AWH 6-1

Chapter 6, Water Vapor 6-1 6 Water Vapor 6.1 Introduction Water vapor is the gaseous form of water and one of the most important of all constituents of the atmosphere. It constitutes only a small percentage of the Earth’s atmosphere, varying from only trace amounts to 4 percent by volume, and its amount varies widely in space and time. Approximately half of all of the atmospheric water vapor is found below 2 km (6,500 ft) altitude, and only a minute fraction of the total occurs above the tropopause. The development of clouds and precipitation can have potential impacts on flight operations. Water vapor is important, not only as the raw material for clouds and precipitation (e.g., rain and snow), but also as a vehicle for the transfer of heat energy and as a regulator of the Earth’s temperatures through absorption and emission of radiation, most significantly in the thermal infrared (i.e., the greenhouse effect). The amount of water vapor present in a given air sample may be measured in a number of different ways, involving such concepts as relative humidity and dewpoint. Before talking about these subjects, the process of water cycling through the Earth-atmosphere system will be discussed.