Energy Balance and Temperature
Atmospheric Influences on Isolation
The atmosphere absorbs some radiation directly and thereby gains heat. Another portion of
radiation disperses as weaker rays going out in many different directions through a process we
Some of the scattered radiation is directed back to space; the remainder is scattered forward as
the light we see from the portion of the sky away from the solar disk.
In either case, the energy that is scattered is not absorbed by the atmosphere and therefore
does not contribute to its heating.
The remaining insolation is neither absorbed nor scattered and passes through the atmosphere
without modification, reaching the surface as direct radiation. But not all the energy reaching
the surface is absorbed. Instead, a fraction is scattered by the atmosphere, it doesn't contribute
to the heating of the planet.
Absorption: a process in which radiation is captured by a molecule. Unlike reflection, absorption
represents an energy transfer to the absorbing molecule.
o Represents an energy transfer to the absorber. This transfer has two effects: the
absorber gains energy and warms, while the amount of energy is delivered to Earth's
surface is reduced.
The gases of the atmosphere are not effective at absorbing sunlight, and different wavelengths
of radiation are not equally subject to absorption.
o For example, Ultraviolet radiation is almost totally absorbed by the ozone in the
stratosphere. Visible radiation, in contrast, passes through the atmosphere with only a
minimal amount of absorption.
Near-infrared radiation, which represents nearly half the radiation emitted by the Sun, is
absorbed mainly by the two gases in the atmosphere- water vapour and (to a lesser extent
This is why direct sunlight in the dessert feels so hot, where as in humid regions the apparent
temperature differences between standing in direct sunlight and standing in shade is relatively
When the humid is high, water vapours absorbs a significant portion of near-infrared radiation,
thereby reducing the amount of energy available to warm your skin.
Reflection and Scattering
Reflection: the process in which radiation arriving at a surface bounces back, without being
absorbed or transmitted. Reflection doesn't heat the reflector, because there is no net energy
transfer to the surface.
Albedo: The fraction of solar radiation arriving at a surface that is reflected.
Specular reflection: when light strikes a mirror, it is reflected back as a beam of equal intensity.
Diffuse reflection (scattering): when a bean is reflect from an object as a larger number of
weaker rays travelling in many different directions.
o When scattering occurs, you can't see an image of yourself on the reflecting surface as
you can in a mirror.
Diffuse Solar Radiation: sunlight that is scattered downward towards the surface.
Direct Solar Radiation: sunlight that passes through the atmosphere without absorption or
The characteristics of radiation scattering by the atmosphere depend on the size of the
scattering agents (the air molecules of suspended particles) relative to the wavelength of the
incident electromagnetic energy. Energy Balance and Temperature
Rayleigh Scattering: the scattering of radiation by agents substantially smaller that the
radiation's wavelength. It primarily affects short wave lengths.
o In the case of the atmosphere, this applies to the scattering of visible radiation by air
o Leads to a blue sky on a clear day, the blue tint of the atmosphere when viewed from
space, and the redness of sunsets and sunrise.
Figure 3.3:The sky appears blue because the gases and particles in the atmosphere scatter some of the
incoming solar radiation in all the directions. Air molecules scatter shorter wavelengths most effectively.
Someone at the surface looking skyward perceives blue light, the shortest wavelenght of the visible portion
of the spectrum.
On the Moon, which has no atmosphere, the “sky” appears black. As a viewer looks toward the
horizon on the Moon, there is no downwards scattered. Light because of the absence of an
atmosphere, and the sky appears little different from the way it does at night. All that can be
seen is the energy reflected off the lunar surface and Earth.
Sunrises and sunsets appear red because sunlight travels a longer path through the atmosphere.
This causes a high amount of scattering to remove shorter wavelengths from the incoming beam
radiation. The result is sunlight consisting almost entirely of longer (e,g. Red) wavelengths.
Mie Scattering: scattering of visible radiation caused by particulates. Energy Balance and Temperature
Mie scattering is predominantly forward, diverting relatively little energy backward to space.
Furthermore, Mie scattering does not have nearly the tendency to scatter shorter-wavelength
that Rayleigh scattering does.
Mie scattering causes sunrises and sunsets to be redder than they would be due to Rayleigh
scattering alone, so episodes of heavy air pollution often results in spectacular sunsets.
Non-selective scattering: scattering of radiation in which all wavelengths are scattered about
equally. This type of scattering causes cloud to appear white.
When solar radiation travels through the vacuum of outer space, there is no modification of its