Earth and the Sun's Energy

Guiding Questions

●How does the sun’s energy reach and move through Earth’s atmosphere?

●How is heat transferred in Earth’s atmosphere?

●What role does the atmosphere play in allowing life to thrive on Earth?

Vocabulary

electromagnetic wave

greenhouse effect

thermal energy

convection

conduction

radiation

Energy from the Sun

Most of the energy that is moving within Earth’s atmosphere and
across Earth’s surface comes from the sun. The sun’s energy
travels to Earth as electromagnetic radiation, a form of energy that
can move through the vacuum of space. Electromagnetic waves
consist of an electric field and a magnetic field.

When you use a microwave oven or watch television, you are
using the energy created by electromagnetic waves. The waves
are classified according to wavelength, or distance between wave
peaks. Most of the electromagnetic waves that travel from the sun
and reach Earth are in the form of visible light, which you can see
in Figure 1, and infrared radiation. A smaller amount arrives as
ultraviolet (UV) radiation

Fill in the Blank

Sunlight (warms / cools) the ice, causing it to melt

Sunlight and the Atmosphere

In order for the sun’s energy to reach Earth’s surface and sustain life, it
must first get through the atmosphere. Earth’s atmosphere is divided into
layers based on temperature. Some sunlight is absorbed or reflected by
the different levels of the atmosphere before it can reach the surface, as
shown in Figure 2.

Some UV wavelengths are absorbed by the topmost layer of the
atmosphere, called the thermosphere. More UV energy, along with some
infrared energy, is absorbed in the next layer, the mesosphere. Below
that, in the stratosphere, ozone absorbs more infrared and UV energy.
Without the ozone layer, too much UV radiation would reach Earth’s
surface and threaten the health of organisms. However, the amount of
UV radiation that reaches Earth’s surface can still be damaging, which is
why humans benefit from wearing clothing, sunscreen, and sunglasses.

By the time sunlight reaches the troposphere, there is some infrared
radiation, some UV radiation, and visible light. Some light has been
reflected into space by clouds. The daytime sky on a cloudless day
appears blue because gas molecules scatter short wavelengths of
visible light, which are blue and violet, more than the longer red and
orange wavelengths.

Open Ended

What would happen if all of the sun’s energy were to reach Earth?

Earth’s Energy Budget

Of the radiation that travels from the sun to the
troposphere, only about 50 percent is absorbed by land
and water and converted, or transformed, to heat. The rest,
as shown in Figure 3, is reflected by clouds and other
particles in the atmosphere (25%), absorbed by gases and
particles (20%), or reflected by the surface itself (5%).
Snow, ice, and liquid water reflect some sunlight back into
the atmosphere, where some will be absorbed by clouds
and particles that the energy missed on the way down.

Only a tiny fraction of the visible light that reaches Earth’s
surface is transformed to chemical energy in plants and
other photosynthetic organisms. The rest is absorbed by
Earth and re-emitted into the atmosphere as infrared
radiation. Earth’s surface absorbs and re-emits equal
amounts of energy so that its energy remains in balance
over time.

Labelling

Label the different percentages of energy that are absorbed or reflected.

Drag labels to their correct position on the image

25%

20%

5%

50%

The Greenhouse Effect

Have you ever been to a greenhouse to buy plants? The glass walls and roof of
a greenhouse allow sunlight inside. Some sunlight is absorbed by plants and
transformed into chemical energy. Most of the sunlight is converted to heat.
Much of the heat is contained by the glass panes of the greenhouse, keeping
the interior at an acceptable temperature for plant growth.

Earth’s atmosphere plays a similar role. Sunlight is absorbed and transformed
into heat within the atmosphere and in the materials at Earth’s surface, such as
rock and water. The surface reradiates all of that energy, and Earth’s total
energy remains in balance over time. (Otherwise, Earth would continually heat
up and turn into molten rock.) Gases in the atmosphere trap some of the heat
near Earth’s surface, while some heat escapes into space. This greenhouse
effect is shown in Figure 4.

Overall, Earth’s atmosphere keeps our planet at a temperature that is adequate to support life.
Organisms are adapted to specific ranges of temperatures. Surface features such as the sea level
and the amounts of trapped ice have been relatively constant for thousands of years. However,
changes to the composition of the atmosphere—those gases that absorb the infrared energy
radiated from Earth’s surface—can result in changes in temperature. Most scientists who study the
atmosphere and the climate think that humans have been enhancing the greenhouse effect by
increasing the amounts of carbon dioxide and methane in the atmosphere. This has caused an
increase in the average temperature of Earth, which in turn is causing changes to sea level and
melting ice in polar regions and in glaciers.

Heat Transfer in the Atmosphere

All matter is made up of particles that are constantly moving.
The faster the particles move, the more energy they have.
Temperature is the average amount of energy of motion of
each particle of a substance. Thermal energy is the total
energy of motion in the particles of a substance.

It may seem odd to think that particles in solids are moving,
but they are vibrating in place. Even the water molecules in a
block of ice, or the atoms of iron and carbon in a steel beam,
are moving ever so slightly.

When a substance reaches its melting point, the substance
has enough energy of motion to reach a new state—liquid.
And when the substance reaches its boiling point, it changes
into a gas, which has even more energy of motion. The
energy that first reaches Earth as sunlight drives many
processes on Earth, including the freezing, melting, and
evaporation of water.

Methods of Heat Transfer

We often talk about heat as though it is the same as thermal
energy. Heat is actually energy that transfers into an object’s
thermal energy. Heat only flows from a hotter object to a
cooler one. Heat transfers in three ways: convection,
conduction, and radiation, as shown in Figure 5.

Heat Transfer at Earth’s Surface

The sun’s radiation is transformed at Earth’s surface into thermal energy.
The surface may get warmer than the air above it. Air doesn’t conduct
heat well. So only the first few centimeters of the troposphere are heated
by conduction from Earth’s surface to the air. When ground-level air
warms up, its molecules move more rapidly. As they bump into each
other, they move farther apart, making the air less dense. The warmer,
less-dense air rises, and cooler, denser air from above sinks toward the
surface.

The cool air then gets warmed by the surface, and the cycle continues. If
the source of heat is isolated in one place, a convection current can
develop. This occurs in Earth’s atmosphere as a result of radiation,
conduction, and convection working together. The horizontal movement
of the convection current in the atmosphere is what we call wind.
Convection currents are especially powerful if Earth’s radiant surface is
much warmer than the air above it. This is related to why storms can
arise much more suddenly and be more severe in warmer regions of
Earth. For example, hurricanes tend to form in tropical areas where the
sea is very warm and the air above it is relatively cool.

Draw

Use different arrows or other illustration techniques to draw the three types of heat transfer that are occurring at this beach. Label each type of heat transfer.

Multiple Choice

What are the three ways that heat can transfer?

• Reflection, absorption, transmission
• Evaporation, convection, radiation

• Expansion, compression, diffusion
• Conduction, convection, and radiation

Open Ended

If the amounts of greenhouse gases such as carbon dioxide and methane continue to increase in the atmosphere, what will happen to the average temperature of Earth? Explain.