Air pressure is created by the weight of the earth's atmosphere. Although we can't see air, the gas molecules still have mass , and gravity acts upon it. The air pressure changes daily due to the heating and cooling of the earth's surface. When air gets warm, it expands, becoming less dense, and therefore pushes with less pressure. We can measure changes in atmospheric pressure by using a barometer .
Some barometers use long glass tubes filled with mercury inverted in a dish.
Air pressing down on the surface of the dish forces the mercury up the tube. Normal air pressure can support a column of mercury about 760 mm high. We use air pressure all the time when we breathe. When our diaphragm moves down, air is pushed into our lungs from the outside, expanding the volume of the chest cavity. A device for measuring air pressure which uses an air-tight box instead of a tube of liquid.