Sky watchers in the northern hemisphere have snapped the first images of this year鈥檚 noctilucent clouds 鈥� silvery blue structures that are the highest clouds to form in Earth鈥檚 atmosphere. This season鈥檚 crop of clouds could be the biggest in years due to the lull in the sun鈥檚 activity.
鈥淣octilucent鈥�, 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.
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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掳, while also growing in number and getting brighter. The reason for the clouds鈥� spread is unclear, but some suspect it could be due to an increase in greenhouse gases. That鈥檚 because the gases actually cause Earth鈥檚 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鈥檚 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.
Because the sun has been abnormally quiet in recent years, noctilucent clouds could be especially bright and numerous this summer in the Northern hemisphere. 鈥淲e expect this year to be a bigger year because of lower solar activity,鈥� says Scott Bailey of Virginia Tech in Blacksburg, VA and a lead scientist for NASA鈥檚 (AIM) spacecraft, which launched in 2007 to study the clouds. The clouds may be about twice as abundant this year as they are when the sun is at the peak of its activity, he says.
Time lag
Although amateur astronomers last week snapped photos of the Northern hemisphere鈥檚 first noctilucent clouds of the season, AIM caught the first hints of the new cloud season the previous week, on 22 May. The hints came from the subtle dimming of sunlight passing through the Earth鈥檚 atmosphere, Bailey told New 杏吧原创.
Northern sky watchers will have the best chance of seeing noctilucent clouds between mid-June and mid-August. The southern hemisphere, which has fewer and dimmer clouds, will have its own peak some six months later.
Still, it鈥檚 not yet clear whether the number of northern clouds will truly crest this summer due to the solar cycle. Noctilucent cloud activity seems to peak roughly a year after solar activity hits its minimum, which researchers believe happened in December 2008 (see Solar cycle will be weakest since 1928, forecasters say).
But the exact time lag between solar minimum and peak cloud activity is uncertain, and researchers aren鈥檛 entirely convinced a lag even exists, Bailey says.
There are no good theories to explain observations of the apparent lag. 鈥淭here鈥檚 no explanation for it,鈥� Bailey told New 杏吧原创. 鈥淓very model says the clouds should respond immediately to what the sun is doing.鈥�
Following the clouds鈥� behaviour in the coming years could help resolve the matter.
