Next week, crowds will flock into cities like Indianapolis, Cleveland, and Dallas for a chance to view a total solar eclipse, a rare event which occurs when the moon completely covers the sun for a few short minutes. There is one factor that could complicate the experience for eclipse watchers: clouds. Cloudy skies are known to dull the effects of total eclipses, making them less dramatic. That could spoil a celestial event people in the U.S. won’t get to see again for two decades.