Introduction Density Altitude Effects of Temperature on Density Effects of Pressure on Density Effects of Humidity (Moisture) on Density This chapter discusses the factors that affect aircraft performance, which include the aircraft weight,  atmospheric conditions, runway environment, and the fundamental physical laws governing the forces  acting on an aircraft.  Since the characteristics of the atmosphere have a major effect on performance, it is necessary to  review two dominant factors—pressure and temperature. The more appropriate term for correlating aerodynamic performance in the nonstandard atmosphere  is density altitude—the altitude in the standard atmosphere corresponding to a particular value of air  density. As the density of the air increases (lower density altitude), aircraft performance increases. Conversely,  as air density decreases (higher density altitude), aircraft performance decreases. A decrease in air  density means a high density altitude; an increase in air density means a lower density altitude. Density  altitude has a direct effect on aircraft performance.  Air density is affected by changes in altitude, temperature, and humidity. High density altitude refers to  thin air while low density altitude refers to dense air. The conditions that result in a high density  altitude are high elevations, low atmospheric pressures, high temperatures, high humidity, or some  combination of these factors. Lower elevations, high atmospheric pressure, low temperatures, and low  humidity are more indicative of low density altitude.  Since air is a gas, it can be compressed or expanded. When air is compressed, a greater amount of  air can occupy a given volume. Conversely, when pressure on a given volume of air is decreased, the  air expands and occupies a greater space. That is, the original column of air at a lower pressure  contains a smaller mass of air. In other words, the density is decreased. In fact, density is directly  proportional to pressure. If the pressure is doubled, the density is doubled, and if the pressure is  lowered, so is the density. This statement is true only at a constant temperature. Increasing the temperature of a substance decreases its density. Conversely, decreasing the  temperature increases the density. Thus, the density of air varies inversely with temperature. This  statement is true only at a constant pressure. In the atmosphere, both temperature and pressure decrease with altitude and have conflicting  effects upon density. However, the fairly rapid drop in pressure as altitude is increased usually has  the dominant effect. Hence, pilots can expect the density to decrease with altitude. The preceding paragraphs are based on the presupposition of perfectly dry air. In reality, it is never  completely dry. The small amount of water vapor suspended in the atmosphere may be negligible  Remote Pilot – Small Unmanned Aircraft Systems Study Guide  21 Chapter 3b:  Effects of Weather on Small Unmanned Aircraft Performance 