Air Pressure, Barometers, and Isobars, What are they?The air that surrounds the Earth creates atmospheric pressure. As you go up into mountains or fly high in an airplane, the air is thinner and the pressure is less. Air pressure is also known as barometric pressure and is measured using a device called a barometer. A rising barometer indicates increasing air pressure; a falling barometer indicates decreasing air pressure.
The following links connect to web resources with additional information and examples of barometers:
- How do Mercury and Aneroid Barometers Work?
- Using an Analog Barometer
- Focus on the Mercury Barometer
Changes in air pressure are also be caused by the difference in air temperature above the Earth. Continental landmasses and ocean waters change the temperature of the air above them. These changes create wind and cause pressure system to develop. The wind moves these pressure systems that change as they pass over mountains, oceans, and other areas.
Meteorologists use a metric unit for pressure called a millibar and the average pressure at sea level is 1013.25 millibars. A line on a weather map connecting points of equal atmospheric pressure is called an isobar.
The illustration to the right depicts a pair of isobars. At every point along the top isobar, the pressure is 996 mb (millibars) while at every point along the bottom isobar, the pressure is 1000 mb. Points above the 1000 mb isobar have a lower pressure and points below that isobar have a higher pressure.
The Relationship Between Air Pressure and WeatherYears ago the French scientist and philosopher Blaise Pascal, discovered that air pressure decreases with height, and pressure changes at ground level at any one place can be related to daily weather changes. Often, weather forecasters refer to a storm or low-pressure area moving toward your region. As air rises, it cools and often condenses into clouds and precipitation. In high-pressure systems the air sinks toward the Earth and warms up, leading to dry and fair weather.
Gather the Materials for the Lesson
- Aneroid barometer (or mercury barometer if available)
- Internet accessible computers with network printing capabilities
- LCD projector (Optional)
Teaching ProceduresDiscuss air pressure as introduced above. Have students predict what the current barometric reading might be (high or low) based on the weather outside. Introduce the aneroid barometer. (Note: it is not as accurate as a mercury barometer, which is still used by most professional meteorologists.)
When the air is dry, cool, and pleasant, the mercury or barometer reading rises. When the air is warm and wet, the barometer reading falls. When the air pressure falls, it usually indicates some type of storm or wet weather is coming. When it rises, it often means clear weather. If the barometer remains steady, there will be no immediate change in the weather.
At this point the students should complete the computer activity Air Pressure and Weather Systems. This program can be accessed over the Internet at the About.com website or download and installed on individual computers.
Modification and AdaptationsIf there are not enough computers available for an entire class, use an LCD projector or a large screen computer monitor and conduct this activity as an interactive presentation.
At-risk and slow learners seem to do well with virtual computer activities because they can easily see the results. These students should be permitted to work with their lab partners to answer the on-screen questions.
Conclusion and AssessmentOnce all the students have completed the activity discuss with your students their results and their rationale for their answers. If this activity is to be used for assessment purposes only, have the students complete the computer exercise then print their results.
Enrichment and Reinforcement ActivityAs an extension assignment and for reinforcement purposes students can construct a homemade barometer and record barometric pressure over several days correlating the readings to actual weather conditions.
Images for Teaching Weather ScienceThe following weather images may be useful for science teachers and students in the classroom. You are free to print these pictures for classroom use.
- An Approaching Cold Front
- An Approaching Warm Front
- Occluded Fronts
- A Weather Map Symbol Tutorial
- Weather Station Model Symbols
- Free Weather Calculators (Not an image, but very useful!)