Air humidity is the water vapor existing in the atmosphere. In this paper it is presented measurements of the relative air humidity inside a closed container where temperature was kept constant. Under these conditions, humidity increases until water vapor becomes saturated. At this moment, humidity reaches its maximum value. The experimental set includes: a closed glass container, a hygrometer, a thermometer and a chronometer. Thus, it is a simple experiment, which is possible to be reproduced in a general physics laboratory and used as a complement to textbooks that do not include this topic.
