The IRS21 is a passive sensor embedded in the monitored road surface, flush with the surface, to provide measurements of actual road conditions. It measures surface temperature, water-film level, freezing temperature for NaCl, and road condition (dry, damp, wet, ice or snow, residual salt, freezing). The IRS21 is a primary component of our road-weather information stations (RWIS). Two IRS21 sensors are typically installed at each measurement site. An SDM-SIO1 or other RS-485 to RS-232 converter is required for communication with Campbell Scientific data loggers.