Aircraft performance is improved in cooler, denser air, which has a lower density altitude. As the density altitude increases, the air becomes less dense and performance is degraded. High density altitude is likely to occur in conditions that include high elevations, low atmospheric pressure, high temperature, and high humidity. Not all of those factors have to be present for the density altitude to be high; any combination is enough to have an effect on your flight. For example, you may experience high density altitude at an airport that's at sea level but with high air temperature and humidity.

Using the chart in your aircraft's flight manual is a simple way to calculate density altitude, which is derived by inputting the pressure altitude and the outside air temperature. At standard temperature, pressure altitude and density altitude are the same, so density altitude is simply the pressure altitude adjusted for nonstandard temperature.

When listening to weather broadcasts from control towers or automated stations, you might have noticed that the density altitude is sometimes included in the remarks. Anytime the density altitude at an airport is 1,000 feet or more above the field's elevation, it will be announced on the automatic terminal information service (ATIS) or automated weather observing system (AWOS) broadcast. The density altitude is the altitude the aircraft "feels" it is at, meaning that a field elevation of 600 feet msl with a density altitude of 1,800 feet msl will result in the aircraft performing as if at 1,800 feet even though the actual elevation is 600 feet.

This is important to consider in your decision making because a high density altitude will increase the distance your aircraft will require for takeoffs and landings, and will result in a reduced rate of climb. The amount of runway required for a takeoff could, in extreme cases, more than double depending on the density altitude. This is critical information to have for any flight, but especially if you’re using a short runway. In that situation, it may be best to fly in the early morning or evening when the air is cooler and density altitude is lower. Remember, sometimes the aircraft may not climb at all, so density altitude should always be a part of the go/no-go decision. How you load the aircraft with fuel or passengers will become one of the most important decisions for such a flight.

For more information about aircraft performance as it relates to density altitude, see this FAA publication as well as Chapter 11 of the Pilot's Handbook of Aeronautical Knowledge.