A passive device for reducing the relative humidity of a flowing gas has a container in which a tube is disposed. A flowing gas passes into the-container-through a inlet and then flows downwardly through a cavity in the container, whereupon the gas transfers heat to the tube and the walls of the container. As a result, liquid water condenses from the water vapor content of the gas. The flowing gas then flows upwardly through a channel in the tube and is then warmed by the heat previously transferred to the tube, whereupon the relative humidity of the flowing gas decreases.