Density Altitude defined (part 1 of 3)

One of the reasons that airplanes fly is because of lift. Lift is created by air molecules (oxygen) flowing over the wings. More air molecules (high density air) means more air molecules flowing over the wings and greater lift. Conversely, fewer air molecules or lower density air means less lift because fewer air molecules are flowing over the wing.

Three factors, either singly or in combination, contribute to low density air: (1) higher altitude, (2) higher temperatures because air expands when it is heated, and (3) higher relative humidity because more water molecules mean fewer air molecules.

Low density air affects engine performance. One of my pilot friends, whom I will call T, explains:

Simply put, the lower the air density, the lower the oxygen level.

For carbureted engines, such as an automobile, less oxygen means less horsepower.  The higher you go in altitude, the air density lowers.

Example:  Suppose you were on a straight highway that went from sea level to 12,000 feet above sea level.  It doesn't matter how long it takes to get to the top, but we'll say 100 miles.  Simple math tells us that the road will rise 120 feet per mile.  As we travel along, we're getting higher in altitude and the oxygen is getting thinner as we go.  After awhile, the car isn't running too well.
       
1.  As there is less oxygen to burn at higher altitudes, the engine can't burn fuel as well as it could at the bottom.
2.  As air density decreases, so does available horsepower.
       
In newer vehicles, the computer module can adjust the fuel mixture a little bit, but not like aircraft.  In an aircraft, the pilot can adjust the fuel mixture the entire range from full rich, to a lean idle cut-off.  This is a necessity due to air density.

Coming up on Oct. 16: density altitude and air tanker performance