A polarization resistance type sensor has been designed and has been evaluated with an Atmospheric Corrosion Rate Monitor (ACRM). A year of hourly average data were evaluated with respect to simultaneously collected environmental data at a site in the Research Triangle Park, NC. The system can be used as a time-of-wetness meter. The primary effect on time-of-wetness is the difference between dew point and surface temperature. Sensor responses to wetness are reproducable between sensors and are sensitive enough to detect significant environmental changes other than the primary effects of surface temperature and air moisture content.