Air Pressure Air Pressure Air consists of atoms

Air Pressure

Air Pressure �Air consists of atoms �And molecules �Which both have mass. �Because air has mass, it also has other properties, including density and pressure.

Air Pressure �Density is the amount of mass in a given volume. �You can calculate density by dividing mass by volume.

Air Pressure �Pressure is the force pushing on an area or surface. �A denser substance has more mass than a less dense one. �So denser air exerts more pressure than less dense air.

Air Pressure Air pressure is the result of the weight of a column of air pushing down on an area. There is a column of air above you all the time. The weight of the air in the atmosphere causes air pressure.

Air Pressure �Why doesn’t air pressure crush your desk? The molecules in air push in all directions. So the air pushing down on the top of your desk is balanced by the air pushing up on the bottom.

Measuring Air Pressure � Have you ever heard a weather report say that the air pressure is falling?

Measuring Air Pressure �Falling air pressure usually indicates that a storm is approaching. �Rising air pressure usually means that the weather is clearing.

Measuring Air Pressure �A barometer is an instrument that is used to measure changes in air pressure. �There are two kinds of barometers: �mercury barometers �aneroid barometers

Measuring Air Pressure �A mercury barometer consists of a glass tube open at the bottom end and partially filled with mercury.

Measuring Air Pressure �The word aneroid means “without liquid. ” �An aneroid barometer has an airtight metal chamber. �The metal chamber is sensitive to changes in air pressure. �As the shape of the chamber changes, the needle on the dial moves.

Measuring Air Pressure

Increasing Altitude �Altitude, or elevation, is the distance above sea level. �Sea level is the average level of the surface of the oceans. �Air pressure decreases as altitude increases. �As air pressure decreases, so does density.

Altitude Affects Air Pressure �Imagine a stack of ten books. Which book has more weight on it, the second book from the top or the book at the bottom? �Air at sea level is like the bottom book. �Air near the top of the atmosphere is like the second book from the top.

Altitude Also Affects Density �If you were near the top of a mountain and tried to run, would you get out of breath quickly? �Why would you have difficulty breathing at high altitudes? �As you go up through the atmosphere, air pressure decreases �As air pressure decreases, the density of air decreases

Pressure and Density based on Elevation