Sullen refers to a mood or demeanor that is gloomy, silent, or withdrawn. People who are sullen often appear unhappy or brooding, showing little interest in engaging with others. This behavior can be a response to disappointment, sadness, or frustration, and it may manifest in facial expressions, body language, or a lack of communication. The term is often associated with a heavy atmosphere, where the environment feels dark or oppressive. For example, a sullen sky filled with gray clouds can create a sense of gloominess, affecting the mood of those who experience it. This feeling can also be reflected in literature and art, where characters or scenes evoke a sense of melancholy.