Introduction  Effect of Temperature on Density Effect of Humidity (Moisture) on Density The manufacturer may provide operational and performance information that contains the operational  performance data for the aircraft such as data pertaining to takeoff, climb, range, endurance, descent,  and landing. To be able to make practical use of the aircraft’s capabilities and limitations, it is essential  to understand the significance of the operational data. The use of this data in flying operations is  essential for safe and efficient operation. It should be emphasized that the manufacturers’ information  regarding performance data is not standardized. If manufacturer-published performance data is  unavailable, it is advisable to seek out performance data that may have already been determined and  published by other users of the same small UA manufacturer model and use that data as a starting  point.  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, a fairly rapid drop in pressure as altitude increases usually has a dominating  effect. Hence, pilots can expect the density to decrease with altitude.  The preceding paragraphs refer to air that is perfectly dry. In reality, it is never completely dry. The  small amount of water vapor suspended in the atmosphere may be almost negligible under certain  conditions, but in other conditions humidity may become an important factor in the performance of an  aircraft. Water vapor is lighter than air; consequently, moist air is lighter than dry air. Therefore, as the  water content of the air increases, the air becomes less dense, increasing density altitude and  decreasing performance. It is lightest or least dense when, in a given set of conditions, it contains the  maximum amount of water vapor. Humidity, also called relative humidity, refers to the amount of water vapor contained in the  atmosphere and is expressed as a percentage of the maximum amount of water vapor the air can hold.  This amount varies with temperature. Warm air holds more water vapor, while cold air holds less.  Perfectly dry air that contains no water vapor has a relative humidity of zero percent, while saturated  air, which cannot hold any more water vapor, has a relative humidity of 100 percent. Humidity alone is  usually not considered an important factor in calculating density altitude and aircraft performance, but  it is a contributing factor.  Remote Pilot – Small Unmanned Aircraft Systems Study Guide  43 Chapter 8:  Determining the Performance of Small Unmanned Aircraft 