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.
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.