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Student Resource Sheet IHA - 11
UNDER PRESSURE
“I wonder if Stormin’ Stan has any more information for me?” you think to yourself. After
wrestling with your brother for control of the computer, you quickly log on and, sure enough,
Stormin’ Stan has sent you an email.
From: [email protected]
To: [email protected]
Subject: Ready for More?
Morning my little TV stars! I hope you finished researching the atmosphere,
because times a wastin’! Here’s your next topic – air pressure! Sure you’ve
felt it before – your ears popping while driving up and over mountains, or
during take-off and landing in an airplane. Boy, my ears feel it every time!
I’m sure you’re wondering what my ears have got to do with weather. Well,
nothing really. But, air pressure has quite a bit to do with weather. I’ve sent
along a really cool demonstration and an article from my files on pressure that
might help you. So, get your head out of the clouds, and get some more work
done!
And as always, this is Stormin’ Stan the Weather Man asking, “Do you have what it takes to
be a star?
“Great!” you think to yourself. “I think I’ll look at the demonstration first, and then read the
article.” In this investigation, you will examine the factors that affect air pressure in order to
understand its affect on weather.
Objective
When you have completed this investigation, you should be able to
1.
identify the factors that affect air pressure in order to explain how air pressure will affect
weather.
Day One
Activity 1
1.
Observe the materials set-up by the teacher.
2.
Sketch the set-up. Be sure to label the different materials used in the demonstration.
3.
Predict what caused the changes you observed during the teacher demonstration.
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Activity 2: Finding Out More
That demonstration was pretty interesting, but you don’t really understand why it happened.
So, you decide you might as well read the article Stormin’ Stan has sent.
4.
Read the selection, “Stop Pressuring Me!” in order to gather information about the factors
that affect pressure and how this relates to weather.
5.
Reread the selection and complete the organizer, “Air Pressure,” to take notes about the
factors that affect air pressure.
Air Pressure
Definition: ___________________________________________________________
____________________________________________________________________
Measured with: ________________________________________________________
Factors that Affect Air Pressure
How? Why?
How? Why?
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How? Why?
Student Resource Sheet IHA - 11
CLASS DISCUSSION: Complete the graphic organizer as your class discusses the information
you gathered on the factors that affect air pressure. Make sure to include arrows to
explain the direction that the air will move.
Warm air _______________
6.
Cool air _______________
______________ pressure
______________ pressure
______________ dense
______________ dense
Review the prediction you made in Activity 1. Determine the accuracy of your prediction.
In your response, be sure to identify and describe the factor(s) that caused the can to be
crushed during the teacher demonstration. Use information from the investigation to support
your response.
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Day Two
Activity 3
7.
Review the information you gathered on Day One. Complete Chart 1, “Factors Affecting
Air Pressure,” by adding arrows to show how pressure will change based on a change in
each factor. An up arrow indicates increasing, and a down arrow indicates decreasing.
Factor
Chart 1
Factors Affecting Air Pressure
Increase or Decrease?
How will air pressure change?
Temperature
Water Vapor
Elevation
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Student Resource Sheet IHA - 11
Activity 4
“Hmmm…,” you think to yourself. “So, these factors affect air pressure; but, what does that
have to do with weather? Why didn’t he send me information about that?”
WWW: Stan? Are you there?
SS: Yeah, I’m here. What’s up now? Don’t you have enough work to do?
WWW: Well, I can’t finish the research you told me to do in that email you sent. I
downloaded the article you told me to read, and got a lot of good information about the
factors that affect air pressure. But, it didn’t say anything about how air pressure relates to
weather?
SS: Oops! Sorry about that! I’ve been so busy planning several trips, and I can’t keep
everything straight. I’ll send the other article over right away.
WWW: Thanks.
8.
Read the article, “There’s a lot of Pressure in the Weather,” to gather information on the
relationship between pressure and weather.
9.
Reread the article and take notes on the type of weather associated with high and low
pressure.
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Student Resource Sheet IHA - 11
10. Observe the weather map outlining the weather across the continental United States.
Bismarck
San
Francisco
Springfield
Charleston
Houston
Monterey
11. Complete Chart 2, “Weather Forecasts,” to identify the type of weather that would be
expected in each of the cities listed. Support your response in the space provided.
Chart 2
Weather Forecasts
City
Type of Weather
Justification
Bismarck, ND
San Francisco, CA
CLASS DISCUSSION: Explain how characteristics such as elevation, proximity to water, and
distance from the equator may affect the type of pressure and weather expected in cities
such as:
 Charleston, South Carolina
 Springfield, Missouri
 Monterey, Mexico
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Analysis
Use the information from the weather report for Houston, Texas, to complete question 1.
“This is Whirlin’ Wally with a quick
update on your Houston weather.
Wheww… it sure is hot out there,
ya’ll! Temperatures are reaching into
the high 90’s, and there is some
serious moisture in the air. Better
keep one eye out behind ya’ for a
thunderstorm. They could pop up
anytime with weather like this! Back
to you Charlene.
1. Identify and describe the type of pressure most likely responsible for the weather
described in the weather report. In your response be sure to include the following:
 A statement supporting the type of pressure responsible for the weather
 Factors that may affect the type of pressure in the area.
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Forecaster’s Forum: Collect some weather maps for about a week from the
city you are researching. Place them in order by date. Identify any patterns
you notice with the pressure systems and weather indicated on each map.
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Student Resource Sheet IHA - 11
Press down on the palm of your hand. What you feel is a force pressing against your
hand called pressure. As you sit here right now, there is a tremendous amount of
pressure pushing against you. No, not pressure from your teacher to get your work
done. It’s actually the pressure of the air that surrounds you. The weight of the
atmosphere exerts a force on all surfaces. Air pressure is a measure of the force
of the air pressing down on Earth’s surface. Air pressure can change from day
to day and location to location. A denser substance has more mass per unit
volume than a less dense one. So, a denser substance exerts more pressure than a less dense
substance. The same is true for air – more dense air exerts more pressure than less dense air.
Okay, so that makes sense, but how do scientists measure pressure? With a device called
a barometer! A barometer works by registering slight changes in the pressure of the air pushing
against it. When the air pressure increases, it exerts more force on the barometer causing the
meter on the device to increase. The opposite is true when the air pressure decreases. Weather
reports use several different units for air pressure. Most weather reports for the general public
use “inches of mercury” when reporting air pressure. For example, the local
meteorologist on television may report the air pressure as “30 inches.”
What he/she means is that the barometer measured 30 inches of mercury.
The National Weather Service indicates air pressure in “millibars.” One
inch of mercury is approximately 33.87 millibars. So, 30 inches of mercury
is equal to approximately 1,016 millibars.
Now you know how air exerts pressure and how scientists measure
pressure. You’re probably wondering what all this has to do with weather.
Well, there’s one more thing you need to know before we can get there – the
factors that affect pressure. As you have already learned, less dense air
exerts less pressure than more dense air. But, what causes the density of air
to change? Three things: temperature, water vapor, and elevation. Let’s start
with temperature. When a substance is heated, the molecules begin to move more rapidly and
spread further apart. When this happens, the substance becomes less dense. In this way,
locations with high temperatures usually register lower pressure than areas with cooler
temperatures. Next, water vapor – at a given pressure moist air is less dense than dry
air. This is because a water molecule has less mass than either a nitrogen or an
oxygen molecule. If there is a lot of water vapor in the air, the air tends to be less
dense. Therefore, the more water vapor in the air, the lower the air pressure in the
area. The third factor is elevation, or altitude. As elevation increases, the air
particles begin to spread further apart. In this way, the air becomes thinner, and
less dense. And, just like we’ve said over and over, less dense air exerts less
pressure.
Adapted from Prentice Hall Science: Exploring Earth’s Weather
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If you look at a weather map, you will see areas marked with an L, and others with an H.
These letters tell you the type of air pressure in those areas. The L is short for “low,” and
indicates an area of relatively low air pressure. Likewise, the H stands for “high,” and indicates
an area where the pressure is relatively high. Sure, that’s simple, but what does that mean in
terms of the weather? Let’s take a look.
As large masses of air move, they will frequently come together in the upper atmosphere.
When this happens, the air pressure may become relatively high. These air masses press down
on the layers of air below. This pressure usually prevents warm, moist air from rising into the
atmosphere. As a result, clouds do not form. And, no clouds generally mean no precipitation.
So, high pressure usually means fair weather.
On the other hand, when large air masses move apart in the upper atmosphere, the air
pressure may become relatively low. This reduces the pressure on the layers of warm air below.
As a result, the warm air is able to rise into the atmosphere. If the warm air is moist, the
moisture may condense into clouds as it is cooled in the atmosphere. So, low pressure can lead
to cloudy, rainy weather. But, remember, there are always exceptions. Figure 1, “Air Pressure
and Weather,” illustrates these ideas.
So, understanding the day to day weather in an area relies heavily on understanding the
pressure that exists in that area at that time. But remember, air pressure can change from one day
to the next. Which might explain why you have to watch the weather report daily!
Figure 1: Air Pressure and Weather
From Simple Weather Experiments With Everyday Materials by Muriel Mandell
Adapted from Prentice Hall: Science Explorer, Weather and Climate
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