All motions that can be attributed to the action of a steady gravitational
field upon variations of density in a fluid may be called convective
motions and thus almost all the kinetic energy of the earth's atmosphere and
oceans emd the bulk of that of the many fluid systems in the known
universe results from convection; other fluid motions are attributable to tidal
and electrodynamic forces. In the atmospheric sciences, however, one gen-
ereilly uses a more restricted definition of convection which encompasses
only a class of relatively small-scale, thermally direct circulations which
result from the action of gravity upon an unstable vertical distribution of
mass, with "vertical" taken to mean "along the gravitational vector." (We
make an exception in the cjise of slantwise convection, which is driven by
gravitational and centrifugal accelerations, though the underlying physics
is the same.) This restricted definition, which will be used throughout this
course, excludes the many motion systems that result from differential
heating in the horizontal, including simple Hadley circulations emd sea breezes,
as well as circulations that arise as a result of unstable distributions of
vorticity. Due to the geometry of the atmosphere of the earth, this
distinction between "convective" emd "nonconvective" motions is rendered less
academic, as the former is generally highly turbulent, whereas the latter
can often be regarded as laminar except near boundaries. The strong
influence of an entire spectrum of smaller scale motions on most forms of
convection helps make this phenomenon among the most perplexing in the
atmospheric sciences, while the question of the interaction of convection
with circulations of larger scale hjis proven every bit as challenging. It is
fair to say that since Archimedes' time, man's understanding of convection
and related phenomena has progressed surprisingly slowly.
Even the restricted definition of convection embraces