Night Shining Clouds

Noctilucent clouds imaged by Anders Jensen near Frederecia, Denmark. ©Anders Jensen, shown with permission

"There was a huge NLC display here on the night from 9th to 10th of July.  One of the best I've seen of my 7 years of observing.  Already at dusk the high clouds were visible, so I went out on my bike and shot pictures for 1 1/2 hours around midnight. Amazing sight!
This picture was at 5 min's before midnight at the little lake close to our place. The light in the middle is the flame from burning waste gas from the nearby oil refinery.

The last few weeks have seen many bright NLC displays. Each year they seem to get more numerous and appear further and further south. Is this real or only an effect of more and more sky observers sharing images? Probably both.

NLCs are Earth's highest clouds. They form 50-53 miles up in the intense cold (below -125 Celsius) and near vacuum of the mesosphere.

The are composed of small (0.1 micron) ice crystals and shine by scattering the high altitude sunlight.

NLCs are found a few miles below the mesopause, the coldest layer of the atmosphere.

At that level temperatures start to be a meaningless concept except for characterising molecular velocities and energies because the pressure is a mere 1 bar, 1/1000 of that at sea level. Ordinary conductive and convective heat transfer becomes insignificant to us at those pressures.

The increased occurrence of NLCs implies an increasingly colder and wetter summer mesosphere. Paradoxically, tropospheric global warming leads to lower mesosphere temperatures.

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