Environmental Engineering Reference
In-Depth Information
based on the concept of a non-rotating earth with
a uniform surface, which was warm at the
equator and cold at the poles (see Figure 2.10a).
The warmth caused the surface equatorial air to
become buoyant and rise vertically into the
atmosphere. As it rose away from its source of
heat, it cooled and became less buoyant, but was
unable to sink back to the surface because of the
warm air rising behind it. Instead, it spread north
and south away from the equator, eventually
returning to the surface at the poles. From there
it flowed back towards the equator to close the
circulation. The air rising at the equator and
spreading polewards carried energy with it,
helping to reduce the energy imbalance between
the equator and the poles. This type of energy
transfer, initiated by differential heating, is called
convection, and the closed circulation which
results is a convection cell. Hadley's original
model, with its single convection cell in each
hemisphere, was eventually replaced by a three-
cell model as technology advanced and additional
information became available, but his
contribution was recognized in the naming of the
tropical cell (see Figure 2.10b). The three-cell
model, continued to assume a uniform surface,
but the rotation of the earth was introduced, and
with it, the Coriolis effect, which causes moving
objects to swing to the right in the northern
hemisphere, and to the left in the southern. Thus,
the winds became westerly or easterly in this new
model, rather than blowing north or south as in
the one cell version. The three cells and the
Coriolis effect, in combination, produced
alternating bands of high and low pressure,
separated by wind belts which were easterly in
equatorial and polar regions, and westerly in mid-
latitudes. Although only theoretical, elements of
this model can be recognized in existing global
wind and pressure patterns, particularly in the
southern hemisphere, where the greater expanse
of ocean more closely resembles the uniform
surface of the model.
In the late 1940s and 1950s, as knowledge of
the workings of the atmosphere improved, it
became increasingly evident that the three-cell
model oversimplified the general circulation.
The main problems arose with the mid-latitude
cell. According to the model, the upper airflow
in mid-latitudes should have been easterly, but
observations indicated that it was
predominantly westerly. The winds followed a
circular path centred on the pole, which led to
their description as circumpolar westerlies.
Observations also indicated that most energy
transfer in mid-latitudes was accomplished by
horizontal cells rather than the vertical cell
indicated by the model. The mechanisms
involved included travelling high and low
pressure systems at the surface plus wave
patterns in the upper atmosphere called Rossby
waves (Starr 1956). Modern interpretations of
the general circulation of the atmosphere retain
the tropical Hadley cell, but horizontal eddies
Figure 2.11
The index cycle associated with the
meandering of the mid-latitude westerlies in the
northern hemisphere
