Weather events are part of every student's experience, and are controlled by basic principles involving the behavior of matter and energy. Despite this, many students have difficulty explaining simple atmospheric phenomena, even after exposure to primary and secondary science curricula. This study investigated the level to which undergraduates understood the formation of clouds in the atmosphere, and how effectively they incorporated fundamental principles of matter and energy into their explanations. Interviews with earth science undergraduates at the University of Maine indicated that many had trouble with the correct identification of water in its different states, and were unable to name the sources of moisture in certain cases of cloud formation. If these misconceptions can be recognized and addressed directly by instructors, the potential exists to lead students to form better and more accurate mental models of weather.