The gas molecules found in the troposphere constantly move around causing air pressure. Air pressure is the force of air against a unit of area. This movement of air creates a push towards the earth. A great way to explain air pressure to children is by using a balloon as an example. When the air is blown into a balloon it will expand as far as the wall, or as far as the balloon will allow it to go. Air in the troposphere acts exactly like that inside the balloon.
Air pressure is measured with an instrument called a barometer. There are two different types of barometers: a mercury barometer and an aneroid barometer. The mercury barometer consists of a glass tubing closed at one end and filled with mercury at the other end. The open end of the glass tube on the barometer is placed in a container of mercury. At sea level, air pushing down on the mercury in the container supports the column of mercury at a certain height in the tube. As the air pressure decreases, the column of mercury drops.
At sea level and 0 degrees centigrade, air pressure is able to support a column of mercury 760 millimeters high. This value is called standard air pressure and is expressed as an atmosphere. Air pressure can also be measured in millibar. Millibar is most commonly used on weather maps. Standard air pressure, or 760 millimeters of mercury is equal to 1013.20 millibar.
A more common type of barometer is called an aneroid barometer. This is the type of barometer which we will be making for our weather station. An aneroid barometer is made up of an airtight metal box from which most of the air has been removed. A change in air pressure causes the needle to move and indicate the new air pressure. By comparing the daily readings on the calibrated scale, the students will be able to tell if the pressure is rising, falling or remaining steady. It is the changes in air pressure, rather than the level of air pressure, which provide the most meaningful indicator of weather. Pressure changes used in connection with the direction of the wind gives the amateur meteorologist the best key to local weather conditions.
Both the mercury and aneroid barometer are expressed in terms of inches of mercury. I have enclosed directions for making both of the previously mentioned barometers in Appendix 4. Homemade barometers are frowned upon by meteorologist because of the inaccuracy of the equipment. For the purposes of this unit, I suggest using a real barometer (usually expensive) or to listen to the local weather channel to get the barometric readings in your area. Then you can compare both pieces of data with your students.