Background Information: Atmospheric Pressure
Humans are subjected to the pressure produced by the weight of the gases of the atmosphere above us. The force exerted on a unit area of surface by the weight of the air above the surface is named atmospheric pressure and expressed in N (Newton) per square meter which is also named Pa (Pascal).
Although unnoticeable in daily life, the atmospheric pressure can be demonstrated in a simple way. A privately conducted trial run, before the class demonstration, is well advised. In order to draw the better attention of the students and highlight the effect of the atmospheric pressure, it is advisable to conduct the following demonstration during the session of this topic in the classroom.
Take a metal oil container of approximately 4-liter capacity and, having removed the cap, heat the container over a fire or burner or torch. When heated, the can should be removed from the source of heat with heavy gloves and the cap refitted to make the seal again. Without paying special attention to the heated can, the lecture should continue to explain that some air inside the can is removed by heating and consequent expansion. Accordingly, the inside air pressure is much reduced when the can cools down. Meanwhile over several minutes, the can will be seen to creek, groan and distort as it implodes under the external atmospheric pressure. This experiment can be a perfect example of the strength of the atmospheric pressure and it clearly shows that the pressure is exerted on the can from all directions.
Measurement of Atmospheric Pressure
An instrument that is widely used to measure the atmospheric pressure is known as a barometer (baro = weight; metric = measure). Among many kinds of barometer, a simple mercury barometer.
The height to which the mercury rises in the 'tube due to the weight of the atmosphere is the measure of atmospheric pressure. The meteorological measurement unit of atmospheric pressure is one-thousandth of a bar rather than the normal usage of N (Newton) per square meter or Pa (Pascal) in physics. It is called the millibar (mb).
A mercury barometer, the pressure of air is able to support the weight of a column of mercury enclosed in a tube which is free of air above the mercury surface.
It is to be noted that Mb (millibar) is still widely used in many meteorological services all over the world. There are a few other units for pressure, such as lb in [pounds per square inch] which are not discussed here.
There is high atmospheric pressure near the earth's surface due to the great number of air molecules squeezed by the weight of air above that surface. The standard atmospheric pressure at sea level is taken as 1013.25 Mb. So with increasing altitude and less air molecules, atmospheric pressure decreases.
