Density Altitude is a number that is calculated using the temperature, barometer and humidity to determine the quality of the air. The numbers are meant to be compared to how the quality of air would be at sea-level on an average day. So on a day where there was 0% humidity, 60 degree temperature and a barometer of 29.92 – the Density Altitude is 1 ft. That means that the air quality is equivalent to a 60 degree day at sea level with an average barometer. As the barometer goes down (as it would with increased elevation) the DA goes up. If the temperature or humidity go up, the DA goes up. Rising DA values represent worsening air quality. If the barometer rises and the temperature and/or humidity are cool, the DA will descend and could go negative so you have a very high quality air. Lower humidity means there is more room for oxygen (which is valuable for and notable for naturally aspirated vehicles) which makes more horsepower.