- Heat (heat energy) is the total kinetic energy of all the atoms
in a substance.
- The Earth's climate system constantly tries to maintain a balance between
the energy that reaches the Earth from the Sun and the energy that is emitted
to space. Scientists refer to this process as Earth's "radiation
The Earth's energy balance
- On the Moon where there is no atmosphere, a surface temperature far below
freezing emits enough radiation to balance the absorbed solar energy.
- Because of the tilt of the Earth's axis, incoming solar radiation is not
evenly distributed on the Earth's surface and seasonal changes occur.
- The Sun is not in the exact center of the Earth's orbit. During the
Southern hemisphere summer the Earth is closer to the Sun than during the
Northern hemisphere summer. The Earth is farthest from the Sun during the
Southern hemisphere winter.
As the Sun's electromagnetic radiation penetrates the Earth's atmosphere
it is selectively absorbed and scattered by molecules of gases, liquids,
The energy coming from the Sun to the Earth's surface is called solar
insolation or shortwave energy.
Both the amount of energy and the wavelengths at which energy is emitted
by any system are controlled by the average temperature of the system's
radiating surfaces. The temperature of the Sun's radiating surface, or
photosphere, is more than 5500 degree C (9900 degree F).
Energy goes back to space from the Earth system in two ways: reflection
- Part of the solar energy that comes to Earth is reflected back out to
space in the same, short wavelengths in which it came to Earth.
- The percentage of solar energy that is reflected back to space is called
- Different surfaces have different albedos. Over the whole surface of the
Earth, about 30 percent of incoming solar energy is reflected back to space.
- Ocean surfaces (26% albedo) and rain forests (15% albedo) reflect only
a small portion of the Sun's energy.
- Deserts however, have high albedos (40%); they reflect a large portion
of the Sun's energy.
- A cloud usually has a higher albedo than the surface beneath it, the
cloud reflects more shortwave radiation back to space than the surface would
in the absence of the cloud, thus leaving less solar energy available to heat
the surface and atmosphere.
- Another part of the energy going back to space from the Earth is the
longwave radiation emitted by the Earth. The solar radiation absorbed by the
Earth increases the planet's temperature. Heat energy is emitted into space,
creating a balance.
A cloud can absorb radiation emitted by the Earth's surface and radiates
in all directions.
The longwave energy emitted from the surface of the Earth and absorbed
by the atmosphere results in an increase in the ambient temperature (i.e., the
greenhouse effect). This absorbed energy is then emitted both to space and back
towards the Earth's surface.
The greenhouse effect is due mainly to water vapor in the atmosphere.
This effect is enhanced by carbon dioxide, methane and other infrared-absorbing