Geography Reference
In-Depth Information
solar energy received by the Earth, only around a half makes it all
the way down to warm the Earth's surface. Some of this energy is
used for processes such as evaporation of water or for plant growth.
However, most absorbed radiation from the Sun (known as short-
wave radiation; an example of short-wave radiation is visible light)
is transformed by the land, oceans and vegetation and emitted back
into the atmosphere as long-wave radiation in the form of heat
energy (invisible
infrared radiation
). Except for the 19 per cent
of incoming solar energy that is temporarily absorbed, the atmo-
sphere is mostly transparent to incoming short-wave radiation. This
means, perhaps surprisingly to many people, that the air is mainly
heated from below by long-wave heat energy emitted by the
Earth's surface. Thus, the atmosphere should be warmer close to
the Earth's surface but cool with altitude in the troposphere. Since
the air is warmed by the surface below, this means that during the
day the air near the surface becomes less dense and more buoyant.
Less dense gases or liquids will naturally seek to rise and more
dense fluids will seek to fall. Hence the less dense air near the
surface seeks to rise above cooler, denser air which in turn sinks
towards the Earth's surface. As the air rises it in turn cools because
it is able to expand due to the lower air pressure at higher altitudes.
The reason it cools is due to a fundamental law of nature which
means that as the pressure of a gas decreases the temperature will
decrease. The result of these processes is that there is large scale
vertical mixing of the air within the troposphere as rising warm air
is replaced by cooler descending air.
The atmosphere is made up of mainly nitrogen (78 per cent)
and oxygen (21 per cent). The remaining 1 per cent is made up of
mainly argon. There are also small concentrations of other gases
such as hydrogen, water vapour (the gaseous form of water),
methane, nitrous oxide, ozone and carbon dioxide. However,
despite their low concentrations some of these other gases are
important for the climate we experience. While the gases of the
atmosphere are almost unaffected by the short-wave radiation pro-
vided by the Sun, some of them readily absorb long-wave radiation
produced by the Earth's surface. Unlike oxygen and nitrogen, some
gases such as carbon dioxide, methane, water vapour and nitrous
oxide absorb the thermal energy emitted by the Earth's surface and
provide a sort of blanket over the Earth. They radiate this energy
