It's All About Air Pressure Part 1 - Overview Pressure is an important variable in so many aspects of our lives. Not only does pressure affect how well the tires on our car wear, but it has important applications to predicting the weather and helps explain why airplane wings allow an airplane to leave the ground. Below is a summary of the basics of air pressure, along with links to more information and activities. When you feel like you are familiar with the concepts of air pressure, move on to the more difficult concepts involved with Bernoulli's Principle. Pressure The definition of pressure is the force divided by the area. The most common English units for pressure are pounds per square inch, or psi.

Air Pressure

We live in an ocean of air, and air does have weight. While this weight may seem to be extremely small, the weight of miles of air above you pushes down with a considerable force. And since air is a fluid, that force pushes in on you in all directions. Roughly, this force is 14.7 psi ! That's right, almost 15 pounds on every square inch of your body. Fortunately, our bodies have evolved to live with this pressure. Problems do arise when the pressure becomes too great, like deep in the ocean, or too small, like at high altitudes.

Air pressure is not the same everywhere and is continually changing. Sometimes the air above you is denser, has more molecules per volume, and the pressure is "high." At other times the density of the air is less and the pressure is "low." There is a relationship between the air pressure and the weather (see below).

As the altitude (or the distance above the ground) increases, the amount of air above, or the air pressure, decreases. At very high altitudes, the air is so "thin" that it becomes difficult to breathe. Just being on a high mountain can make it more difficult to perform athletic events.

Air pressure can be measured using a number of different instruments and in a number of different units. Examples of those units are k Pa (kilopascals), psi (pounds per square inch), and in Hg (inches of Mercury).

 Links NASA Air Pressure A basic definition of air pressure. NASA Air Pressure & Altitude An interactive Java applet that shows the relationship between altitude and air pressure. Air Pressure and Units Shows conversion factors for common units of pressure. Check Local Air Pressure Click on the map to find the air pressure where you live. Soda Can Activity: Use a soda can to illustrate the strength of air pressure. Word Search Game Game: Find the pressure and air pressure terms in this word search game.

Measuring Pressure

There are two basic instruments used to measure air pressure: the mercury barometer and the aneroid barometer. The mercury barometer is a simple tube that has been inverted into a container filled with mercury. The air pressure pushes the mercury up the tube a given height, which can be measured. Other liquids, such as water, can also be used. An aneroid barometer contains no liquid, but small cells that expand and contract with the changes in air pressure. By attaching a needle to the cells, the air pressure can be measured.

 Links Diagram Mercury Barometer Gives nice diagrams of a typical mercury barometer. Water barometer Activity: Build your own water barometer. Aneroid Barometer Activity: Build you own aneroid barometer.
 Air Pressure & The Weather Again, air pressure is a very useful variable in weather prediction. When you watch the weather report on television, you will see maps with the high and low pressures indicated. This is done by taking the pressure measurements from all over the country and plotting them on the map. They then connect places that have the same pressure with lines called isobars. These isobars then form areas that have higher than average pressure, the "highs," and areas that have lower than average pressure, or "lows." As you probably already know, low pressure areas are associated with rain, and high pressure areas are associated with sunny skies. Meteorologists have devised ways to predict how the air will move between the high and low pressure areas, which allow them to predict wind speeds and other factors that lead to more accurate weather prediction. This analysis of high and low pressure systems leads to what are called warm and cold fronts, lines of changing weather conditions.
 Links Plotting Highs and Lows Activity: Print out, plot isobars, and identify high and low pressure areas. (Requires Shockwave Plug-In) Fronts and Weather Discusses the definition, meaning, and uses of warm and cold fronts. Forces and Winds More detailed discussions on pressure gradients, boundary layer winds, and the Coriolis force.

 Part 2 - Bernoulli's Principle National Science Education Content Standards [9-12] Content Standard B: Motions and Forces[9-12] Content Standard E: Abilities of Technological Design[9-12] Content Standard G: Science as a Human Endeavor, Historical Perspectives Principles and Standards for School Mathematics [9-12] Measurement Standard: Apply tools and formulas to determine measurements.[9-12] Data Analysis and Probability Standard: Develop and evaluate predictions based on data.[9-12] Connections Standard: Apply mathematics in contexts outside of mathematics.