Understanding air density and its effects

Understanding air density and its effects

By Jack Williams, USATODAY.com

In simple terms, density is the mass of anything - including air - divided by the volume it occupies.

In the metric system, which scientists use, we usually measure density in terms of kilograms per cubic meter.

The air's density depends on its temperature, its pressure and how much water vapor is in the air. We'll talk about dry air first, which means we'll be concerned only with temperature and pressure.

In addition to a basic discussion of air density, we will also describe the effects of lower air density – such as caused by going to high altitudes – on humans, how humidity affects air density – you might be surprised – and the affects of air density of aircraft, baseballs, and even racing cars.

The molecules of nitrogen, oxygen and other gases that make up air are moving around at incredible speeds, colliding with each other and all other objects. The higher the temperature, the faster the molecules are moving. As the air is heated, the molecules speed up, which means they push harder against their surroundings.

If the air is in a balloon, heating it will expand the balloon, cooling it will cause the balloon to shrink as the molecules slow down. If the heated air is surrounded by nothing but air, it will push the surrounding air aside. As a result, the amount of air in a particular "box" decreases when the air is heated if the air is free to escape from the box. In the free atmosphere, the air's density decreases as the air is heated.

Pressure has the opposite effect on air density.

Increasing the pressure increases the density. Think of what happens when you press down the handle of a bicycle pump. The air is compressed. The density increases as pressure increases.

Altitude and weather systems can change the air's pressure. As you go higher, the air's pressure decreases from around 1,000 millibars at sea level to 500 millibars at around 18,000 feet. At 100,000 feet above sea level the air's pressure is only about 10 millibars. Weather systems that bring higher or lower air pressure also affect the air's density, but not nearly as much as altitude.

We see that the air's density is lowest at a high elevation on a hot day when the atmospheric pressure is low, say in Denver when a storm is moving in on a hot day. The air's density is highest at low elevations when the pressure is high and the temperature is low, such as on a sunny but extremely cold, winter's day in Alaska. (Related: Understanding air pressure)

Effects of lower density on humans

If you go high enough, either by climbing a mountain or going up in an airplane that does not have a pressurized cabin, you will begin feeling the effects of lower air pressure and density.

As air pressure decreases oxygen continues to account for about 21% of the gasses in the air as it does at sea level. But, there is less oxygen because there is less of all of the air's gasses. For instance, by the time you go to 12,000 feet the air's pressure is about 40% lower than at sea level. This means that with each breath you are getting about 40% less oxygen than at the lower altitude.

These effects aren't felt in airliners because the cabins are pressurized to keep the air density inside about the same as it would be about 6,000 or 7,000 feet above sea level.