2 The air is made up of molecules. Ar N2N2 N2N2 O2O2 O2O2 CO 2

3 Air molecules are everywhere.

4 Although invisible, air molecules have mass and take up space (like all matter).

5 Gravity pulls the air molecules toward the earth thus giving them weight.

6 The weight of the air molecules pressing down on us is called atmospheric pressure or air pressure.

7 The force of air pressure at sea level is equal to 14.7 pounds per square inch. at SEA LEVEL One unit of air pressure is called an atmosphere.

8 1atm (atmosphere) = 14.7/in 2 (per square inch) mb (millibars) 29.92in of Hg (inches of mercury) at SEA LEVEL

9 Air pressure is equal in all directions.

10 So why aren’t we crushed by atmospheric pressure?

11 It’s always been around us - we’re accustomed to it. Although we can detect sudden changes in it, if the pressure increased or decreased too much, we would DIE!

12 As elevation goes up Barometric pressure goes down. This is an inverse relationship.

13

14 High altitudes = lower pressure Low altitudes = higher pressure Fewer air molecules More air molecules Lower pressure Higher pressure

Air Pressure can be affected by temperature. Warm air rises; pressure drops Low Pressure system Cold air sinks; pressure increases High Pressure system 15

16 The woods were dark and foreboding, and Alice sensed that sinister eyes were watching her every step. Worst of all, she knew that Nature Abhorred a Vacuum.

17 Evangelista Torricelli invented the barometer in Torricelli’s tube

18 Torricelli’s barometer used a glass column suspended in a bowl of mercury.

19 The pressure of the air molecules pushed the mercury up into the glass tube.

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

21 He concluded that the weight of the mercury in the tube was equal to the weight of the air pressing down on the mercury in the dish.

22 Standard atmospheric pressure: 1 atm = 760mmHg or 1 atm = mb

23 The Mercury Barometer Simple to construct Highly accurate Glass tube is fragile Mercury is very toxic! Is there a better way to measure air pressure?

24The Aneroid Aneroid Barometer Barometer No fragile tubes! No fragile tubes! No toxic chemicals! No toxic chemicals! No batteries! No batteries! Never needs winding! Never needs winding! Get yours today!!

25 HOWITWORKS

26 Inside the barometer is a cylinder that contains a partial vacuum. As pressure outside of it changes, the cylinder expands or contracts causing the springs to move which, in turn makes the dial move.

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

28

Since pressure changes with altitude… … how do scientists account for the differences in the readings that are reported from one place to another?

30 … there are 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.

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

32 Changing Pressure A rising barometer = increasing air pressure. This usually means:

33 Changing Pressure A falling barometer = decreasing air pressure. This usually means:

For Help: The GLOBE ProgramAMSTI-GLOBE Robin Nelson Lynn Vaughan AMSTI-GLOBEAMSTI-GLOBE AdministratorResource Specialist Jerry Cobbs AMSTI-GLOBE Technology Specialist