Following a delayed start to the northern hemisphere's 2017 Noctilucent clouds season, the magnificent, electric-blue clouds are now back in full effect.
Noctilucent, or night-shining, clouds are a seasonal phenomenon, occurring most often in late spring and summer. They are composed of tiny cristals of water up to 100 nm in diameter and are formed at a height of about 85 km (54 miles), higher than any other clouds. In the northern hemisphere, the best time to observe them would be during local summer, between mid-May and the end of August.
This year, they were briefly spotted late May over Denmark, Germany and UK, but that's when something happened and they mysteriously disappeared.
"As I am following closely reports and pictures of NLC appearances, very few have been claimed on NLC network and in my network. Very dim and low on the horizon clouds have been spotted mainly from Germany and the UK, but the polar mesospheric clouds had somehow come to a standstill, whereas they would generally be fully on-going by now," photographer Adrien Mauduit said June 15.
The delayed start was apparently caused by an unexpected heat wave in the mesosphere that temporarily wiped out all of them. As reported by Dr. Tony Phillips of SpaceWeather.com on the same day, the mesosphere heat wave was discovered by Lynn Harvey of the University of Colorado's Laboratory for Atmospheric and Space Physics, using temperature data from the Microwave Limb Sounder onboard NASA's Aura satellite.
"In early May, the summer mesosphere was cooling down as usual, approaching the low temperatures required for NLCs," Harvey said. "But wouldn't you know it? Right after May 21st the temperature stopped cooling over the pole! In fact, it warmed a degree or two over the next week. The warming resulted in 2017 being the warmest summer mesopause in the last decade."
"We don't know why the mesosphere warmed up," Professor and Chair of the University of Colorado Department of Atmospheric and Oceanic Sciences, Cora Randal, said "It's probably a complex process involving the propagation of atmospheric gravity waves, which affect the flow of air and heat in the upper atmosphere. We're looking into it."
Whatever happened, the mesosphere started heating up again during the second half of June and by July 1, the magnificent noctilucent clouds were shining brightly across much of the northern hemisphere.
While beautiful to see, there is more to noctilucent clouds than meets the eye.
The first known observation dates back to 1885, two years after the 1883 eruption of Krakatoa. At first, they were only seen at higher latitudes, but have recently started appearing ever lower in latitudes.
Why? Science still doesn't provide a definite answer. Some say it's connected to climate change, others, disagree and provide different theories. In any case, they are a beautiful sight to see and an interesting research quest. Just take a look at the video below... truly mesmerizing. Use headphones.
If you want more, here is another great NLC timelapse made by Adrien Mauduit during the 2016 NLC season. Adrien gathered his best time-lapse shots and compiled them into this 'little' documentary video on July 8:
Featured image: Noctilucent clouds over Wien, Austria on July 2, 2017. Credit: Forces of Nature © David Gepart