Temperature, Pressure, and Relative Humidity
Typical meteorological quantities measured close to the surface include temperature, pressure, and relative humidity. These quantities are part of the “standard” set of surface observations. In the United States, such observations are collected regularly by Automated Surface Observing Stations. A time series of surface temperature may be used to provide a bottom boundary condition for simulations, and temperature and pressure are used together to estimate air density, which directly affects wind power production. Surface air density at sea level is, on average, 1.225 kg m-3, but at altitudes of approximately 1.6 km, surface air density decreases to nominally 1.0 kg m^-3.
Temperature, pressure, and relative humidity measurements are typically considered at a technology readiness level (TRL) of > 8 and have been so for several years. As summarized by U.S. Environmental Protection Agency (2000), sensors for monitoring ambient temperature include wire bobbins, thermocouples, and thermistors, but platinum resistance temperature detectors are most widely used in the atmospheric science community. Temperature and relative humidity probes are typically coupled together and housed within an aspirated radiation shield, such as that shown in Figure 2, and mounted at a standard altitude of approximately 2 meters (m) or 10 m above the surface. A barometer to measure pressure is typically mounted nearby, preferably at the same altitude as the thermometer.
Temperature is measured in K or °C.
Pressure is measured in ATM, bar, kPa inHg, or mmHg.
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.