Abstract: Provided is a hand-held, portable, battery operated, reliable, and inexpensive, ultrasonic, visible vapor generator or fog generator for use in contamination sensitive areas, such as clean rooms. The ultrasonic fog generator is useful for visualizing laminar airflow between rooms, around equipment, around ventilation systems, and the like. The ultrasonic fog generator includes a housing and a battery pack sized to be carried by a user. The fog generator of the present invention, therefore, is capable of being easily moved around a room and does not take up valuable floor space in the clean room.