Pressure, temperature and humidity all affect air density. And you can think of air density as the mass of air molecules in a given volume.
How much does air density decrease with altitude?
Density and pressure/temperature
Air density will decrease by about 1% for a decrease of 10 hPa in pressure or 3 °C increase in temperature.
How does dew point affect density altitude?
Dew point isn’t used in calculating density altitude because density altitude is pressure altitude, corrected for non-standard temperature. The temperature could remain the same, but pressure will change as fronts move over an airport, thus changing density altitude.
