Air Density

The Atmosphere

Your aircraft will be flying through atmosphere at a certain elevation, so we might as well start there.  Think back to your early science classes when you were taught about the environment.  You may not remember the exact mixture, but I’m sure you recall that our atmosphere is a mixture of gases.  What you might not recall is that gases have weight, and are thus subject to the laws of physics.  Thus, we can determine some basic facts about the air based on the temperature, humidity, and pressure of the air.  Why is this important?  Because temperature, humidity, and pressure affect air density – and air density is a key component of aircraft performance!

Density Decreases with Altitude (Generally)

The standard sea level pressure is 14.7 pounds per square inch or 29.92 inches of mercury displaced.  At a constant temperature, density will increase proportionally with pressure.  Put simply, if you double the pressure the density too will double.  However, at a constant pressure, density varies inversely to temperature.  If the temperature rises the density falls, and vice versa.  As most of us probably know, as you go to higher altitudes the air is “thinner” (lower pressure if the temperature remains constant).  But the temperature doesn’t remain constant at higher elevations; it becomes cooler (higher density if the pressure remains constant).  So which one wins?   There are forces trying to influence the density both higher and lower.  The heading gives it away – overall, density decreases with altitude.  Again, this is why people generally know the air is “thinner” at higher elevations.  But wait, let’s not forget humidity!

Humidity also Plays a Role

While temperature and pressure are important, humidity is another leg of the air density stool (so to speak).  Air humidity has an inverse relationship with air density.  Because water vapor is lighter than air, air density tends to decrease with increases in humidity.  Although humidity does play a role in the performance of an aircraft, it is not as pronounced of a difference as compared to differences in temperature and pressure (2, p180).

The Importance of Air Density

Air density is an important concept to consider when flying.  We’ll talk more about this concept at a later point when we begin to discuss density altitude; but for now just take this away with you: air density affects the performance of your aircraft.  A lower density will degrade performance.  Not all air is created equal, and the air is nearly never the same in a given location day in and day out!  Not knowing this could one day cause you to run short of runway on takeoff.

Watch the following video to illustrate the latter point more clearly.