Teacher’s page BACKGROUND INFORMATION: A barometer is an instrument used to measure air pressure.  One way air pressure is measured is in inches of mercury.  Air pressure is highest as sea level, making the air also heaviest. At sea level, air pressure is 29.53 inches of mercury.  This means that at sea level, it takes 14.7 pounds of pressure per square inch of air to raise mercury in a hollow tube to 29.53 inches in height.  What is 14.7 pounds of pressure?  Try pushing down on a scale at home until it reads “14.7.” You have exerted 14.7 pounds of pressure on that scale!   A barometer is an instrument that measures the pressure of the atmosphere at a particular location.  There are two types:  a mercurial barometer which uses mercury, and an aneroid barometer that uses a balloon that expands or contracts when the atmospheric pressure changes.   You may not notice in your experiment that the barometer reading changes in response to temperature as well as direct changes in pressure.  Temperature changes pressure, but only in a tightly sealed space (your classroom is not tightly sealed to the outside.) Warmer air taking up the same amount of space as cold air will create more pressure.  This is because warm molecules push away from one another instead of moving towards one another.  The warm molecules try to take up more space than they have, creating a greater pressure pushing out of the container. Measuring atmospheric pressure is very important for predicting and preparing for storms. Barometers allow weather forecasters to estimate maximum hurricane wind speeds.  Since air is always flowing from high pressure toward low pressure, air moves inward toward the low pressure center in a hurricane.  By using a barometer to find the pressure in the center of the hurricane, forecasters can estimate the fastest wind speeds that the hurricane is capable of producing.   Usually, when barometric pressure (or, pressure as shown on a barometer) falls rapidly, it means a storm is coming.  A steady barometer shows stable, fair weather.  The barometer saves lives since it helps predict the approach of poor weather.  Back when RADAR and satellites didn’t exist, crude barometers were the main way people predicted the approach of bad weather like a hurricane or a major snow storm.   PRE-LAB TEACHING STRATEGIES: 1.   Demonstrate that air actually has pressure (or takes up space) by blowing up a plastic baggie and sealing it to the mouth of a jar with a rubber band.  Hold the outside of the bag where it is attached to the jar and have a student try to push the baggie into the jar. Discuss with the students that the air pressure inside the jar prevents them from being able to push the baggie in- something’s already there! 2.   Initiate a discussion about pressure.  Why do your ears hurt when you dive deep into a pool? Does air weigh anything?  Why don’t we feel air pressing down on us?  Why does air push out of our ears making them “pop” when we go up in an airplane? 3.   Discuss pressure in general with your class.  Other than air, what other things  exert pressure?