A Howler was an enchanted letter in a red envelope which could be sent to a recipient. It relayed the message in the voice of the writer at high volume, and if the letter wasn't opened quickly enough, the volume increased, or would explode if unopened entirely.

Howlers were typically sent to deliver angry or unpleasant messages, particularly disappointed or displeased parents to students. Notably, Ron Weasley received one from his mother, Molly Weasley, after the Flying Ford Anglia crashed into the Whomping Willow.