfer.
The Radiation Balance
Solar radiation flow in the atmosphere
Reflectivity, albedo
Source: http://www.cgd.ucar.edu/ccr/aboutus/staff/kiehl/EarthsGlobalEnergyBudget.pdf
Global
distribution
of heat flow
• Tropics receives more
energy than the poles
• Heat must be transported
from tropics to polar
regions
(a) Annual radiation
gains and losses
through the top of
the atmosphere as a
function of latitude.
The vertical axis gives
heat gain or loss per
degree of latitude,
which allows for a
better comparison
between latitudes.
(b) Horizontal energy
transport by ocean
and atmosphere.
Positive values are
for northward heat
transfer, whereas
negative values
denote transport to
the south. The
atmosphere curve
includes both latent
and sensible transfer.
Heat Transfer
• Conduction
– Molecule-to-molecule transfer
• Convection
– Energy transferred by movement
• Advection
– Horizontally dominant movement
• Radiation
– Energy traveling through air or space
Heat Transfer
Figure 3.7
Greenhouse with and without
ventilation
Greenhouses are warm because the heated air is confined within (a). A little ventilation drastically
lowers temperature despite little effect on the transmission of radiation.
Greenhouse effect of the atmosphere
Cartoon depiction of the greenhouse effect. Because of clouds and greenhouse gases, emission to space is effectively from a
cold location high in the atmosphere (a). Adding greenhouse gases raises the emission altitude, resulting in less emission to
space (b). Eventually warming throughout the column leads to a new equilibrium when emission again balances absorbed
solar radiation (c).