Geoscience Reference
In-Depth Information
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.
Observations from both kites and balloons also
revealed that strong inversions extending up to
about 1000m are a near ubiquitous feature of the
Arctic in winter.
must be an overall tranasfer of energy from
lower to higher latitudes via the atmospheric and
oceanic circulations. Put differently, while the
differential solar heating gives rise to the equator-
to-pole temperature gradient, the poleward
energy transports work to reduce this gradient.
Later and more refined calculations showed that
the poleward flow (or flux) of atmospheric energy
reaches a maximum around latitudes 30
,
with the maximum ocean transport occurring at
lower latitudes. The total poleward transport in
both hemispheres is in turn dominated by the
atmosphere. The amount of solar energy being
received and re-radiated from the earth's surface
can be computed theoretically by mathematicians
and astronomers. Following Schmidt, many such
calculations were made, notably by Meech (1857),
Wiener (1877) and Angot (1883) who calculated
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 (1928-1929) calculated values of the
insolation balance over the earth's surface, were
important contributions to understanding astro-
nomic controls of climate. Nevertheless, the solar
radiation received by the earth was only accurately
determined by satellites in the 1990s.
°
and 40
°
B
SOLAR ENERGY
Differential solar heating of low and high latitudes
is the mechanism which drives the earth's large-
scale atmospheric and oceanic circulations. Most
of the energy from the sun entering the atmos-
phere as short-wave radiation (or insolation)
reaches the earth's surface. Some is reflected back
to space. The remainder is absorbed by the surface
which then warms the atmosphere above it. The
atmosphere and surface together radiate long-
wave (thermal) radiation back to space. Although
the land and ocean parts of the surface absorb
different amounts of solar radiation and have
different thermal characteristics, the differential
solar heating between low and high latitudes dom-
inates, fostering an equator-to-pole gradient in
atmospheric and upper ocean temperatures.
Although increased solar heating of the tropical
regions compared with the higher latitudes had
long been apparent, it was not until 1830 that
Schmidt made a key calculation, namely heat gains
and losses for each latitude by incoming solar
radiation and by outgoing longwave radiation
from the earth. This showed that equatorward of
about latitudes 35° there is an excess of incoming
solar over outgoing longwave energy, while pole-
ward of those latitudes the longwave loss exceeds
solar input. If, at each latitude, the longwave loss
to space equaled the solar radiation input (termed
radiative equilibrium), this pattern would not be
seen. That it exists is direct evidence that there
C GLOBAL CIRCULATION
While differential solar heating of the surface
and the atmospheric temperature gradient that
it generates fosters the large-scale transport of
energy from equatorial to polar regions, what are
the mechanisms by which this atmospheric
transport is accomplished? While we now
know that the transport is accomplished by
the Hadley circulation in lower latitudes and in
higher latitudes through disturbances in the basic
westerly (west to east) flow in the form of transient
cyclones and anticyclones, it is fascinating to
briefly outline how our modern view of the global
circulation emerged.
