Air Pressure Notes

Properties of Air Nitrogen (N2) Oxygen (O2) CO2, H2O, and others

Properties of Air Because air is made up of atoms of nitrogen, oxygen, carbon, and many others, it has mass. Because air has mass, it also must have density. Remember, density is the amount of mass in a certain volume (or how tightly packed particles are). Density = mass/volume m or d v

Properties of Air Also because air has mass, it has pressure. Pressure is the force pushing on an area or a surface. Air has pressure because air’s mass has weight due to gravity. It may not seem like a lot, but it is there. This weight presses down. Remember. mass x gravity = weight Air Pressure is the pressing down of the weight of air on an area. Denser air has more air pressure then less dense air.

The weight of all of this air presses down onto the surface, creating air pressure.

Measuring Atmospheric Pressure A barometer is an instrument that measures air pressure. One kind of barometer is a mercury barometer. A mercury barometer is made up of a glass tube that is open at the bottom end. The open end of the tube sits in a dish of mercury. As the air pressure increases on the dish of mercury, the level in the tube will rise.

Measuring Atmospheric Pressure An aneroid barometer has a closed space surrounded by metal. Air pressure pushes the metal in and out. As the shape of the space changes, a needle moves to show what the air pressure is. If you have a barometer in your house, hanging on the wall, this is the same type of barometer.

Measuring Atmospheric Pressure Weather reports use several different units for atmospheric pressure. Most weather reports for the general public use inches of mercury (inHg). For example, if the column of mercury in a mercury barometer is 30 inches high, the air pressure is 30 inHg. Some systems use mmHg, or millimeters of Hg, as well. Hg is the chemical symbol for mercury. National Weather Service maps indicate air pressure in millibars (mb). One inHg is approximately 33.87 mb, so 30 inHg is approximately equal to 1,016 mb.

Altitude and Atmospheric Pressure Altitude is the distance above sea level. Altitude is also known as elevation. Elevation is usually used if the object being referenced is still touching the ground. Sea level is the average level of the ocean’s surface.

Altitude and Atmospheric Pressure Air at sea level carries the weight of all of the air above it. As you go higher, the air has less air on top of it, and therefore it has less weight. So, air pressure at sea level is greater than the air at any place above it.

Pressure will decrease if air is rising for any reason. Pressure will increase if air is sinking. More on this later. Sea Level

Notice how the weight would decrease as the pyramid goes higher and higher.

Altitude and Atmospheric Pressure Altitude will also affect the density of the air. Because gravity is affected by both mass AND distance, gravity can more easily hold gases near the surface due to their low masses. As you go higher, the air becomes less and less dense. As you get closer to sea level, the air becomes more and more dense.

Altitude and Atmospheric Pressure Because the air is less dense, there are fewer molecules of nitrogen and oxygen in a given area. This is why, at the top of a mountain, it becomes very difficult to breathe and air tanks are required.