The Sun's Energy on Earth
Have you ever been to the beach on a hot summer day and tried to walk across the sand on your
bare feet? It can be so hot that you run to the water and step in the ocean to cool off your burning
feet. If the sand and water are exposed to the same heat source, why is there a difference in the
way they feel? Have you ever taken a walk on that same beach later that night? The sand feels
chilly and refreshing on your feet, while the water feels warm in comparison. How can this be
explained?
Heat vs. Temperature
Before investigating heat transfer from the Sun to Earth, you must understand what is meant by
heat. Heat and temperature are often used interchangeably, but the two terms have different
meanings.
Heat is a form of energy caused by the motion of atoms. The faster atoms move, the more heat
energy they contain. Heat is a measure of all the kinetic energy of all atoms in a sample of matter.
Temperature is a measure of the average speed of atoms, and it is measured in units
called degrees. The faster the atoms are moving, the higher the temperature is.
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The Sun's Energy on Earth
Which contains more heat: a large pot full of boiling water or a small mug full of boiling water?
Which contains more heat: a 25-yard swimming pool heated to 75°F, or a large pot of boiling water
heated to 212°F?
Which has a higher temperature: a large pot of boiling water or a small mug of boiling water?
Which has a higher temperature: a 25-yard swimming pool heated to 75°F, or a large pot of boiling
water (212°F)?
Many people do not understand how heat energy is transferred. Have you ever heard someone
say, “Quick, shut the door. Don’t let the cold air in.”? In reality, they should say, “Quick, shut the
door. Don’t let the heat out.” Heat energy is transferred between objects with differences in
temperature. Heat energy moves from objects with higher temperatures to objects with lower
temperatures in an effort to reach equilibrium.
Think back to the scenario from the Reflect section. Since the Sun is
the main source of energy for Earth, how does the Sun transfer its heat
energy to Earth? And why does it appear to transfer heat unevenly or at
different rates? Heat is transferred between bodies in three different
manners: radiation, conduction, and convection.
Radiation
Radiation refers to heat energy transfer that occurs through
electromagnetic radiation. Radiation travels in waves. An example of
heat transfer from radiation is the heat you feel from a fire. If you stand
near a campfire, you feel the heat radiating from it, even though you are
not actually touching it.
How does radiation affect how the Sun warms Earth? Radiation is the
method by which the Sun’s heat reaches Earth. It travels through outer
space in waves. Radiant heat from the Sun is the source of most of
Earth’s heat energy.
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The Sun's Energy on Earth
Conduction
Conduction refers to the transfer of heat energy from one object to
another through physical contact. Heat is passed from one object to
another when objects are touching and there is a temperature difference
between objects. Heat energy passes from an object with a higher
temperature to an object with a lower temperature until the objects are
the same temperature.
Conduction is an especially effective method for heating solids because
atoms in solids are packed close together and solids have a fixed
structure that keeps atoms contained. An example of conduction is a
metal frying pan placed on an electric burner. The atoms of the pan are
heated by conduction. First, the atoms touching the burner heat up and
begin to vibrate faster. Then the heated atoms touch atoms next to them,
which heats those atoms and cause them to vibrate faster. This action is
repeated over and over until all of the atoms in the metal are the same
hot temperature—like a crowd at a football game doing the wave.
The Sun heats Earth by conduction in this same manner. Although
sunlight travels through space as radiant energy, once sunlight falls on
the surface of Earth and heats it, heating by conduction begins. Once an
atom has been heated, it speeds up and heats the atoms that touch it,
and they heat the atoms they touch.
How could the method of conduction result in uneven heating for solids
versus liquids or gases? The answer lies in the density and the structure
of the atoms in each substance. Because atoms of solids are packed so
closely together and are fixed in place, it is easier for atoms to become
heated, vibrate more rapidly, touch the atoms next to them, transfer heat energy, and repeat this
process quickly, furthering the heating process. Properties of liquids mean the atoms in liquids are
not confined to a certain shape or structure and are not packed as closely together as solids. As
liquids flow, it takes longer for them to “bump into” an atom that is a heat source and gain heat
through direct contact. This is even more apparent with gases, in which the atoms are spread even
farther apart since gases have no fixed shape or volume. Therefore, the heat transfer of conduction
works fastest to heat solids; liquids are heated more slowly; and gases are heated the most slowly
by conduction. The same is true in reverse as well. Solids lose heat rapidly when a heat source is
removed.
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The Sun's Energy on Earth
When we apply what we know about conduction to the Sun’s heating of Earth, the same principles
about the rates of heating solids, liquids, and gases apply. For instance, sand (a solid) is heated
more rapidly than water (a liquid), and water is heated more rapidly than air (a gas) when exposed
to direct sunlight. The heating of Earth’s environment is not that simple, however, because
conduction typically does not happen in isolation. It is usually one aspect of heat transfer that
occurs in tandem with convection and radiation.
Convection
Convection refers to the transfer of heat energy through the
motion of fluids, such as air and water. For example, to boil a
pot of water, conduction is used to heat the pot, but the water in
the pot is heated by convection. The water molecules at the
bottom of the pot receive heat energy, move faster, and expand.
When the heated water expands, it becomes less dense
because it has the same number of molecules (or mass) but
takes up a larger space (or volume). Since density is mass per
volume, the density of heated water is less than the density of
cooler water. Because the heated water becomes less dense
than the cooler water at the top of the pan, the heated water rises, causing the cooler water at the
surface to sink because it is denser. The cooler water that sinks to the bottom then becomes
heated and rises, resulting in convection currents that circle in the pot and heat the water.
How does convection relate to the Sun’s heating of Earth? Even though the Sun’s energy travels to
Earth as radiant energy, convection plays a major role in the heating and cooling of Earth, due to
Earth’s atmosphere being composed of gases and Earth’s surface being largely covered with
water. Both air and water are good candidates for heat transfer through convection since they both
flow.
The Sun heats the surface of Earth, and the air near the surface of Earth is heated by conduction.
This air warms, expands, and rises, taking the place of cooler air. Where warm air is rising, air
pressure is lessened, allowing for cooler air from an area of higher pressure to rush into the
vacated spot. Eventually, the warm air that rose cools (because it is away from the heated surface
of Earth), and the cool air that sank is heated (because it is close to Earth’s heated surface). The
newly heated air rises, while the risen air that has cooled sinks, repeating the convection process
and causing convection currents.
The surface of the ocean is warmed by radiant heat from the
Sun, causing water molecules to heat and evaporate into the air
as water vapor, joining in the process of convection with the air.
This also activates the water cycle. In addition, the heating of
the oceans causes convection to occur, which is one of the
causes of ocean currents that help regulate Earth’s climate.
Heat transfer through convection is not successful with solids because molecules in solids are fixed
in place and cannot rise and fall when heated and cooled. Because solids do not flow, they cannot
experience heating through the convection process.
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The Sun's Energy on Earth
Methods of heat transfer work together.
Radiation, conduction, and convection often work together to
efficiently heat matter. Think of how sap is boiled to make maple
syrup. A fire is built, and the pot is hung over the fire. Radiation
from the fire heats the pot. The liquid at the bottom of the pot is
heated by conduction. The rest of the liquid is then heated by
convection until it is boiling and uniformly heated.
The same “cooperation” happens with heat transfer from the
Sun to Earth, involving radiation, conduction, and convection.
Radiation transports the Sun’s heat energy to Earth and heats
the surface of Earth. Conduction then transfers heat energy through atoms that make physical
contact. Conduction is most effective with solids, such as sand at the beach or sidewalks or
streets, because atoms are close together. Once the surface of a solid or liquid is heated, other
matter (solid, liquid, or gas) that comes into contact with it is also heated. Convection then occurs
with liquids and gases as changes in density happen due to uneven heating. This, in turn, results in
such phenomena as the water cycle, winds, ocean currents, and other processes here on Earth.
Look at the images below. Convection near coastlines causes “sea breezes” to occur during the
day and “land breezes” to occur at night. Write a paragraph to explain your reasoning behind this
phenomenon. Hint: Think of unequal heating of liquids and solids caused by conduction. Also,
remember that when air rises, air pressure in that area drops, allowing for air from an area of
higher pressure to rush in.
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