Abstract: A humidity sensor system includes a probe in the form of a hollow tubular housing which is attached to a fixed surface. The remote end of the housing has a sensing cap mounted thereto. A liquid coolant supply tube is disposed in the housing and includes a spray nozzle which is located within the cap, but spaced from the inner surface of the cap. The cap is made of a porous material capable of absorbing the liquid coolant sprayed from the nozzle. A temperature detector is mounted to the cap for detecting the temperature change. Also included in the humidity sensor system is a microcomputer which controls the sensor probe, and calculates humidity from the temperature data it generates.