Measurements of relative and absolute humidity are helpful in wind energy studies when considering downwind impacts of wind farms and wind turbines. Several of the instruments mentioned earlier that measure temperature via measuring the speed of sound (e.g., sonic anemometers, RASS) actually measure a temperature influenced by humidity measurements. Further, when measurements of atmospheric stability are desired, the contribution of moisture to stability can be important (Friedrich et al. 2012) as a situation may be conditionally unstable if moisture is considered but neutral or stable if the effects of moisture are neglected. Measurements of moisture profiles are possible with microwave profiling radiometers.
Further, Satellite-based Measurements of moisture can be useful. The Global Positioning System (GPS) consists of 31 satellites and a small number of ground stations. The large number of satellites and the different viewing angles from a detector to each satellite allow the delay in the GPS signal because of the atmosphere to be estimated. This delay can be related to the water content of the atmosphere above the detector, providing the total amount of water in a column from the ground to space (Bevis et al. 1992). Because most of this moisture is concentrated in the lower part of the atmosphere (the troposphere), the data can be used as input to atmospheric models.
Absolute humidity is measured in mass of water per unit volume of air, which is a function of the ambient air temperature.
Relative humidity is conveyed as a percent of the maximum absolute humidity possible at a given air temperature.