A dew warning, also known as a dew alarm or dew signal, is an error indication on VCRs and camcorders if the VCR/camcorder develops dew inside the unit from being exposed to extreme temperature and/or humidity changes.
The presence of moisture between the tape and the rotating head drum increases friction which prevents correct operation and can cause damage to both the recording device and the tape. In extreme cases, if the dew sensor fails to function and stop the video recorder, moisture can cause the tape to stick to the spinning video head. This can pull a large amount of tape from the cassette before the head drum stops spinning. The tape will be extensively damaged, the video heads will often become clogged, and the mechanism may be unable to eject the cassette.
The dew sensor itself is mounted very close to the video head drum. Contrary to how one might expect this to behave, the sensor increases its resistance when moisture is present. Poor contacts on the sensor can therefore be a cause of random dew sensor warnings.
Usually, a "DEW" indicator or error code lights up on the display of most VCRs/camcorders, and on some, a buzzer may sound.