Noctilucent clouds are the highest clouds in Earth’s atmosphere. They form under extremely cold conditions—below -120°C—when water vapor freezes onto “smoke” particles left behind by meteors or volcanic dust. Because the mesosphere is incredibly dry, these clouds are very thin and only become visible when the lower atmosphere is in shadow. Their increasing frequency in recent years is a subject of scientific study, as some researchers believe they are linked to rising methane levels in the upper atmosphere, which breaks down to form the water vapor necessary for these ice crystals to grow.