figure 1
Earth at aphelion and perihelion.
(Lutgens, F.K., and Tarbuck E. J., 1986: The Atmosphere:
An Introduction to Meteorology. Reprinted by permission of
Prentice Hall, Upper Saddle River, N. J., 07458.)
figure 2
Power input from the Sun.
figure 3
Transport and radiation.
The earth moves in an elliptical orbit around the sun, being closest to the
sun, a distance of 1.47 x 108 kilometers, in December. The distance to the sun
is maximum at about 1.52 x 108 kilometers in June. The eccentricity of this
elliptical orbit is about 0.016. The time of closest approach is called
perihelion (see figure 1) and occurs when the Northern Hemisphere is having its winter, and we call this
time the winter solstice. The earth is at aphelion, the furthest distance from the
sun, in July when the Southern Hemisphere is having its winter solstice and the
Northern Hemisphere has its summer solstice. The midpoints between solstices
are called the vernal equinox (equal length of day and night) in spring and
autumnal equinox in autumn. The plane of the earth's equator is tipped at an
angle of 23.5° to the plane of its orbit around the sun. The earth wobbles
slightly so that the tilt actually changes between about 22 and 25 degrees
cyclically with a period of about 41,000 years.
The earth/atmosphere/ocean system can be considered as a very large
thermodynamic engine that takes energy from the sun, converts it to many other
forms, and then releases it back to outer space. The intensity of the sun's
radiation reaching the "top" of the atmosphere is 1,380 Wm-2.
More power per square meter reaches the earth at low latitudes (closer to the
equator) than in the polar latitudes. A simple calculation shows that
the power of this engine, shown in figure 2, is about 1.76 x 1011 megawatts. A large power plant in a major
city might produce 100 megawatts, so the sun provides the earth with the
equivalent of about 2 billion such power plants. Wallace and
Hobbs (1977) have calculated magnitudes of natural and anthropogenic heat sources that might alter
this engergy balance at the earth's surface.
This energy from the sun is absorbed preferentially in low latitudes in the
tropical regions and subtropics (see figure 3). It is transmitted from the tropics toward the
polar regions as thermal energy or as latent heat in the form of water vapor.
Eventually, this energy is radiated back to outer space in amount equal to the
input, giving the earth/atmosphere/ocean system as a whole a thermodynamic
balance. The global warming we will discuss later in the course is not a matter
of the atmosphere gaining more energy than it is losing, but rather a change in
the redistribution of energy in the atmosphere. The earth is not observed to be
heating up or cooling down rapidly, and even if we changed the composition of
the gases in our atmosphere we don't change this fact that the earth loses the
same amount of energy it receives from the sun. When we change the gases in
the earth's atmosphere, we change the processes of redistribution: the surface
warms, but the stratosphere actually cools.
