OPOD - Bishops Ring

OPOD - Bishops Ring: Exploring the Phenomenon of Noctilucent Clouds

Noctilucent clouds (NLCs) have become a captivating sight in recent years, captivating sky observers with their mesmerizing displays. These ethereal clouds, which appear to glow in the night sky, have been observed further and further south, raising questions about the cause behind this phenomenon. Is it a result of increased sky observers sharing images or a genuine shift in their occurrence? It seems to be a combination of both.

NLCs are Earth's highest clouds, forming at an astonishing altitude of 50-53 miles in the mesosphere. This region of the atmosphere is characterized by intense cold, with temperatures plummeting below -125 degrees Celsius, and near vacuum conditions. It is within this extreme environment that these unique clouds take shape.

Composed of minuscule ice crystals measuring only 0.1 micron in size, NLCs owe their luminosity to the scattering of high-altitude sunlight. These delicate ice crystals interact with the sun's rays, creating a stunning display of light and color that captivates observers worldwide.

The mesopause, a few miles above NLCs, marks the boundary with the coldest layer of the atmosphere. At this level, conventional temperature measurements lose their meaning due to the incredibly low pressure—only 1/1000th of that at sea level. Consequently, ordinary conductive and convective heat transfer mechanisms become insignificant. The mesosphere becomes a realm of molecular velocities and energies rather than temperature gradients.

Interestingly, the increasing frequency of NLC displays indicates a colder and wetter summer mesosphere. This paradoxical relationship is attributed to tropospheric global warming, which leads to lower temperatures in the mesosphere. As our planet's lower atmosphere warms, it creates a chain reaction that influences the upper reaches of our atmosphere, resulting in a chilling effect on the mesosphere.

The captivating image of NLCs illuminated by the flame from a nearby oil refinery, as captured by Anders Jensen near Frederecia, Denmark, showcases the awe-inspiring beauty of these nocturnal clouds. The image was taken during a particularly remarkable NLC display on the night of July 9th to 10th. This event stands out among Jensen's seven years of observing, leaving an indelible impression of the sheer magnificence and splendor that NLCs can possess.

The growing popularity of NLC observations can be attributed to a combination of factors. With the advent of digital photography and the ease of sharing images online, sky observers have been able to document and spread awareness about these atmospheric wonders. As a result, more people are actively seeking out opportunities to witness and capture the beauty of NLC displays. This heightened interest has undoubtedly contributed to the perception that NLCs are appearing more frequently and in previously uncharted territories.

In conclusion, the enchanting phenomenon of Noctilucent clouds continues to captivate our imagination. These ethereal formations, situated at Earth's highest altitudes, defy conventional atmospheric conditions and mesmerize us with their luminescent beauty. As we delve further into understanding the intricate relationship between tropospheric global warming and mesosphere temperatures, we gain valuable insights into the complex dynamics of our atmosphere. Whether through the lens of a camera or with our naked eyes, the sight of NLCs illuminating the night sky is a reminder of the immense wonder and mystery that lies above us.

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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  • "OPOD - Bishops Ring". Atmospheric Optics. Accessed on March 28, 2024. https://atoptics.co.uk/blog/opod-bishops-ring/.

  • "OPOD - Bishops Ring". Atmospheric Optics, https://atoptics.co.uk/blog/opod-bishops-ring/. Accessed 28 March, 2024

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