Who doesn’t like a pretty cloud in the sky? As familiar as the Sun and the Moon, clouds assume a wide array of shapes and sizes—from ominous cumulonimbus to cottony altocumulus, feathery cirrus to leaden stratus, and various hybrids in-between. We see them year-round worldwide, boding fair and foul weather alike. The great majority of clouds inhabit the troposphere, the lowest atmospheric layer ranging from the ground up to 12 kilometers. But two uncommon varieties dwell far higher, in the stratosphere and mesosphere. Respectively called nacreous and noctilucent clouds, they are amongst the most stunning meteorological phenomena.
Nacreous clouds are technically a Type II polar stratospheric formation, visible in both hemispheres between latitudes 50◦ and 85◦. Appearing during civil twilight as undulating veils of pink, orange, and green, they derive their name from the Latin word for mother-of-pearl. Nacreous clouds are composed of microscopic ice crystals that diffract and scatter sunlight at an altitude of 25 kilometers. Owing to their height, nacreous clouds glimmer well after sunset in the polar winter months.
Type I formations are somewhat similar to nacreous cloud, though different in chemical composition. They scatter sunlight less efficiently; hence, Type I clouds are less iridescent. Their formation depends upon stratospheric nitrogen oxide, sulfur dioxide, and water vapor chilled to about -78◦ C.
In contrast, noctilucent clouds form in the mesosphere, about 80 kilometers high. At -125◦ C, upwelling water vapor crystallizes on minute dust particles, scattering sunlight into diaphanous streamers of icy blue and ruddy gold. Noctilucent clouds shine high and bright through the night-- hence their Latinate name. Appearing northwest just before astronomical twilight, they gradually rise overhead during the polar summer months.
Noctilucent clouds were first noticed over Europe in 1885, two years after the cataclysmic Krakatoa eruption massively ejected volcanic ash and water vapor into the upper atmosphere. Volcanic activity contributes to their formation, as does meteoric disintegration and rocket exhaust.
Scientists regard polar clouds as bellwethers of environmental change. Type I formations are hypothesized to produce chlorine monoxide, a volatile ozone-depleting gas. Ozone protects the Earth’s surface from harmful levels of ultraviolet solar radiation. Noctilucent clouds are correlated with methane, a greenhouse gas associated with global warming. Over the last century, the increased prevalence of noctilucent clouds (especially at lower latitudes) suggests rising methane levels.
For the sake of the biosphere, one may hope for ever fewer nacreous and noctilucent clouds. Yet, their ghostly resplendence cannot-- and should not --be denied. Should you be lucky to see one someday, be mindful of their equally striking beauty and warning.