The effects of humidity on density altitude are quantified in detail and graphically represented as a function of temperature and dew-point temperature for ease of use in professional aviation education. A ten-year climatology of dew-point temperatures for various representative locations throughout the United States is created to provide a basis for comparison and use with the graphical displays. Density altitude is demonstrated to be a function only of dew-point temperature for a given pressure altitude. The absolute errors between density altitude calculations that incorporate humidity to those that do not are combined with linear regression techniques to create a simple rule of thumb for diagnosing the impact of humidity on density altitude. The rule of thumb for the correction (in feet) is simply twenty times the dew-point temperature in Celsius, or colloquially, “double the dew point and add a zero.” This rule of thumb is shown to limit the percent error in density-altitude calculations to within five percent for the range of dew-point temperatures between 5°C to 30°C and elevations below 6,000 compared to over 20 percent for the same conditions using the dry case alone. The effect of humidity is also shown to create larger absolute errors in density-altitude calculations for the same dew-point temperature at higher pressure altitudes; however, the percent error decreases quickly with pressure altitude.