Nacreous clouds are rare and form in the lower stratosphere during the winter months in polar regions. Because the stratosphere is extremely dry, these clouds only appear when temperatures drop low enough for the tiny amount of available water vapor to freeze into ice crystals. These crystals refract sunlight, creating a shimmering, rainbow-like effect. Despite their beauty, they are scientifically significant because they facilitate the conversion of harmless chlorine compounds into highly reactive forms. When sunlight returns in the spring, these reactive chemicals destroy ozone molecules, contributing directly to the formation of the Antarctic ozone hole.