The fog that constantly surrounds Chief and the patients on the ward is, Chief claims, "made" by Nurse Ratched. Because we know that Chief is schizophrenic and sees things that are not literally there, we recognize that the fog may be medicinally induced and is a fog of the mind rather than a literal fog. It keeps the patients from rising up in rebellion against Nurse Ratched, but it also keeps them satisfied with their lives and prevents them from ever thinking about anything real. It both helps them to live this way and prevents them from ever trying to improve their life situation. As Chief says, the men hide behind the fog because it is comfortable.