On calm, clear nights, the surface temperature of leaves can drop well below air temperature due to a net loss of longwave radiation from the leaf into the air. A radiation frost occurs when frost forms at the surface of the leaf before the air temperature reaches freezing (0 C). Under cloudy and/or windy conditions, radiation frost events do not occur. The Apogee radiation frost detector contains a high accuracy thermistor mounted in a durable housing. The thermistor is designed to mimic a leaf, providing estimates of leaf temperatures and a direct means of monitoring radiation frost events.