Air pressure is the weight (or force) of the atmosphere at any particular point. Although you can't feel it or see it, air pressure plays an important part in weather, and can be demonstrated with some really fun experiments.
"What type of pressure is there around you right now? Is your area surrounded by a high or low pressure? Unlike temperature, moisture, and wind, you can't easily determine the pressure through your five senses. Because it is difficult to perceive pressure through our senses, it is tempting to assume that pressure is not very important. This assumption would be very incorrect. Pressure is extremely important, and has a dramatic effect on our weather." This illustrated geology lesson for elementary and middle-school students includes songs ("Why Does the Wind Blow") and an introduction to atmospheric pressure, density, wind patterns, and the Coriolis effect.
In addition to the short lesson, this NASA Kids Earth site includes three air pressure experiments, a word search puzzle, five discussion questions ("What do you think causes wind?") and a couple of online experiments. One of these, called Take a Balloon Ride, demonstrates what happens to a hot air balloon as it rises from sea level to 10,000 meters. Can you guess what happens to the balloon when it is six miles above the earth? "As you move the mouse over the altitude scale to the left, you'll notice that the balloon changes shape and size."
Today's lesson from the National Weather Service Online Weather School is all about air pressure, and this online classroom is my pick of the day because of the six do-it-yourself experiments or labs, called Learning Lessons. Heavy Air is the first, and it demonstrates that air has weight by balancing a yardstick with two balloons to each end. If one balloon was heavier than the other, what would happen to the yardstick? Would the heavier end move up or down?
"Gravity shapes and influences all atmospheric processes. It causes the density and pressure of air to decrease exponentially as one moves away from the surface of the Earth." This illustrated chapter on atmospheric pressure for high school and college students is from the online textbook "Fundamentals of Physical Geography" by Michael Pidwirny, Associate Professor, University of British Columbia. It is hyperlinked to a large glossary of terms (from abiotic to zooplankton), so clicking on any bolded word will take you to its definition. At the bottom of the page, you'll find links to a study guide, bibliography, and related websites.
"Air is constantly moving to seek an equilibrium between areas of more air molecules (higher pressure) and those with less (lower pressure). You have probably experienced this by opening a container that has been vacuum packed." Professor of Geography Michael Ritter, from University of Wisconsin, Stevens Point, provides another excellent online textbook for high-school and college students. It is illustrated, includes a glossary, and links to external movies, podcasts and interactive activities. For the multimedia extras, follow the "Contents" link at the bottom of every page, and you'll find them listed by chapter.