The season for spotting Noctilucent Clouds or the “night-shining” clouds has begun, NASA said in a statement in early July.

Noctilucent Clouds are composed of tiny ice crystals 40 to 100 nanometers wide, which is just the right size to scatter blue wavelengths of sunlight, a NASA scientist explained.

According to NASA, the best time and location to search for these breathtakingly beautiful phenomenon would be between mid-May and the end of August in the northern hemisphere. However, with no explanation yet found, these glowing, mystical clouds have been seen even as far south as Utah and Oregon and Denver, Colorado in the recent years.

Noctilucent Clouds were first observed in 19th century but their appearance has increased over time and are seasonal, appearing most often in late spring and summer every year.

NASA’s AIM (Aeronomy of Ice in the Mesosphere) satellite, which was launched in 2007, continues to study the clouds, while scientists strive to know if noctilucent clouds signal climate change.

They were a high-latitude phenomenon, but in recent years, AIM has spotted the clouds appearing ever lower in latitudes, but just why is not yet known,” NASA said.

Noctilucent Clouds first appeared in the nineteenth century, the era of Industrial Revolution; they are spreading in the recent years, which has puzzled scientists to the possible connection of the clouds with weather and climate, it added.

As this “beautiful mystery” refuses to unravel itself, check out some of the startling pictures from Noctilucent Clouds season 2011: