Sky watchers in the northern hemisphere have snapped the first images of this year’s noctilucent clouds — silvery blue structures that are the highest clouds to form in Earth’s atmosphere. This season’s crop of clouds could be the biggest in years due to the lull in the sun’s activity.
“Noctilucent”, or night-shining, clouds float dozens of kilometres higher than other clouds, at an altitude of about 80 km. Because of their height, they can be seen glowing before sunrise or after sunset as the sun illuminates them from below the horizon.
The clouds were first seen above polar regions in 1885, suggesting they may have been caused by the eruption of Krakatoa two years before. But in recent years the clouds have spread to latitudes as low as 40°, while also growing in number and getting brighter. The reason for the clouds’ spread is unclear, but some suspect it could be due to an increase in greenhouse gases. That’s because the gases actually cause Earth’s upper atmosphere to cool, and the clouds need cold temperatures to form.
Although the average number of noctilucent clouds has been increasing in recent decades, their abundance also seems to rise and fall with the sun’s 11-year cycle of activity. The clouds thrive when the sun is quiet and spews less ultraviolet radiation, which can destroy water needed to form the clouds and can keep temperatures too high for ice particles to form.