How Solar Radiation Heats Earth’s Surface and Atmosphere
Energy from the sun travels to Earth and heats the surface. Different parts of the surface
receive different amounts of solar energy. Generally, the equator receives the most energy
because the sun’s rays strike the surface most directly at the equator. The amount of solar
radiation received at higher latitudes depends on the season. The seasons are caused by the
direction Earth’s axis of rotation is pointing relative to the sun. The axis is always tilted 23.5
degrees relative to Earth’ plane of orbit around the sun, but the north and south poles may tilt
toward or away from the sun.
When it is summer in the Northern Hemisphere, the north pole tilts toward the sun. As a result,
the sun’s rays strike the Northern Hemisphere more directly at this time of year. Light from the
sun is more concentrated where the rays are more direct, so the Northern Hemisphere is
heated more during the summer than at other times of year. The opposite is true when it is
winter in the Northern Hemisphere. The north pole tilts away from the sun, so the sun’s rays
strike the Northern Hemisphere less directly. Sunlight is less concentrated where the rays are
less direct. As a result, Northern Hemisphere is heated less in the winter than at other times of
year. The Southern Hemisphere has seasons for the same reasons, but the seasons are reversed
relative to the Northern Hemisphere.
Much of the heat that Earth’s surface receives by radiation from the sun is radiated back into
the atmosphere where it heats the troposphere. Radiation is the transfer of energy between
two objects by electromagnetic waves. Heat from Earth’s surface also travels to the
troposphere by conduction. In conduction, heat is transferred between two objects that are in
direct contact. Molecules of the warmer object vibrate rapidly and collide with nearby
molecules, transferring their energy. In the atmosphere, conduction is more effective at lower
altitudes where air density is higher and gas molecules are closer together.
Heat is transferred throughout the troposphere by convection. Convection occurs when
molecules of a substance—in this case, gases in the atmosphere—move in currents because of
differences in density. Air near the surface warms because of heat transferred from the surface.
The warm air is low in density, so it rises. The rising warm air cools, becomes denser, and sinks
to the surface again. These air movements form convection currents that mix air throughout
the troposphere.
Reading Questions
1. Explain why the Northern Hemisphere is warmer in the summer than in the winter.
2. Describe how heat from Earth’s surface is transferred to the atmosphere.
3. How is heat transferred throughout the troposphere?
Circle the letter of the correct choice.
1. Water in a lake stays cooler than nearby land on a hot sunny day because water has a higher
b. insolation.
c. latent heat.
d. specific heat.
2. Earth’s seasons occur because of changes in the
a. direction Earth is tilting.
b. speed of Earth’s revolution.
c. distance of Earth from the sun.
d. solar radiation produced by the sun.
4. Heat transfer by the movement of molecules in currents is called
a. radiation.
b. conduction.
c. reflection.
d. convection.
5. Only about half of the solar radiation that strikes the top of the atmosphere reaches the
ground because of
a. absorption.
b. reflection.
c. scattering.
d. all of the above
6. Which of the following is the best analogy for how greenhouse gases affect Earth’s climate?
a. wool blanket
b. heating pad
c. radiator
d. candle flame
7. All of the following greenhouse gases are naturally occurring in the atmosphere except
a. CO2.
b. H2O.
c. NO2.
d. CFCs
Definitions
_____ 1. heat that is taken in or released when matter changes state
_____ 2. amount of solar radiation that reaches a given area in a given time
_____ 3. measure of how fast atoms of a material are vibrating
_____ 4. transfer of energy by electromagnetic waves
_____ 5. measure of the total energy of a substance
_____ 6. amount of energy needed to raise the temperature of 1 gram of a substance by 1 °C
Terms
b. heat
c. latent heat
d. specific heat
e. insolation
f. temperature
g. radiation