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The Sun and Convection Currents
SECTIONS
The Sun's Energy
Why Air Moves - Part 1: Pressure
Belts
Why Air Moves - Part 2:
Coriolis E!ect
Types of Wind
Currents in the Ocean
The Sun's Energy
Have you ever stood on a sidewalk in a hot, sunny day while now wearing shoes and
felt the warmth of the sidewalk? The sidewalk was heated as it absorbed the Sun's
energy. The Earth's atmosphere is heated in by the transfer of energy from the Sun.
Earth receives its energy from the Sun. The Sun's energy travels to Earth in the form
of radiation. Radiation is the transfer of energy as electromagnetic waves. Radiant
energy reaches the Earth in the form of both visible and invisible electromagnetic
waves. Although the Sun releases huge amounts of energy, the Earth receives only a
small amount of this energy. The diagram below shows what happens to the Sun's
energy once it enters the atmosphere.
Source:
Questionable Choices,
sldownard, Flickr
What percent of the Sun's energy is absorbed by Earth's surface?
Check Your Answer
After the energy from the Sun has reached the Earth, it is transferred to other objects in one of two ways.
Conduction is the transfer of thermal energy (heat) from one material to another by direct contact. This type of
transfer is like the sidewalk example from above. Thermal energy or heat always moves from warm to cold
areas.
On a hot day, the sidewalk heats up from the Sun. When you walk across the sidewalk in your bare feet, your
feet touch the sidewalk and the sidewalk passes heat to your feet through conduction.
Most thermal energy in the atmosphere moves by convection. Convection is the transfer
of thermal energy by the movement of a liquid or gas. Convection works when a liquid
or gas is unevenly heated. Hot liquids (and gases) are less dense and rise, causing
convection currents. The warmer section of the material will rise while the cooler part
sinks. This creates a current of warmer material going up and a current of cooler material
going down. An example of this is boiling water in a pan. The pan heats up the bottom
of the water. The warmer water then moves to the top of the pan, while the cooler water
sinks.
Why Air Moves - Part 1: Pressure Belts
Warm air is less dense and rises while cool air is denser and sinks. Look at the diagram below. Imagine that the
circles in the Erlenmeyer flasks are air molecules. The image on the left shows the molecules when the air is
cool, and the image on the right shows the molecules when heated. When the molecules are heated, they move
faster and move apart, causing them to be less dense and rise.
Source: Cool and heated gas, NOAA
When the warm air rises and the cool air sinks, convection currents are created. Since gas molecules are not
visible, it is hard to imagine convection currents. If you place food coloring in the bottom of a container and
apply heat under the food coloring, you can see convection currents in action. Watch this video below to see a
demonstration of convection currents.
Convection Current Demo
Source: Convection Current Demo, Telsclear, YouTube
Wind is the moving of air and is created by differences in air pressure. The
greater the differences in air pressure, the faster the wind moves. The differences
in air pressure are caused by the unequal heating of Earth. If you have ever
watched the weather, you know that the Earth does not heat evenly. In the
summers in Texas, the temperatures can be well above 100°F, while in the northern states, the temperatures are
in the 70s. Due to the tilt of the Earth, the air around the equator is consistently warmed. This warm air is less
dense and rises. As the air rises, it creates an area of low pressure. At the poles, the air is colder and more dense
so the air sinks. This cold, sinking air creates areas of high pressure. Pressure differences in the atmosphere
cause the air to move. Winds generally move from the poles to the equator because air moves from areas of high
pressure to areas of low pressure.
Source Adapted From: Earth, National Energy Education Development Project
The pattern of pressure is not as simple as the diagram above. As warm air rises, it begins to cool and eventually
stops rising. Then some of the cool air begins to sink. This happens at about 30° north and 30° south latitude.
This sinking air causes high pressure belts in these latitudes.
Cold air sinks at the poles. As the air moves away from the poles and along Earth's surface, it begins to warm
and the pressure drops. This drop in pressure creates low pressure belts around 60° north and 60° south latitude.
Convection cells, which are circular patterns of wind, are caused by this rising and sinking of air. The image
below shows the convection cells.
Source Adapted From: Earth, National Energy Education Development Project
Directions: Complete the following statements by selecting the correct terms.
Well Done!
Submit
1. Warm air is less
dense and rises
while cool air is more
2. The rising and sinking of air creates convection
3. The greater
Reset
dense and sinks.
currents.
the difference in air pressure, the faster the wind moves.
4. When air rises, it creates an area of low
When air sinks, it creates an area of high
5. Air moves from areas of high
pressure.
pressure.
pressure to areas of low
pressure.
Why Air Moves - Part 2: Coriolis E!ect
Remember that Earth is not still - it rotates. The movement of wind is affected by the rotation of Earth. Because
of this, winds do not blow directly north or south. The Coriolis effect causes the winds to change direction.
Watch this video for an introduction to the Coriolis effect.
Coriolis Effect
Watch later
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Source: Coriolis Effect, ParkerScience, YouTube
Because of the Coriolis effect, winds in the Northern Hemisphere curve to the right while winds in the Southern
Hemisphere curve to the left. To see how the Coriolis effect changes the winds, watch this short video.
coriolis effect (2-11)
Source: Coriolis Effect (2-11), samwsm1, YouTube
The Coriolis effect is the perceived change in the direction of the currents caused by Earth's rotation. We have
to look at the direction that the wind was moving due to uneven heating. Click the boxes below to look again at
the diagram of the wind direction because of uneven heating. Remember, because of the Coriolis effect, winds
in the Northern Hemisphere curve to the right while winds in the Southern Hemisphere curve to the left from
the direction that the wind was moving.
Coriolis Effect
Watch later
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Directions: Complete the following statements by selecting the correct terms.
coriolis effect (2-11)
Types of Wind
There are two main types of winds: global winds and local winds. Both types of winds are caused by uneven
heating of the Earth's surface and by pressure differences. Let's take a look at global winds first.
Global Winds
Global winds appear all over the world because of the following reasons:
Uneven heating of the Earth's surface
Pressure differences
Coriolis Effect
Global winds are part of a pattern of air circulation that moves across Earth. The winds travel longer distances
than local winds and each type of global wind travels in a specific direction. The three global winds are trade
winds, westerlies, and polar easterlies.
To learn more about the types of winds, click on the boxes below.
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Use what you learned about the global winds to label the following diagram. Drag the terms to
the correct location. Some have already been done for you.
WESTERLIES
SOUTHEAST TRADE
WINDS
HORSE LATITUDES
NORTHEAST TRADE
WINDS
POLAR EASTERLIES
DOLDRUMS
Local Winds
Local winds are influenced by the geography of an area. An area's local geography, such as a shoreline or a
mountain, produces temperature differences that cause local winds, such as land and sea breezes. Uneven
heating of the earth can happen on a local level as well as a global level.
During the day, air above land heats more quickly than
air above the water. The warmer air is less dense and
the cooler, denser air moves toward the coast, causing
a sea breeze.
At night, the air over land cools more quickly,
reversing the process. The cool air moves toward the
water and producing a land breeze.
Mountain and valley breezes are another example of local winds caused by the geography of an area. During the
day, the sun heats the valley floor and warms the air above it. Warm air from the valley moves upward and
creates a valley breeze. At night, the mountains cool faster than the valleys. Cold air sinks form the mountain
peaks, creating a mountain breeze.
Source: Valley and Mountain Breezes, Kids Geo.com
Currents in the Ocean
The Sun also provides the energy that drives convection in the ocean
and produces ocean currents. There are two main types of ocean
currents: surface currents and deep currents.
Surface currents are stream-like movements of water that occur at or
near the surface of the ocean. Surface currents flow in distinct
patterns around the Earth. Surface currents are controlled by four
factors: global winds, the Coriolis effect, continental deflections, and
the water temperature.
Global Winds
Near the equator, the winds blow ocean water east to west, but closer to the poles, ocean water is blown west to
east.
Coriolis Effect
Just as with winds, the rotation of the Earth causes surface currents to move in curved paths rather than straight
lines. Because of the Coriolis effect, currents in the Northern Hemisphere turn clockwise while currents in the
Southern Hemisphere turn counterclockwise.
Continental Deflections
If Earth's surface were covered only with water, currents would travel in a fixed pattern across the Earth, but
Earth’s surface is not covered only with water. When currents meet landforms, they change direction.
Water Temperature
Warm water currents begin near the equator and carry warm water to other parts of the ocean. Cold water
currents begin closer to the poles and carry cool water to other parts of the ocean. The map below shows the
Earth's surface currents. Warm water currents are shown in red, and cold water currents are shown in blue.
Source: Earth's Surface Winds, TERAA Environmental Research Institute
Deep currents are stream-like movements of ocean water far below the ocean surface. Deep currents are not
controlled by wind or the Coriolis effect. Deep currents are mainly controlled by increases in water density.
Temperature and salinity affect the density of ocean water. Ocean water gets denser when it becomes saltier or
gets colder. Cold air chills the water molecules at the surface, causing them to move closer together. As a result,
the volume of the water decreases, and the water becomes denser. Starting at a depth of about 200 meters, the
water temperature becomes colder as the depth increases. The denser water moves along the ocean floor and
eventually travels toward the equator in the form of a deep current. Less-dense water always flows on top of
denser water.
The diagram below shows how deep currents and surface currents interact and trade places.
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