Almost all planets in our solar system have an atmosphere, a shell of gases
that surrounds the solid mass of the planet. In general, the planets located
in the outer part of the solar system have an atmosphere consisting mostly of
hydrogen and helium, or a mixture of these two gases with methane. Earth is
located between two planets, Venus and Mars, that have atmospheres of mostly
carbon dioxide. Mercury, which is closest to the Sun, is the only planet without
an atmosphere. Earth is also unusual since it is the only planet with an atmosphere
of mostly nitrogen and oxygen. An explanation of why these differences exist
between the planets was discussed in chapter 17. The earth's atmosphere has a unique composition because of the cyclic flow
of materials that takes place between different parts of the earth. These cycles,
which do not exist on the other planets, involve the movement of materials between
the surface and the interior (see chapter 20) and the building-up and tearing-down
cycles on the surface (see chapters 21-22). Materials also cycle in and out
of the earth's atmosphere. Carbon dioxide, for example, is the major component
of the atmospheres around Venus and Mars, and the early Earth had a similar
atmosphere. Today, carbon dioxide is a very minor part of Earth's atmosphere.
It has been maintained as a minor component in a mostly balanced state for about
the past 570 million years, cycling into and out of the atmosphere. Water is also involved in a global cyclic flow between the atmosphere and
the surface. Water on the surface is mostly in the ocean, with lesser amounts
in lakes, streams, and underground. Not much water is found in the atmosphere
at any one time on a worldwide basis, but billions of tons are constantly evaporating
into the atmosphere each year and returning as precipitation in an ongoing cycle. The cycling of carbon dioxide and water to and from the atmosphere takes
place in a dynamic system that is energized by the sun. Radiant energy from
the sun heats some parts of the earth more than others. Winds redistribute this
energy with temperature changes, rain, snow, and other changes that are generally
referred to as the weather. Understanding and predicting the weather is the subject of meteorology.
Meteorology is the science of the atmosphere and weather phenomena, from understanding
everyday rain and snow to predicting not-so-common storms and tornadoes (Figure
24.1). Understanding weather phenomena depends on a knowledge of the atmosphere
and the role of radiant energy on a rotating Earth that is revolving around
the Sun. This chapter is concerned with understanding the atmosphere of the
earth, its cycles, and the influence of radiant energy on the atmosphere. This
understanding will be put to use in the next chapter, which is concerned with
weather and climate. |
2002 McGraw-Hill Higher Education