A pulse storm is a single cell thunderstorm that is usually not very strong; when it is of substantial intensity, it only produces severe weather for short periods of time. Such a storm weakens and then generates another short burst -- hence "pulse." Pulse storms ordinarily form in environments with low wind shear and moderate instability, with the low wind shear contributing a short average lifespan of less than an hour. Severe weather in a pulse storm will most often occur during a momentarily forceful updraft, taking the form of hail or particularly damaging winds brought about by downbursts. Rarely, a weak tornado develops in association with a pulse storm.