Density Altitude
The problem with atmospheric conditions is that you are forced to juggle more than one variable when evaluating air density. Perhaps you've seen an old air-density gauge that calculated density as a percent based on air temperature and barometric pressure. The problem with that gauge is that it ignores humidity (vapor pressure). Pilots were the first to work out a way to juggle two of the three variables by using a system called density altitude. For aviation purposes, vapor pressure is ignored. The formula (which is way too complex to dive into here) takes temperature and pressure conditions into account to come up with a single equivalent elevation above sea level.
Standard motorsports temperature and pressure are established to be 60 degrees F, 29.92 inches of mercury pressure (which equals 14.7 psi), and zero humidity. Aviation standard temperature is 59 degrees, which is why you will see that figure used sometimes. That is considered zero-density altitude. Any temperature or vapor-pressure increase or pressure decrease will contribute to raise the density altitude. This ultimately means a decrease in overall air density.
According to the folks at Altronics, there is one more variable here that we have not touched upon. The science books tell us that the oxygen content of air is roughly 21 percent. But the reality is that this can vary by a few tenths of a percent. This can have a slight effect on performance, and it's significant if you are a bracket racer where every hundredth of a second must be tracked in order to make the car as consistent as possible. For the rest of us mere mortals, however, keeping track of density altitude will suffice to make us better tuners than most of our friends.
The Weatherman Lies
Have you ever wondered why the weatherman in Denver will tell you with a smile on his face that it's going to be a beautiful day with the pressure at 30.12 when your uncorrected mercury barometer tells you it's actually something like 24.80? The government decided that regardless of the altitude, atmospheric pressure readings from the National Weather Service should be altitude-corrected based on similar conditions that would exist at sea level.
In most cases this would not be important, except that you cannot plug the weatherman's data into your density-altitude calculations because the data is incorrect for the altitude. This is why if you are attempting to do density-altitude calculations, you must use what is called uncorrected station pressure. This is actual atmospheric pressure at a given altitude and can be obtained from the nearest airport by asking for uncorrected station pressure. The higher the elevation, the greater the disparity will be between those numbers and what the weatherman gives you.