Radio acoustic sounding systems (RASS) measure (virtual) temperature remotely, usually looking vertically from the ground. RASS uses a radar to measure the speed of propagation of an acoustic disturbance and derive therefrom the virtual temperature. Virtual temperature is the temperature of dry air at the same density as the moist ambient air, and is the appropriate measure for air density. It is up to a few degrees larger than (kinetic) temperature in very humid (tropical) conditions.
Probably the most common application for RASS is in field measurements for air quality studies. Approximately 30 to 50 wind profiler systems with RASS capability are in use throughout the world for these applications. Many of these are deployed on a campaign basis, while others are at fixed locations such as power plants. Important parameters measured by RASS in air quality applications are mean temperature, temperature profiles, and mixing depth.
The most common wind profiler RASS can measure virtual temperature from approximately 150 m above ground level.The height coverage is limited by the attenuation of the acoustic signal, which at these acoustic frequencies (2-3 KHz) can be as much as 20 dB km-, and by the advection of the acoustic signal out of the radar beam by the wind. The coverage therefore depends on temperature, humidity, and wind speed.
The temperature affects the rates of chemical reactions determining air quality. The vertical profile of temperature can also aid in diagnosis of air transport and other meteorological phenomena affecting air quality.