Your Life....Under Pressure Barometric Pressure Your Life....Under Pressure

The air is made up of molecules.

. Air molecules are everywhere

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.

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 things change air pressure in the atmosphere. 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.

Pressure analogy Imagine a stack of books. The bottom book feels all the weight of the all the books stacked above it, while the second one up feels slightly less weight. The higher in the stack, the less pressure one would feel.

Air pressure is equal in all directions. 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 Approximately 14.7 lbs/ in2

A. Pressure Which shoes create the most pressure?

A. Pressure Key Units at Sea Level 101.325 kPa (kilopascal) 1 atm 760 mm Hg 14.7 psi

B. Boyle’s Law When the volume of a gas decreases, its pressure increases (at constant temp). P1V1 = P2V2 P V INVERSE

B. Boyle’s Law

C. Charles’ Law When the temperature of a gas increases, its volume also increases (at constant pressure). Because the formula is equal to a constant, it is possible to solve for a change in volume or temperature using a proportion: V1/T1 = V2/T2 V T DIRECT

Elevation Changes Air Pressure As elevation goes up Barometric pressure goes down. This is an inverse relationship.

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

Inflating example 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.

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?

Temperature As air temperature increases, the molecules move more rapidly. The expands, becomes less dense, and rises. This rising air is exerting less force on the ground and pressure lowers

Water Vapor As water vapor (storms) move 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

Changing Pressure A rising barometer = 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.

One time in the movie “Twister” Changing Pressure A falling barometer = decreasing air pressure. This usually means: Wet Weather One time in the movie “Twister” 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.

A Barometer is used to measure air pressure.

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

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?

As atmospheric pressure increases… The mercury in the tube rises.

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?

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

An aneroid barometer uses a cell which has had most of the air removed. 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.

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.

Why would this be important? Most aneroid barometers have a needle which can be set to remember the previous reading. Why would this be important? 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.