The Winds that Struck Pella Thursday morning were caused by a rare meteorological event, called a heat burst. Jim Lee with the National Weather Service explains that a heat burst occurs when a thunderstorm collapses and a dry layer of air heats up as it’s pulled downward. The burst are common in the high plains, but uncommon in Iowa. Still, they do happen a heat burst in Southwest Iowa last year produced 100 mph winds. The highest measured gust in our area was 60 mph, recorded in Bussey, also hit by the burst.