It's All About Air Pressure
Part 1 - Overview |
Pressure
is an important variable in so many aspects of our lives. Not only does
pressure affect how well the tires on our car wear, but it has important
applications to predicting the weather and helps explain why airplane
wings allow an airplane to leave the ground. Below is a summary of the
basics of air pressure, along with links to more information and activities.
When you feel like you are familiar with the concepts of air pressure,
move on to the more difficult concepts involved with Bernoulli's Principle.
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Pressure
The definition
of pressure is the force divided by the area. The most common English
units for pressure are pounds per square inch, or psi.
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Air Pressure
We live in an
ocean of air, and air does have weight. While this weight may seem
to be extremely small, the weight of miles of air above you pushes
down with a considerable force. And since air is a fluid, that force
pushes in on you in all directions. Roughly, this force is 14.7 psi
! That's right, almost 15 pounds on every square inch of your body.
Fortunately, our bodies have evolved to live with this pressure. Problems
do arise when the pressure becomes too great, like deep in the ocean,
or too small, like at high altitudes.
Air pressure
is not the same everywhere and is continually changing. Sometimes
the air above you is denser, has more molecules per volume, and the pressure
is "high." At other times the density of the air is less
and the pressure is "low." There is a relationship between
the air pressure and the weather (see below).
As the altitude
(or the distance above the ground) increases, the amount of air above,
or the air pressure, decreases. At very high altitudes, the air is
so "thin" that it becomes difficult to breathe. Just being
on a high mountain can make it more difficult to perform athletic
events.
Air pressure
can be measured using a number of different instruments and in a number
of different units. Examples of those units are k Pa (kilopascals),
psi (pounds per square inch), and in Hg (inches of Mercury).
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Links
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A basic definition of air pressure.
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An interactive Java applet that shows the relationship between altitude and air pressure.
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Shows
conversion factors for common units of pressure.
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Click on the map to find the air pressure where you live.
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Activity: Use a soda can to illustrate the strength of air pressure.
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Game:
Find the pressure and air pressure terms in this word search game.
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Measuring Pressure
There are two
basic instruments used to measure air pressure: the mercury barometer and the aneroid barometer. The mercury barometer is a simple tube
that has been inverted into a container filled with mercury. The
air pressure pushes the mercury up the tube a given height, which
can be measured. Other liquids, such as water, can also be used. An
aneroid barometer contains no liquid, but small cells that expand
and contract with the changes in air pressure. By attaching a needle
to the cells, the air pressure can be measured.
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Links
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Gives
nice diagrams of a typical mercury barometer.
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Activity:
Build your own water barometer.
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Activity:
Build you own aneroid barometer.
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Air Pressure
& The Weather
Again, air pressure
is a very useful variable in weather prediction. When you watch the
weather report on television, you will see maps with the high
and low pressures indicated. This is done by taking the pressure measurements
from all over the country and plotting them on the map. They then
connect places that have the same pressure with lines called isobars.
These isobars then form areas that have higher than average pressure,
the "highs," and areas that have lower than average pressure,
or "lows."
As you probably
already know, low pressure areas are associated with rain, and high
pressure areas are associated with sunny skies. Meteorologists have
devised ways to predict how the air will move between the high and
low pressure areas, which allow them to predict wind speeds and other
factors that lead to more accurate weather prediction. This analysis
of high and low pressure systems leads to what are called warm and
cold fronts, lines of changing weather conditions.
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Links
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Activity:
Print out, plot isobars, and identify high and low pressure areas. (Requires Shockwave Plug-In)
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Discusses the definition, meaning, and uses of warm and cold fronts.
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More detailed discussions on pressure gradients, boundary layer winds, and the Coriolis force.
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Part 2 - Bernoulli's Principle
National Science Education Content Standards
[9-12] Content Standard B: Motions and Forces [9-12] Content Standard E: Abilities of Technological Design [9-12] Content Standard G: Science as a Human Endeavor, Historical Perspectives
Principles and Standards for School Mathematics
[9-12] Measurement Standard: Apply tools and formulas to determine measurements. [9-12] Data Analysis and Probability Standard: Develop and evaluate predictions based on data. [9-12] Connections Standard: Apply mathematics in contexts outside of mathematics.
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