Geoscience Reference
In-Depth Information
practice until the 1920s. Equally vital was the establish-
ment of networks of observing stations, following a
standardized set of procedures for observing the weather
and its elements, and a rapid means of exchanging the
data (the telegraph). These two developments went
hand-in-hand in Europe and North America in the 1850s
to 1860s.
The greater density of water, compared with that of
air, gives water a higher specific heat. In other words,
much more heat is required to raise the temperature
of a cubic metre of water by 1°C than to raise the
temperature of a similar volume of air by the same
amount. In terms of understanding the operations of the
coupled earth-atmosphere-ocean system, it is inter-
esting to note that the top 10-15 cm of ocean waters
contain as much heat as does the total atmosphere.
Another important feature of the behaviour of air and
water appears during the process of evaporation or
condensation. As Black showed in 1760, during evap-
oration, heat energy of water is translated into kinetic
energy of water vapour molecules (i.e. latent heat),
whereas subsequent condensation in a cloud or as fog
releases kinetic energy which returns as heat energy.
The amount of water which can be stored in water
vapour depends on the temperature of the air. This
is why the condensation of warm moist tropical air
releases large amounts of latent heat, increasing the
instability of tropical air masses. This may be con-
sidered as part of the process of convection in which
heated air expands, decreases in density and rises,
perhaps resulting in precipitation, whereas cooling air
contracts, increases in density and subsides.
The combined use of the barometer and thermometer
allowed the vertical structure of the atmosphere to
be investigated. A low-level temperature inversion
was discovered in 1856 at a height of about 1 km on
a mountain in Tenerife where temperature ceased
to decrease with height. This so-called Trade Wind
Inversion is found over the eastern subtropical oceans
where subsiding dry high-pressure air overlies cool
moist maritime air close to the ocean surface. Such
inversions inhibit vertical (convective) air movements,
and consequently form a lid to some atmospheric
activity. The Trade Wind Inversion was shown in the
1920s to differ in elevation between some 500 m and
2 km in different parts of the Atlantic Ocean in the
belt 30°N to 30°S. Around 1900 a more important
continuous and widespread temperature inversion was
revealed by balloon flights to exist at about 10 km at
the equator and 8 km at high latitudes. This inversion
level (the tropopause) was recognized to mark the top
of the so-called troposphere within which most weather
systems form and decay. By 1930 balloons equipped
with an array of instruments to measure pressure,
temperature and humidity, and report them back to earth
by radio (radiosonde), were routinely investigating the
atmosphere.
B SOLAR ENERGY
The exchanges of potential (thermal) and kinetic energy
also take place on a large scale in the atmosphere as
potential energy gradients produce thermally forced
motion. Indeed, the differential heating of low and
high latitudes is the mechanism which drives both
atmospheric and oceanic circulations. About half of
the energy from the sun entering the atmosphere as
short-wave radiation (or 'insolation') reaches the earth's
surface. The land or oceanic parts are variously heated
and subsequently re-radiate this heat as long-wave
thermal radiation. Although the increased heating of
the tropical regions compared with the higher latitudes
had long been apparent, it was not until 1830 that
Schmidt calculated heat gains and losses for each
latitude by incoming solar radiation and by outgoing re-
radiation from the earth. This showed that equatorward
of about latitudes 35° there is an excess of incoming
over outgoing energy, while poleward of those latitudes
there is a deficit. The result of the equator-pole thermal
gradients is a poleward flow (or flux) of energy, inter-
changeably thermal and kinetic, reaching a maximum
between latitudes 30° and 40°. It is this flux which
ultimately powers the global scale movements of the
atmosphere and of oceanic waters. The amount of solar
energy being received and re-radiated from the earth's
surface can be computed theoretically by math-
ematicians and astronomers. Following Schmidt, many
such calculations were made, notably by Meech
(1857), Wiener (1877), and Angot (1883) who calcu-
lated the amount of extraterrestrial insolation received
at the outer limits of the atmosphere at all latitudes.
Theoretical calculations of insolation in the past by
Milankovitch (1920, 1930), and Simpson's (1928
to 1929) calculated values of the insolation balance
over the earth's surface, were important contributions
to understanding astronomic controls of climate.
Nevertheless, the solar radiation received by the earth
