photo #7624914

The vast majority of Earth’s clouds form in the lowest 8 miles (12 km) or so of the atmosphere, which is known as the troposphere and is where weather happens. Nacreous clouds are an exception. They’re extreme-altitude clouds that form up in the region above, the stratosphere. They’re colourful wintertime clouds forming up at altitudes of 9-16 miles (15-25 km). They tend to form at higher latitudes, particularly towards polar regions, because they only appear when the temperatures in the stratosphere are lower than –85°C (–120°F). Extreme cold like this is needed to form the ice crystals of nacreous clouds in a region of the atmosphere much drier than the troposphere below it.When they do form, the ice crystals are particularly small. And that’s why nacreous clouds can have vivid mother-of-pearl colours. The minuscule and uniformly sized crystals diffract, or scatter, the sunlight, separating it into wavy bands of pastel hues. These shine out against the darkened sky when the Sun’s just below the horizon. Roger Hewstone (Member 43,631) spotted these nacreous clouds over Liverpool Bay, Lancashire, England – further south than they usually appear. His multicoloured scene was punctuated by the silhouetted forms of Cumulus and Stratocumulus clouds lurking in the shadows of the sky below.