1. What is barometric pressure?
Use your Air Pressure Guide Notes with these slides.
Barometric Pressure
Barometric Pressure i
a MEASUREMENT of AIR PRESSURE
What is barometric pressure?

2. The air is made up of molecules.

3. .
Air molecules are all around us

4. Like all matter, air molecules have mass.kg
Recall that mass and weight are not the same. Mass refers to an object’s inertia, how hard the object is to accelerate or decelerate. If you were in space and got hit in the head with a “weightless” hammer, it would still hurt, because the hammer still has mass. The hammer is still hard to stop.

5. Weather changes as air pressure changes.
Gravity pulls the air molecules toward the earth, giving them weight. The weight of the air molecules all around us is called the air pressure.
Weather changes as air pressure changes.
Three factors change air pressure in the atmosphere. Stay alert to learn what they are.
Your weight is the result of gravity pulling your mass down on the bathroom scales. Note that weight has units of a force, such as pounds.
Now answer #3 and #4. Why does Earth have a blanket of air on the outside? Why doesn’t the moon have an atmosphere?

6. Who feels the most pressure?
Imagine a pile of bodies. The bottom person feels all the weight of the all the bodies stacked above it, while the second one up feels slightly less weight. The higher in the stack, the less pressure one would feel.

7. Where is the most weight?
in a stack of bodies?
in a stack of books
in a stack of air?

8. (Psst! The arrows are a hint.)
Same answer for each—at the bottom. But air is a fluid, so there is a difference. Guess what it is (#9).
Stack of air
Stack of bodies or books
vs.
(Psst! The arrows are a hint.)

9. #9 Answer:
Air pressure (fluid pressure) is from all directions, not just downward.
Because air is a fluid, force applied in one direction is distributed equally in all directions. Thus the downward pull of gravity on air molecules produces air pressure in all directions.
Pressure = force per unit area
Air pressure at sea level is ~ 14.7 lbs/ in2

10. Barometric pressure goes down.
What makes air pressure change?
As elevation goes up
Barometric pressure goes down.
This is an inverse relationship.
We can graph it!

11. High altitudes = lower pressure
Air pressure can be thought of as the column of air rising above us. As we go up in altitude, we get closer to the top of the column. Thus there are fewer molecules of air above us to be pulled down by gravity, so the air “weighs” less. Therefore, pressure always decreases as one goes up.
Low altitudes = higher pressure.... See?

12. #11, 12, 13 Check:
What is altitude?
In what units is it measured?
What is air pressure?

13. #15: Since pressure changes with altitude…
As we have noted earlier, higher elevations have fewer air molecules pressing downward, and so atmospheric pressure is lower. This means a barometer will read lower as it is carried to a higher elevation. Airplanes use a special type of barometer, called an altimeter, to measure altitude.
…how does changing altitude affect a barometer?

14. #15 Answer

15. Inflating a ball Air inside a ball pushes against the sides.
The more air we put in a ball, the more the molecules push.
So the pressure increases as we add air.

16. #17—Temperature change
What makes air pressure change?
As air temperature increases, the molecules move more rapidly.
The air expands, becomes less dense, and rises.
This rising air is exerting less force on the ground and pressure lowers!
But what about this?
Does pressure lower when the ball gets hotter?

17. Back to the atmosphere Here comesWater Vapor
As water vapor moves into an area, the light molecules of water force out the heavier oxygen, nitrogen, and other gases.
This results in a drop in air pressure
On a weather map, you may notice these symbols L H

18. Review: What three factors change air pressure. (You just learned this
Δ altitude
Δ temperature
Δ water vapor 1 2 3

19. #21 What does all this have to do with weather?
You can predict weather based on changing air pressure.
How? See the next two slides.

20. Changing Pressure A rising barometer reading
= increasing air pressure.
This usually means clear weather.
Rising barometer readings indicate that a high pressure system is approaching. Higher atmospheric pressure is usually associated with fair weather and clearing skies.

21. I have an umbrella and you don’t.
Changing Pressure
A falling barometer = decreasing air pressure.
This usually means wet weather.
(Does the barometer really fall?)
I have an umbrella and you don’t.
Falling barometer readings usually indicate the approach of an area of low pressure. Low pressure readings are usually associated with storm systems. Tornadoes and hurricanes can produce very low barometric readings.

22. A Barometer
is used
to measure air pressure.

23. In 1643, Evangelista Torricelli invented the barometer
How can I represent the downward force of air?
Torricelli didn’t actually build a barometer, but he gave detailed instructions on how to build one, so he is given credit for the invention. He was actually trying to prove the existence of a vacuum. Many scientists in his day didn’t believe that a vacuum could exist, hence the phrase, “nature abhors a vacuum.”
In 1643, Evangelista Torricelli invented the barometer

24. Torricelli’s barometer used a glass column suspended in a bowl of mercury. The pressure of the air molecules pushed the mercury up into the glass tube.
The weight of the mercury in the tube was equal to the weight of the air pressing down on the mercury in the dish.
The abbreviation “Hg” is the chemical symbol for mercury. Some kinds of pressure reading instruments, including some barometers, use the abbreviation “mmHg,” meaning “millimeters of mercury.” 760 mmHg is considered the standard “normal” atmospheric pressure at sea level. This unit is called a “torr,” after Torricelli.
To construct a mercury barometer, fill a tube with a liquid. Invert then tube in a dish of liquid holding your thumb over the top of the tube until the the tube is immersed in the bowl of liquid, the atmospheric pressure will keep the liquid in the tube from emptying such that the weight of the liquid in the tube equalize with the atmospheric pressures. (Do not do this with mercury because of its toxicity)
Mercury was used because it is a very heavy liquid, so the tube could be relatively short. The tube in a mercury barometer still has to be over a meter long. Students may want to try building a barometer using colored water. How high would the tube need to be? Merucy is about 11 times more dense than water. What if they used milk or some other liquid, would the height be the same?

25. As atmospheric pressure increases…
The mercury in the tube rises.
That’s why it is called a mercury barometer. (#22)

26. Is there a better way to measure air pressure?
The Mercury Barometer
Good:
Bad:
Simple to construct
Highly accurate
Glass tube is fragile
Mercury is very toxic!
Although mercury has been used for hundreds of years, its toxic effects have only been fully realized in the last few decades. Students should NEVER handle mercury or broken mercury thermometers or barometers. Mercury should also never be thrown in the trash or washed down the drain, since it moves easily up the food chain from fish to humans. A local health department or environmental professional can assist with disposal of old or broken mercury instruments. GLOBE instrument specifications call for organic-fluid (non-mercury) or digital thermometers, except for the analog min/max thermometer, which is mounted in a shelter and is not handled by the students.
Is there a better way to measure air pressure?

27. Introducing... The Aneroid Barometer!! Get yours today!!
No fragile tubes!
No toxic chemicals!
No batteries!
Never needs winding!
Get yours today!!

28. An aneroid barometer uses a cell which has had most of the air removed (hence the name).
MILLIBARS
As the air pressure around the cell increases, it presses on the cell, which causes the needle to move.
The word “aneroid” means “no air,” and refers to the partial vacuum inside the cell. The aneroid cell is shaped like a bellows, so that it can flex as air pressure changes. Increasing air pressure compresses the cell, causing the needle to register a change. Decreasing pressure allows the cell to expand, causing the needle to move in the opposite direction.
The use of inches of mercury is a hold-over from the days of mercury barometers. It refers to the actual height of mercury in the glass tube. Millibars are metric system units, and as such are readily understood by scientists around the world.
Television weather forecasters usually give barometric pressure in inches of mercury. However, meteorologists measure atmospheric pressure in millibars or bars.

29. #25 Two types of barometric pressure measurements:
Station pressure is the actual pressure at the recording location. It is affected by the local altitude.
Sea level pressure is referenced to sea level, so it has the same altitude anywhere in the world.
Station pressure on a mountain top will be lower than station pressure in a valley. Scientists need a fixed point of reference in order to compare barometer readings in different locations. That is why barometer readings are sometimes adjusted for elevation above sea level at the station location.

30. Most aneroid barometers have a needle which can be set to remember the previous reading.
Knowing how the air pressure is changing is as important as knowing the actual barometric reading. The set needle allows students to compare the current reading to the previous one. If the current reading is less than the previous one, the barometric pressure is falling. If the current reading is more than the previous one, the pressure is rising. If it is the same as the previous reading, the pressure is said to be steady. Weather forecasters often use the phrases “falling barometer,” “rising barometer,” or “steady barometer” as a way of referring to the change in atmospheric pressure.
#26 Why would this be important?

31. Finish your guided notes!
Get credit for your work—
Give your name, period, record #.
Follow instructions for turning it in.
Raise your hand if you have questions.