Heat Transfer in the Atmosphere
Heat can move in three different ways. We’ve already examined radiation, in which electro-
magnetic waves transfer heat between two objects. Conductionis a type of heat transfer
that occurs when heat moves from areas of more heat to areas of less heat by direct contact.
Warmer molecules vibrate more rapidly than cooler molecules. They collide directly with
other nearby molecules, giving them some of their energy, which transfers heat. When all the
molecules are moving at the same rate, the substance is the same temperature throughout.
Heat in the atmosphere is transferred by conduction. This is more effective at lower altitudes
where air molecules are packed more densely together. Conduction can transfer heat upward
to where the molecules are spread further apart. It can also transfer heat laterally from a
warmer to a cooler spot, where the molecules are moving less vigorously.
The most important way heat is transferred in the atmosphere is by convection currents.
Convection is the transfer of heat by movement of heated materials. The radiation of heat
from the ground warms the air above it. This warmer air is less dense than the air above
it and so it rises. As the heated air rises it begins to cool, since it is further from the heat
source. As it cools, it contracts, becomes denser and sinks. Air moves horizontally between
warm, rising air and cooler, sinking air. This entire structure is aconvection cell.
Heat at Earth’s Surface
Not all energy coming in from the Sun makes it to the Earth’s surface. About half is filtered
out by the atmosphere. Besides being absorbed by gases, energy is reflected by clouds or
is scattered. Scattering occurs when a light wave strikes a particle and bounces off in some
other direction. Of the energy that strikes the ground, about 3% is reflected back into the
atmosphere. The rest warms the soil, rock or water that it reaches. Some of the absorbed
energy radiates back into the air as heat. These infrared wavelengths can only be seen by
infrared sensors.
It might occur to you that if solar energy continually enters the Earth’s atmosphere and
ground surface, then the planet must always be getting hotter. This is not true, because
energy from the Earth escapes into space through the top of the atmosphere, just as energy
from the Sun enters through the top of the atmosphere. If the amount that exits is equal to
theamountthatcomesin, thenthereisnoincreaseordecreaseinaverageglobaltemperature.
This means that the planet’s heat budget is in balance. If more energy comes in than goes
out, the planet warms. If more energy goes out than comes in, the planet cools.
To say that the Earth’s heat budget is balanced ignores an important point. The amount of
incoming solar energy varies at different latitudes (Figure15.17). This is partly due to the
seasons. At the equator, days are about the same length all year and the Sun is high in the
sky. More sunlight hits the regions around the equator and air temperatures are warmer.
At the poles, the Sun does not rise for months each year. Even when the Sun is out all day