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Barometric PressureWhat is barometric pressure?1Barometric PressureUse your Air Pressure Guide Notes with these slides.i1 a MEASUREMENT of AIR PRESSURE

The air is made up of molecules.

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Air molecules are all around us

33Like all matter, air molecules have mass.

kg44Recall that mass and weight are not the same. Mass refers to an objects 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.

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.5Now answer #3 and #4. Why does Earth have a blanket of air on the outside? Why doesnt the moon have an atmosphere?5Your 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.

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.

6Where is the most weight?

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in a stack of bodies? in a stack of booksin a stack of air?

7Same answer for eachat the bottom. But air is a fluid, so there is a difference. Guess what it is (#9).Stack of airStack of bodies or books8(Psst! The arrows are a hint.)vs.#9 Answer: Air pressure (fluid pressure) is from all directions, not just downward.Pressure = force per unit areaAir pressure at sea level is ~ 14.7 lbs/ in2

99Because 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.

As elevation goes upBarometric pressure goes down.This is an inverse relationship.We can graph it!

10What makes air pressure change?10

High altitudes = lower pressure

Low altitudes = higher pressure.... See?

1111Air 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.#11, 12, 13 Check:What is altitude?In what units is it measured?

What is air pressure?In what units is it measured?12#15: Since pressure changes with altitudehow does changing altitude affect a barometer?13

13As 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.14

#15 AnswerInflating a ballAir 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.

15#17Temperature changeAs 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!

16But what about this?Does pressure lower when the ball gets hotter?

What makes air pressure change?Back to the atmosphere Here comesWater VaporAs 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

17Review: What three factors change air pressure?(You just learned this.)123 altitude temperature water vapor18#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.19

Changing PressureA rising barometer reading = increasing air pressure.This usually means clear weather.

2020Rising barometer readings indicate that a high pressure system is approaching. Higher atmospheric pressure is usually associated with fair weather and clearing skies.

Changing PressureA falling barometer = decreasing air pressure.This usually means wet weather.(Does the barometer really fall?)

I have an umbrella and you dont.

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21Falling 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.

to measure air pressure.A Barometeris used2222

In 1643, Evangelista Torricelli invented the barometerHow can I represent the downward force of air?

2323Torricelli didnt 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 didnt believe that a vacuum could exist, hence the phrase, nature abhors a vacuum.

Torricellis 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.2424The 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?

As atmospheric pressure increasesThe mercury in the tube rises.Thats why it is called a mercury barometer. (#22)2525The Mercury BarometerGood:Bad:Simple to constructHighly accurateGlass tube is fragileMercury is very toxic!Is there a better way to measure air pressure?2626Although 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.

Introducing...The Aneroid Barometer!!No fragile tubes!No toxic chemicals!No batteries!Never needs winding!

Get yours today!!

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An aneroid barometer uses a cell which has had most of the air removed (hence the name).As the air pressure around the cell increases, it presses on the cell, which causes the needle to move.Television weather forecasters usually give barometric pressure in inches of mercury. However, meteorologists measure atmospheric pressure in millibars or bars.MILLIBARS2828The 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.#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. 2929Station 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.

Most aneroid barometers have a needle which can be set to remember the previous reading.#26 Why would this be important?3030Knowing 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. Finish your guided notes!Get credit for your workGive your name, period, record #. Follow instructions for turning it in.Raise your hand if you have questions.31