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Air: The Medium for Weather & Climate
A very mature storm system actively spinning off funnel
clouds and tornadoes
Lesson 1
Air Pressure
Exploration Phase:
Examine the diagram below. Suppose you placed a jar over a lit candle in a pan of
water. What do you think would happen?
Write your predictions below.
After you have completed your predictions, perform the above experiment. Write
your observations below.
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If you were careful with your observations you noticed that several things
occurred. Two questions that might come to mind are (1) What caused the flame to
go out, and (2)What caused the water to rise?
What are some possible reasons for each of these observations? Carefully repeat
the experiment several times only after you have provided several reasons for
the phenomenon. Write additional observations below.
Reasons
Additional Observations
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Consider the following hypotheses...
Hypothesis 1
If oxygen is being used and thereby decreasing the
volume of air in the jar,
Experiment
and we compare the water rise with 1 candle and 3
candles,
Expected Outcome
then the water level should be the same for both 1
candle and 3 candles because the amount of oxygen
remains constant.
In other words, Oxygen is being used thereby decreasing the volume of air in the
jar, then increasing the number of candles should not increase the level of the
water. Explain why this statement is or is not true.
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Suppose that when oxygen is being used, carbon dioxide is being produced, thereby
maintaining the amount of air in the jar and the heat/cool process is causing water
to rise…
Hypothesis 2
If the heat from the candles causes the air to expand,
pushing air out, and the air cools after the candle goes
out causing a partial vacuum, allowing water to be
pushed back in
Experiment
and we compare the water rise with 1 candle and 3
candles,
Expected Outcome
then the water level should rise higher with 3 candles
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than 1 candle because of the greater amount of heat
produced.
In other words when the jar is placed over the candle the temperature of the air
increases causing the air to expand. Air moves out of the jar which decreases the
pressure inside the jar. Water moves into the jar because the pressure in the jar
is less than the pressure outside the jar. If this true, what would happen if you
increased the number of candles? Explain.
Perform the above experiment. Record your results below.
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Do the results of your experiment support the first or second hypothesis? Explain.
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CONCEPT INTRODUCTION
The water moved into the jar because of a change in air pressure. Scientists use
the molecular theory of gases to explain air pressure, which assumes that air is
composed of moving particles that have weight and can bounce into objects (such
as water) and push them out of the way. Explain the exploration activity using the
terms air pressure and particles of air.
A common misconception is that water is sucked up into the jar. The molecular
theory of gases implies that suction does not exist. Particles can not pull they can
only push. The water was pushed into the jar by the higher pressure moving
particles on the outside of the jar rather than being sucked upward by some
nonexistent force inside the jar. Therefore, science does not "SUCK" (a joke).
Suck is a common word in our vocabulary that helps us explain common everyday
events. Suck is not a scientific term, it is a common everyday term.
Explain, using scientific terms, how one removes water from a cup using a straw.
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APPLICATION 1
Use the same set up as in exploration minus the candles. With a syringe and piece
of tubing fill the jar with water while leaving the neck of the jar in the pan of
water. Explain how you filled the jar using scientific terms.
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APPLICATION 2
Fill a glass completely with water. Cover the glass firmly with the palm of your
hand and quickly turn the glass upside down as illustrated below. Remove the palm
of your hand carefully from below the cardboard and record your observation.
Force pushing down
Force pushing up
Repeat the experiment with the glass half full of water. Did you obtain the same
result? Now replace the cardboard with a piece of aluminum sheet metal, fill the
glass completely with water and repeat the experiment. Do the experiment a
fourth time but with the glass half full of water and record your observations.
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Thinking Activity:
1. There are two forces acting on the index card; one pushes up and one pushes
down (see the arrows in the above diagram).
a. What is producing the force that is pushing down on the card?
b. What is producing the force that is pushing up on the card?
c. How do you explain the “mystery” of the water remaining in place inside the
inverted glass?
d. Why was the result for the experiment was different the fourth time when
the glass was filled halfway and a piece of aluminum was used instead of
cardboard?
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Terms: Combustion, oxygen, carbon dioxide, heat, pressure, expansion,
contraction, vacuum, temperature, air pressure, air particles, molecular theory,
molecules, sunction, cardboard, aluminum sheet metal,
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Teacher Notes:
Explanation: An increase in volume obtained from the bulge in the cardboard
causes the necessary pressure drop in the glass and allows water to remain in the
glass. The experiment works just as well when the glass is full, half full, or almost
empty. But if the cardboard is replaced with a piece of aluminum sheet metal, the
water will remain in the glass only when the glass is full. This is caused by the
difference in flexibility between the cardboard and the aluminum sheet. The
cardboard bulges more easily than the aluminum sheet and thus causes a greater
change in the volume of air in the glass. When an aluminum sheet is used, the
increase in air volume is caused by the slight descent of the water from the
glass.
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Lesson 2
Air Pressure and Altitude
Exploration:
Activitiy 1: Using a hammer and nail, make three holes of the same size in the side
of a tall tin can. You can also use a 1 liter plastic soda bottle if you have a drill to
make holes in the side of the plastic bottle. Punch a hole near the top, another hole
in the middle, and a third one near the bottom of the can. Pour water from a
faucet into the can or bottle and keep it full while the water is running out through
the holes as show in the diagram below.
Q1. What did you observe?
Q2. Explain your observation using the concept of pressure.
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Activity 2: Using a hammer and nail, punch 3 or 4 evenly spaced holes around the
base of a tin can or plastic soda bottle. Fill the can with water over the a sink and
leave the faucet running as shown in the diagram below.
Q1. What did you observe?
Q2. Explain your observation using the concept of pressure.
Activity 3: Insert a small metal or glass tube into a stopper and connect one-end of
a rubber tubing to the glass or metal tube. Make sure the connection is air tight.
Next, fill a flask about a quarter way with water and cork the flask with the
stopper. Connect the other end of the rubber tubing to a hand operated vacuum
pump. Make sure that the connection is air tight. Place the flask on hot plate and
bring the water to boil. Carefully remove the flask from the hot plate just when
the water starts boiling and set it on a pad on your lab table for about a minute.
Begin to operate the hand vacuum as vigorously as you can and watch what happens
to the water in the flask. Record your observations.
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Teacher Notes:
Q1. What does your experiment teach us about the relationship between
temperature and air pressure?
Concept Introduction:
A critically important aspect of air pressure is its systematic decrease with
altitude. As we drive up into the mountains or take off in an airplane, our ears
usually pop several times. This popping happens as our ears adjust to the lower air
pressures at higher elevations. Our ears also pop when we drive down a
mountainside or descend toward the airport in an airplane--our ears are adjusting
to the higher air pressures at lower elevations. You have to be at sea level to
experience an air pressure of 14.2 pounds per square inch. Those who live “a mile
high” (at around 5300 ft), experience an air pressure of only about 13 pounds per
square inch. The higher pressure at lower levels of the atmosphere occurs partly
because air is quite easy to compress. We do it every time we pump air into a
bicycle tire. When we raise the handle, we let in fresh air. When we push the
handle down, we squeeze that air, forcing it into the tire. As we repeat the
process, we squeeze more and more air into the same amount of space (the inside
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of the tire). Liquids, such as the oceans, do not compact as much with increasing
pressure. Gases, on the other hand, are perfectly “compressible.” This means that,
the harder you squeeze them, the smaller and denser they get. REMEMBER: it is
NOT the individual molecules that are expanding and contracting--it is the
distances between them that are getting larger and smaller. This is true no matter
what is causing the expansion/contraction (temperature or pressure changes).
Application Phase:
Combine one teaspoon of baking soda in a beaker or flask with about 100 ml of
vinegar to produce carbon dioxide. Stand 3 or 4 short birthday cake candles in a
line inside a narrow cardboard trough. Seal one end of the cardboard trough and
tilt it at an angle of about 30 – 45 degrees. Use matches to light the candles and
carefully pour the carbon dioxide from the flask into the trough. Observe and
record what happens.
Q1. What do you was responsible for extinguishing the candles?
Q2. Now imagine a similar line up lit candles in front of the freezer or fridge.
What do you think will happen to the candle when the freezer or fridge door is
opened? Explain your answer
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Terms: Pressure, altitude, boiling point, heat, vacuum pump, temperature, carbon
dioxide, baking soda, vinegar, combustion,
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Teacher Notes:
Water boils at 100° C at sea level (that would be 212° F). The boiling point drops
about 1° C for every 165 meters of elevation (1° F for every 889 feet). This means
that the higher you are, the cooler the water is when it boils, so that is why you
have to take longer to cook food at higher elevations (and a pressure cooker helps
you cook faster).
Combining baking soda and vinegar creates a chemical reaction that produces
carbon dioxide gas (CO2). Unlike oxygen gas (O2), CO2 is not readily combusted by
the flame. It is also heavier than the other gasses which make up our atmosphere.
Nitrogen gas (N2 - ~75% of the atmosphere) has an atomic weight of 30; O2 - a
weight of 32 (~20% of air). CO2 has a weight of 44 (C = 12 a.m.u + (2 O = 2X16
a.m.u) = 44). It thus sinks to the bottom of your jar when evolved from the
vinegar/baking soda mixture. If enough is created, it will reach the level of the
flame. When the candle can no longer continue combusting its material with oxygen,
it goes out.
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Lesson 3
Air Pressure & Air Movement
Exploration Phase:
Activity 1:
Equipment:
2 tubes of clear plastic about 3 feet long
2 caps for the tubes
2 ring stands
2 test tube clamps for ring stands
1 pkg of incense sticks (for smoke source)
2 glass beakers
1. Assemble the equipment as indicated in the diagram below.
2. Put caps on top of the tubes. Light incense and fill the tubes with smoke from
the bottom by tilting tubes and inserting stick 3-4 inches up the column.
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3. Tilt the smoke-filled tubes and gently place the glass beaker under them. Fill
one beaker with ice and fill the other with hot water as shown in Figure below.
4. Record your observations for about 5 to 10 minutes. Caps may be taken off
after five minutes to and observe what happens to the smoke.
Questions
Q1. Does the smoke behave air would? Compare the movement of the air in the
tubes.
Q2. Did you observe that the air in one tube sank and the air in the other rose?
Use the concept of atmospheric pressure to explain the two air motions.
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Q3. After this exploration activity, what else can you say about how any cooled air
and any warmed (hot) air should move? Use your observations to support your
answer.
Activity 2:
Equipment:
1 ¾ inch diameter tube of clear plastic about 3-4 feet long
2 caps for the tubes
2 ring stands
2 test tube clamps for ring stands
1 pkg of incense sticks (for smoke source)
1 propane cylinder (or similar source of flame)
1. Set up the tube such that is lies horizontal and clamped to the test tube
stands. Put caps on one end of the tube.
2. Light incense and fill one side of the tube with smoke by inserting the
incense stick 3-4 inches into the tube. Remove the stick after a reasonable
amount of smoke accumulates and place the send cap on the open end of the
tube.
3. Carefully ignite a flame using the propane cylinder and move it close to the
opposite end of the tube. Gently heat the air in the tube for about 5
minutes. Observe and record what happens.
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Q1. Using what you've learned from this experiment, explain what causes the wind
to blow.
Q2. Why do you think that we sometimes have gentle breezes and sometimes have
gale-force winds?
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Concept Introduction:
Air pressure is not the same everywhere. It varies from place to place and
that variation is gradual, not abrupt. For example, if the air pressure is 1007 mb in
one place and 990 mb in another, the air pressure will not suddenly jump from 990
mb to 1007 mb in one spot. Rather, there will be a more-or-less gradual change in
pressure, called a pressure gradient, between the two places (See the diagram
below). The air pressure gradient is analogous to the slope of the ground between a
hilltop and a valley bottom.
A 1007 mb
. 1006 mb
Air Pressure Gradient
. 1005 mb
. 1004 mb
. 1003 mb
. 1002 mb
.1001 mb
. 1000 mb
. 999 mb
. 998 mb
. 997 mb
1007 feet
. 996 mb
A Hilltop
. 995 mb
. 994 mb
. 993 mb
. 992 mb
slope
. 991 mb
B 990 mb
990 feet
B Valley
Analogy to the ground surface
Whenever there is a pressure gradient between two locations, air tries to
move in such a way as to equalize the pressure. For example, the air inside a fully
inflated bicycle tire has a pressure of 65–100 pounds per square inch (depending on
the type of tire). At sea level, the air surrounding this tire has a pressure of only
about 14.2 pounds per square inch. Thus there is a significant difference in air
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pressure inside and outside of the tire. This difference can be maintained only as
long as the tire has no leaks. But if you pop a tire on your bicycle, allowing air to
flow in and out of the tire, air will rush out of the tire until the air pressure inside
the tire is the same as the air pressure outside the tire. Similarly, when you blow
up a balloon and pinch the opening closed, the air pressure inside of the balloon is
higher than the air pressure outside of the balloon. As soon as you let go of the
opening, air rushes out of the balloon until the pressure inside the balloon is the
same as the pressure outside. Similarly, a vacuum-packed can, such as a can of
coffee grounds or tennis balls, has almost no air inside of it and therefore the air
pressure inside the can is extremely low. As soon as you open the can, air suddenly
rushes inside the can until the air pressure inside the can is the same as the air
pressure outside the can.
In summary, air tends to flow from a region of high pressure to a region of
low pressure. This is an incredibly important principle in meteorology. It is THE
explanation for why the wind blows--much more on this later. Whenever there is a
pressure difference between one location and another, we call the force that
tends to move the air from the region of high pressure to the region of low
pressure the pressure gradient force. The steeper the pressure gradient (i.e. the
more rapidly air pressure changes with distance), the stronger the pressure
gradient force and the faster the air will flow. This is analogous to the slope of a
ski hill. The steeper the slope of the hill, the faster your snow board or skis will
carry you down it.
Earth's atmosphere is made of four layers. We will concentrate on just the first
two lowest layers.
(1) The troposphere is the lowermost layer of the atmosphere; it is where we live
and where most interesting “weather” (clouds, wind, storms, etc.) occurs.
(2) The stratosphere is immediately above the troposphere; it contains the ozone
layer.
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The boundary between the troposphere and the stratosphere (called the
tropopause) acts like an almost impenetrable barrier to air; very little air crosses
this boundary. 1 Why? Because the bot-tom of the stratosphere is warmer than the
top of the troposphere and you know how effectively that kind of a temperature
gradient acts to prevent vertical air circulation.
The near impenetrability of the boundary between the troposphere and the
stratosphere is very important when it comes to understanding weather. For
example, when air rises in the troposphere, it hits the tropopause. It cannot push
its way upward into the stratosphere, so it is forced to spread out sideways (see
diagram below). Meanwhile, the rising air leaves behind a region of extra-lowdensity air near the ground that pulls in air from the surrounding area. When air
sinks, the opposite happens (see below). So, in many ways the troposphere behaves
like a pan of water (very little water “jumps” up into the air above; very little air
penetrates down into the water).
Stratosphere
Troposphere
Rising Air
Sinking Air
Troposphere
Ground
1Meteorites, jet planes, rockets and weather balloons have no trouble crossing this
barrier, but air molecules do.
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Application Phase:
In this activity, we model the air movements that occur when air is cold in one
place (for example, at the poles) and hot in another (for example, at the equator).
We use water to represent air in the lower part of the atmosphere (i.e. the
troposphere) because it's easier to observe the motion of water than it is to
observe the motion of air. Keep in mind, however, what each part of the model
represents:
Part of Model
Real Thing That it Represents
Bottom of cake pan
The Earth's surface
Water in the cake pan
The lowermost layer of the atmosphere (the
troposphere)
Air above the cake pan
The stratosphere (the layer of the atmosphere
directly above the troposphere)
Bag of ice
Cold ground near the north (or south) pole
Candles
Warm ground near the equator
Materials: large rectangular clear glass cake pan (15x10x2)
red and blue food coloring
2 eye droppers
3 small candles (the kind that are used to keep food warm)
box of matches
three Styrofoam cups
ice
gallon-size Ziploc bag
red and blue colored pencils
Activity
1. Fill the cake pan with water.
2. Place the three Styrofoam cups, upside down on the lab table, forming a
triangle that the cake pan can rest on and remain stable. Place the cake pan on
the three cups.
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3. Light the candles and place them in a line underneath one end of the cake pan.
This end of the cake pan represents the warm equatorial regions of the earth.
4. Put some ice in the large Ziploc bag and place the bag of ice in the cake pan on
the side opposite the candle. This end of the cake pan represents the cold polar
regions of the earth. The set up should look like the diagram below.
Bag of Ice
Candles
Cup
Cup Cup
Questions
1. On the diagram above, use arrows to show any (invisible) motion of the water
that you think may be occurring, due to the temperature differences across the
cake pan.
2. Explain why you think the water is moving this way.
More Activity
5. After the cake pan has rested undisturbed for a few minutes, place several
drops of blue food coloring in a line along the bag of ice, near where it touches
the water (see diagram below).
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6. Carefully place several drops of red food coloring into the water in a line along
the BOTTOM of the cake pan directly above the candles (see diagram below).
7. Watch the motion of the colored water and answer the following questions.
More Questions
3. On the diagram below, use arrows and colored pencils to show the motion of the
red and blue water in the cake pan.
4. Did the motion you observed match your predictions (Question #1 above)?
If not, explain why the motion that you DID observe occurred.
5. What can you conclude about the density of the water near the candle as
compared to the density of the water near the ice? Where is the water more
dense? Why?
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For questions 6–13 below, imagine that you are a tiny water-breathing person,
usually walking around on the bottom of the cake pan but sometimes “flying” up in
the water in an airplane.
6. As you fly around in the “air” (the water in the cake pan), near the top of the
troposphere, where do you feel the highest water pressure, near the ice or near
the candle? Why? Hint: remember the recent lecture on wind.
7. On the diagram of the cake pan (previous page), place an “H” where the
pressure aloft (i.e. near the top of the water) is highest and an “L” where the
pressure aloft (i.e. near the top of the water) is lowest.
8. As you walk around on the “ground” (the bottom of the cake pan), where do you
feel the highest water pressure, near the ice or near the candle? Why? 2
9. On the diagram of the cake pan (previous page), place an “H” where the surface
pressure (i.e. at the “ground”) is highest and an “L” where the surface pressure
(i.e. at the “ground”) is lowest.
10. When you “fly” (in an airplane) up to the upper part of the water
“atmosphere” near the center of the cake pan, which way do you feel the “wind”
blow? (circle your answer)
2In
water as well as in air, the pressure you experience is proportional to the weight of the water or air above you. In
other words, it is proportional to the number of molecules of air or water above you.
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from the pole (ice) to the equator (candle) / from the equator (candle) to
the pole (ice)
11. When you stand on the “ground,” at the center of the cake pan, which way do
you feel the “wind” blow ? (i.e. which way is water flowing?). Circle your answer.
from the pole (ice) to the equator (candle) / from the equator (candle) to
the pole (ice)
12. When you stand on the “ground”, near the “equator,” do you feel much wind?
Why or why not? (Hint: remember that “wind” is defined as a horizontal flow of
air)
13. When you stand on the “ground”, near the “pole,” do you feel much wind? Why
or why not?
Questions 14–18 below ask you to apply what you've learned from the behavior of
the water in the cake pan experiment to an analysis of the behavior of the air in the
upper part of the troposphere.
14. Where the air is rising, the ALOFT air pressure is
at the same level wherever air is not rising.
higher / lower than it is
15. Where the air is sinking, the ALOFT air pressure is higher / lower than it is
at the same level wherever air is not sinking.
16. ALOFT, the polar regions are characterized by relatively
atmospheric pressure.
17. ALOFT, the equator is characterized by relatively
pressure.
18. Winds ALOFT tend to blow
equator to the pole.
high
high / low
from the pole to the equator
/
low
atmospheric
/
from the
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Questions 19–23 below ask you to apply what you've learned from the behavior of
the water in the cake pan experiment to an analysis of the behavior of the air in the
troposphere as experienced by people living on the Earth's surface.
19. Where the air is rising, the SURFACE air pressure is
it is where air is not rising.
20. Where the air is sinking, the SURFACE air pressure is
it is where air is not sinking.
21. The polar regions are characterized by relatively
atmospheric pressure.
22. The equator is characterized by relatively
atmospheric pressure.
higher
/ lower than
higher
/ lower than
high /
high
/
low
low
SURFACE
SURFACE
23. Winds near the ground tend to blow from the pole to the equator / from the
equator to the pole.
24. On the model earth below, use arrows to show the air movement (i.e.
atmospheric convection) that you think would result from the contrast in air
temperatures between the equator and the poles. To keep things simple, show
the air movement just on the outside “edges” of the earth--where the “people”
are shown. Mark each region of low pressure with an “L” and each region of high
pressure with an “H.”
North Pole
60°N
30°N
Equator
30°S
60°S
South Pole
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Note: This is a useful model, but it is not correct. Here are the reasons why:
First, as air flows aloft from the equator toward the pole, it loses a lot of heat. By
the time it gets near 30° N and 30° S latitude, a great deal of that air has
cooled enough that it becomes too dense to stay aloft--thus it sinks there-most doesn't make it all the way to the pole.
Second, Earth rotates on its axis, modifying the direction that the wind blows (at
least from the perspective of any one place on earth.
See the Teacher Notes (below) for the correct and full description of global winds.
Note: This activity and the accompanying questions were originally developed by
Dr. Ann Bykerk-Kauffman at California State University, Chico.
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Terms: air movement, pressure, pressure change, pressure gradient, temperature,
sinking, rising, high pressure, low pressure, troposphere, stratosphere, density,
global winds, convection, wind, plus five more of your own terms.
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Teacher Notes:
What are the global wind patterns?
The equator receives the Sun's direct rays. Here, air is heated and rises,
leaving low pressure areas behind. Moving to about thirty degrees north and
south of the equator, the warm air from the equator begins to cool and sink.
Between thirty degrees latitude and the equator, most of the cooling sinking
air moves back to the equator. The rest of the air flows toward the poles.
What are the trade winds?
The trade winds are just air movements toward the equator. They are warm,
steady breezes that blow almost continuously. The Coriolis Effect makes the
trade winds appear to be curving to the west, whether they are traveling to
the equator from the south or north.
What are the doldrums?
The doldrums is an area of calm weather. The trade winds coming from the
south and the north meet near the equator. These converging trade winds
produce general upward winds as they are heated, so there are no steady
surface winds.
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What are the prevailing westerlies?
Between thirty and sixty degrees latitude, the winds that move toward the
poles appear to curve to the east. Because winds are named from the direction
in which they originate, these winds are called prevailing westerlies. Prevailing
westerlies in the Northern Hemisphere are responsible for many of the
weather movements across the United States and Canada.
What are the polar easterlies?
At about sixty degrees latitude in both hemispheres, the prevailing westerlies
join with the polar easterlies to reduce upward motion. The polar easterlies
form when the atmosphere over the poles cools. This cool air then sinks and
spreads over the surface. As the air flows away from the poles, it is turned to
the west by the Coriolis effect. Again, because these winds begin in the east,
they are called easterlies.
What is a sea breeze?
On a warm summer day along the coast, this differential heating of land and
sea leads to the development of local winds called sea breezes. As air above
the land surface is heated by radiation from the Sun, it expands and begins to
rise, being lighter than the surrounding air. To replace the rising air, cooler air
is drawn in from above the surface of the sea. This is the sea breeze, and can
offer a pleasant cooling influence on hot summer afternoons.
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What is a land breeze?
A land breeze occurs at night when the land cools faster than the sea. In this
case, it is air above the warmer surface water that is heated and rises, pulling
in air from the cooler land surface.
Cool Internet Sites about Weather:
http://www.hpc.ncep.noaa.gov/
(Current weather maps)
http://www.goes.noaa.gov/
(Current satellite images with links to archives of old images)
http://www.hpc.ncep.noaa.gov/dwm/dwm.shtml
(Daily weather maps for the recent past)
http://cirrus.sprl.umich.edu/wxnet/maps.html
(Univ. of Michigan weather site; Weather Channel weather maps, jet stream
analyses, etc.)
http://www.nws.noaa.gov/
(The National Weather service web site, local forecasts. technical information)
www.globe.gov
Weather Data collected by K-12 schools around the world is on the GLOBE
website:
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