Night sky photographer Ruslan Merzlyakov captured bright blue-white noctilucent clouds dancing over Nykøbing Mors in Denmark just before dawn on July 1.

Earth's polar skies have shined with eerie blue-white glowing clouds slowly twisting and undulating in the twilight sky every summer since the late 19th century. These mystifying clouds are ...

I love the different forms they take. There are the puffy cotton ball-like clouds against the blue sky — those iconic cumulus clouds. There are curlicue cirrus clouds wafting high above made of ice.