Chapter 3b: Effects of Weather on Small Unmanned Aircraft Performance  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 the temperature; warm air can hold more water vapor, while colder  air can hold less. Perfectly dry air that contains no water vapor has a relative humidity of zero  percent, while saturated air that cannot hold any more water vapor has a relative humidity of 100  percent. Humidity alone is usually not considered an essential factor in calculating density altitude  and aircraft performance; however, it does contribute.  Performance is a term used to describe the ability of an aircraft to accomplish certain things that make  it useful for certain purposes. Performance Climb Performance Factors Measurement of Atmosphere Pressure To provide a common reference, the International Standard Atmosphere (ISA) has been established.  These standard conditions are the basis for most aircraft performance data. Standard sea level  pressure is defined as 29.92 "Hg and a standard temperature of 59 °F (15 °C). Atmospheric pressure is  also reported in millibars (mb), with 1 "Hg equal to approximately 34 mb. Standard sea level pressure is  1,013.2 mb. Typical mb pressure readings range from 950.0 to 1,040.0 mb. Surface charts, high and low  pressure centers, and hurricane data are reported using mb. Since weather stations are located around the globe, all local barometric pressure readings are  converted to a sea level pressure to provide a standard for records and reports. To achieve this, each  station converts its barometric pressure by adding approximately 1 "Hg for every 1,000 feet of  A change in an aircraft’s weight produces a twofold effect on climb performance. An increase in  altitude also increases the power required and decreases the power available. Therefore, the climb  performance of an aircraft diminishes with altitude.  Weight has a very pronounced effect on aircraft performance. If weight is added to an aircraft, it  must fly at a higher angle of attack (AOA) to maintain a given altitude and speed. This increases the  induced drag of the wings, as well as the parasite drag of the aircraft. Increased drag means that  additional thrust is needed to overcome it, which in turn means that less reserve thrust is available  for climbing. Aircraft designers go to great lengths to minimize the weight, since it has such a  marked effect on the factors pertaining to performance. The primary factors most affected by performance are the takeoff and landing distance, rate of climb,  ceiling, payload, range, speed, maneuverability, stability, and fuel economy.  Since weight, altitude and configuration changes affect excess thrust and power, they also affect  climb performance. Climb performance is directly dependent upon the ability to produce either  excess thrust or excess power. 22  Remote Pilot – Small Unmanned Aircraft Systems Study Guide 