"You'd better open it, Ron. It'll be worse if you don't. My gran sent me one once, and I ignored it and — it was horrible."
—Neville talking to Ron about his Howler[src]

A Howler is a magical letter in a red envelope which enchants the written message into the writer's voice, usually at a very high volume. The physical temperature of the Howler begins to rapidly increase upon delivery, and it will explode if left unopened for too long. This mechanism ensures that the recipient will open the Howler, even though he or she knows that it contains an unpleasantly loud message. Once the message has been received, the envelope bursts into flames, leaving only ashes.

The purpose of the Howler is to deliver a message expressing anger or great displeasure in a manner which standard writing cannot adequately convey. As such, a howler will convey the displeasure of its author/sender even if left unopened, for it will shower the recipient with insults and cursing upon exploding.

People who have been known to receive Howlers