1. Air Pressure

2. Properties of Air
Since air has mass, it also has other properties like density and pressure.
Density
Density is the amount of mass in a given volume (d=m/v).
the greater mass, the greater the density.

3. Pressure Pressure is the force exerted in a given area.
Air pressure is the result of the weight of a column of air pushing down on an area. The molecules in air push in all directions and that is why air pressure does not crush objects.
Denser air exerts more pressure than less dense air.

4. Measuring Air Pressure
A barometer is an instrument used to measure air pressure. The most common are mercury barometer and aneroid barometers.
Mercury barometer
Consists of a glass tube open at the bottom end and partially filled with mercury. The open end rests in a dish of mercury and the space in the tube above the mercury is a vacuum.
The air pressure that pushes down on the mercury in the dish is equal to the weight of the column of mercury in the tube. At sea level, the column is on average 76 cm high.

5. View of a Mercury Barometer

6. Aneroid Barometer
An aneroid barometer has an airtight metal chamber that is sensitive to changes in air pressure.
The thin walls of the chamber flex in and out as the air pressure changes, which causes the needle on the dial to move.

7. Units of Air Pressure
In weather reports, air pressure is usually given in inches of mercury. National Weather Service maps use millibars.
One inch of mercury equals millibars. (25 = )

8. Altitude and Air Pressure
Altitude, or elevation, is the distance above sea level. As altitude increases, air pressure decreases. As air pressure decreases, so does density.
Air at sea level has the weight of the whole atmosphere pushing down on it so air pressure at sea level is the highest.
At the top of a mountain the air pressure is less and so is air’s density. This is why it is difficult to breathe, since there is less oxygen in each cubic meter of air. 8