This system monitors temperature and air moisture in multiple rooms and warns when relative humidity is above 60%--a climate in which mold and dust mites thrive. Up to three remote indoor sensors (one included; additional sensors sold below) and the base station measure temperature and humidity in the rooms they are located. The remote sensors wirelessly transmit climate readings to the base station's backlit 4" sq. LCD, which flashes a warning icon to alert you to dangerous humidity levels. The system also comes with a wireless outdoor sensor that monitors the current weather and 24-hour forecast. The base station automatically synchronizes its calendar and clock with the atomic clock in Fort Collins, Colorado. Remote sensors have a 100' range. Includes six AAA batteries and one AA battery to power the base station and sensors. Base station: 5 1/3" sq. x 1" D. (10 1/2 oz.) Indoor sensor: 3 2/3" L x 2 1/2" W x 3/4" H. (2 1/4 oz.) Outdoor sensor: 2" L x 1" W x 3 3/4" H. (2 1/2 oz.)