not move as much, so they do not take up as much space. When the molecules are warm,
they move vigorously and take up more space. With the same number of molecules in this
larger volume, the air is less dense and air pressure is lower. This warmer, lighter air is more
buoyant than the cooler air above it, so it rises. The cooler air then sinks down, since it is
more dense than the air beneath it. The rising of warmer air and sinking of cooler air is a
very important concept for understanding the atmosphere.
As you learned in the previous section, the composition of gases is mostly the same through-
out the first 100 km of our atmosphere. This means if we measure the percentages of different
gases throughout the atmosphere, it will stay basically the same. However the density of the
gases and the air pressure do change with altitude; they basically decrease with increasing
altitude. The property that changes most strikingly with altitude is air temperature. Unlike
the change in pressure and density, changes in air temperature are not regular. A change in
temperature with distance is called atemperature gradient.
The atmosphere is divided into layers based on how the temperature in that layer changes
with altitude, the layer’s temperature gradient (Figure15.4). The temperature gradient of
each layer is different. In some layers, temperature increases with altitude and in others it
decreases. The temperature gradient in each layer is determined by the heat source of the
layer. The different temperature gradients in each of the four main layers create the thermal
structure of the atmosphere.
There are several layers of the atmosphere. The first layer is thetroposphere. It is the
closest to the ground and is sometimes referred to as the lower atmosphere. The second
layer is thestratosphere, and is sometimes referred to as the upper atmosphere. Most of
the important processes of the atmosphere take place in one of these two layers.
Troposphere
About three-fourths of the gases of the atmosphere are found in the troposphere because
gravity pulls most of the gases close to the Earth’s surface. As with the rest of the atmo-
sphere, 99% of the gases are nitrogen and oxygen.
The thickness of the troposphere varies around the planet. Near the equator, the troposphere
is thicker than at the poles, since the spinning of the Earth tends to shift air towards the
equator. The thickness of the troposphere also varies with season. The troposphere is thicker
in the summer and thinner in the winter all around the planet. At the poles in winter, the
atmosphere is uniformly very cold and the troposphere cannot be distinguished from other
layers. The importance of this feature of the atmosphere will become clear when we learn
about ozone depletion.
Earth’s surface is a major source of heat for the troposphere. Where does the heat come
from? Nearly all the heat comes from the sun, either directly or indirectly. Some incoming
sunlight warms the gases in the atmosphere directly. But more sunlight strikes the Earth’s