The Planet Earth

Earth is a planet with a radius of about 6,000 km, moving around the sun once a year in an orbit that is almost circular, although not precisely so. Its farthest distance from the sun, or aphelion, is about 152 million km, and its closest distance, perihelion, is about 147 million km. This ellipticity, or eccentricity, is small, and for most of the rest of the book we will ignore it. (The eccentricity is not in fact constant and varies on timescales of about 100,000 years because of the influence of other planets on Earth's orbit;

these variations may play a role in the ebb and flow of ice ages, but that is a story for another day.) Earth itself rotates around its own axis about once per day, although Earth's rotation axis is not parallel to the axis of rotation of Earth around the sun. Rather, it is at an angle of about 23°, and this is called the obliquity of Earth's axis of rotation. (Rather like the eccentricity, the obliquity also varies on long tim-escales because of the influence of the other planets, although the timescale for obliquity variations is a relatively short 41,000 years.) Unlike the ellipticity, the obliquity is important for today's climate because it is responsible for the seasons, as we will see later in this chapter.

Earth is a little more than two-thirds covered by ocean and a little less than one-third land, with the oceans on average about 4 km deep. Above Earth's surface lies, of course, the atmosphere. Unlike water, which has an almost constant density, the density of the air diminishes steadily with height so that there is no clearly defined top to the atmosphere. About half the mass of the atmosphere is in its lowest 5 km, and about 95% is in its lowest 20 km. However, relative to the ocean, the mass of the atmosphere is tiny: about one-third of one percent of that of the ocean. It is the weight of the atmosphere that produces the atmospheric pressure at the surface, which is about 1,000 hPa (hectopascals), and so 105Pa (pascals), corresponding to a weight of 10 metric tons per square meter, or about 15 lb per square inch. In contrast, the pressure at the bottom of the ocean is on average about 4 X 107Pa, corresponding to 4,000 metric tons per square meter or 6,000 lb per square inch!

Table 1.1

Main Constituents of the Atmosphere

Table 1.1

Main Constituents of the Atmosphere


Molecular weight

Proportion by volume

Nitrogen, N2



Oxygen, O2



Argon, Ar



Water vapor, H2O


~0.4% (average)

~1%-4% (at surface)

Carbon dioxide, CO2


390 ppm (0.039%)

Neon, Ne


18.2 ppm

Helium, He


5.2 ppm

Methane, CH4


1.8 ppm

Molecular weight is the molar mass, measured in grams per mole. In addition, there are trace amounts of krypton, hydrogen, nitrous oxide, carbon monoxide, xenon, ozone, chlorofluorocarbons (CFCs), and other gases.

Molecular weight is the molar mass, measured in grams per mole. In addition, there are trace amounts of krypton, hydrogen, nitrous oxide, carbon monoxide, xenon, ozone, chlorofluorocarbons (CFCs), and other gases.

The atmosphere is composed of nitrogen, oxygen, carbon dioxide, water vapor, and a number of other minor constituents, as shown in table 1.1. Most of the constituents are well mixed, meaning that their proportion is virtually constant throughout the atmosphere. The exception is water vapor, as we know from our daily experience: Some days and some regions are much more humid than others, and when the amount of water vapor reaches a critical value, dependent on temperature, the water vapor condenses, clouds form, and rain may fall.

Earth's temperature is, overall, maintained by a balance between incoming radiation from the sun and the radiation emitted by Earth itself, and, slight though it may be compared to the ocean, the atmosphere has a substantial effect on this balance. This effect occurs because water vapor and carbon dioxide (as well as some other minor constituents) are greenhouse gases, which means that they absorb the infrared (or longwave) radiation emitted by Earth's surface and act rather like a blanket over the planet, keeping its surface temperature much higher than it would be otherwise and keeping our planet habitable. However, we are getting a little ahead of ourselves—let's slow down and consider in a little more detail Earth's radiation budget.

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