Stupid Weather Facts

OK, these are not stupid weather facts, nor is it just gee-whiz weatherman and pilot stuff. This is stuff every hotrodder should know, and every drag racer must know if they want to be consistent. Indulge me a long rambling post as I endeavor to explain.

A barometer and an altimeter are basically the same instrument. We just read them different depending on what we want to know. In either case, they are typically constructed of a sealed bellows connected to an indicator on a scale, which indicates atmospheric pressure. In an aircraft we want to know our altitude, so we correct our altimeter to the current atmospheric conditions. With a barometer, we want to know pressure so we correct to our actual MSL altitude.

As you discovered, on a "Standard Day" at sea level the atmospheric pressure is 29.92" Hg (inches of mercury) and the temperature is 15 degrees Celsius (59 Fahrenheit). Using this as a standard point of reference we not only have a pressure altitude of 0' MSL, but a Density Altitude of 0' MSL as well. As you also noted, atmospheric pressure decreases (less air above you pushing down) as altitude increases. Temperature also decreases (approx 2 degrees C per 1000 feet), which is equally important. Density Altitude is what we are concerned with.

So to recap this far, Pressure Altitude is where you are, corrected for barometric (weather) conditions, and Density Altitude is Pressure Altitude corrected for temperature. Or more simply, Density Altitude is the actual amount of air (specifically oxygen) available for our engines or lungs.

As Density Altitude increases, engine performance decreases in a basically linear and predictable fashion (assuming correct fuel mixture). Horsepower ratings (SAE net) are based on standard atmospheric conditions, and good dyno operators should either correct to standard day conditions or publish their numbers as "uncorrected". This way we can compare apples to apples.

Most race tracks have a weather station, and many serious racers have their own. This way they always know what the Density Altitude is. By keeping good notes, a racer can look at Density Altitude, make an informed prediction regarding jet choice and dial-in. Really handy if you travel to different tracks. Also nice for keeping an eye on the general health of your race car.