Geoscience Reference
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small. The overall sense of the mean circulation is westerly, positive with respect
to the Earth's rotation, so that the atmosphere is in the mean in superrotation about
the solid Earth. The strength of the superrotation varies strongly with the season,
because of the high difference between the winds in winter and those in summer
over the northern hemisphere, much of which has a so-called continental climate
(see Schindelegger et al. ( 2013 ) in this topic).
In addition, winds have northerly and southerly components, so that the air masses
travel in waves around the planet. Large-scale waves are generally baroclinic in effect
and are part of the patterns of migrating weather systems. In the northern hemisphere
middle latitudes, we generally have weather moving from the west to the east, though
whether they come from the northwest or southwest often depends on which phase
of a planetary wave the region is in at that time.
Variability in the winds may indicate a total change in the angular momentum
of the atmosphere, which in the closed Earth system is transferred to either the
ocean or solid Earth below. In fact, most of the transfers to the ocean are then
quickly transferred to the solid Earth itself, and impacts the changes in the Earth's
rotation rate and vector. These torques happen because of a variety of tangential forces
from surface winds, normal forces of atmospheric pressure against mountains, and
gravitational forces. The atmospheric angular momentum and torques are covered
by Schindelegger et al. ( 2013 ) in this topic.
The atmosphere has been modeled for the purpose of analysis, short-termweather
forecasts, and climate projections. Such models, which are produced and used by the
world's major research and weather forecast centers, are based on the principles of
atmospheric physics which frame an analysis of the weather and climate system.
Physical equations for the atmosphere include the equation of motion, a form
of Newton's second law, the equation of total mass continuity, and of continuity of
its constituents, particularly water substance and the equation of thermodynamics
(reviewed by Salstein ( 1995 )). These may be thought of as representing the conser-
vation laws of momentum, mass, and energy, respectively. Together with information
about gas physics, chemistry, and radiative transfer, these equations are the basis of
numerical weather prediction. To analyze the basicmeteorological quantities, namely
temperature, pressure or geopotential height, wind, and moisture, and to use these
meteorological fields as initial conditions for forecasts, a variety of observational data
concerning the atmosphere are assimilated into an analysis-forecast system. Such
data are derived from weather observations on the surface, and upon the radiosonde
network, from aircraft-based measurements, and from both geostationary and polar
orbiting satellites.
The atmosphere has been thought of as a heat engine, in which solar energy in
different forms is applied to the air to first provide heating for the atmosphere. These
forms are sensible heating of direct contact, say from the warm land or warmer air
layers, below, latent heating from the condensation of water vapor (reduced by its
opposite, latent cooling from evaporation), and the form of heating by radiational
transfer through gases, mostly from the near infrared, visible, and near ultraviolet
parts of the spectrum. When these forms of heating are applied, overturning cir-
culations of the atmosphere result, creating vertical motions and horizontal wind
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