For those too lazy to look it up: ˘ is called a "breve", and it is rounded, not even pointy as the typical "hat" is in physics and mathematics literature.

Obviously, a rounded hat like that will stay on top of a human head only for a brief amount of time. Not sure if that is the actual reason (people in this thread are fairly pedantic about causality), but it may help to remember that a breve is used to signify short vowels.

Breve means short (in duration, as in brevity) too, if it helps to remember it.