In severe weather situations, the air moisture content can vary considerably between locations. When humidity levels are very high, the excessive moisture in the atmosphere can cause lightning to appear soft, even if the focus is tack sharp. Since moisture levels in the elevated terrain of the western and central United States are typically drier in spring and summer than in areas farther east, lightning bolts in those areas often appear more distinct and dramatic.

From Night & Low-Light Photography Professional Techniques from Experts for Artistic and Commercial Success by Jill Waterman (AMPhoto Books, 2008, $30)