Still shrouded in mystery, these "night-shining clouds" were photographed illuminating the sky over Budapest, Hungary, on June 15.
They are among the first such noctilucent clouds pictured this year.
The clouds form in the upper layer of the Earth's atmosphere in summer.
Scientists don't know much about how the clouds form, or why they have been appearing more often at lower latitudes and glowing brighter.
However, the clouds "could represent an early warning that our Earth environment is being changed.
Source: National Geographic
Tags: Night Shining | Clouds