Experience the rare and mysterious noctilucent clouds in this timelapse film. Noctilucent clouds (or literally ‘night-shining clouds’ from Latin) only form when the right conditions are met – but they are one of nature’s most intriguing shows on Earth.
All shots in this film are made in Denmark in the summer 2019 – between June 17 and July 16. The noctilucent clouds (NLC's) are an elusive astronomical phenomenon happening during the bright summer nights in the upper northern hemisphere. They can be seen in the twilight between latitudes 45 and 65 degrees north. Denmark is located at 55 degrees north and is thus one of the best locations for observing the noctilucent clouds. The noctilucent clouds can also form in the Southern Hemisphere, but they're dimmer and less frequent.
AT THE EDGE OF SPACE
Noctilucent clouds are the highest clouds on Earth at the edge of space, and while all other tropospheric clouds remain plunged in darkness noctilucent clouds can still catch the sunlight because of their high position in the atmosphere. But the noctilucent clouds are so faint that they are only visible to us when the rest of the atmosphere is in darkness.
The noctilucent clouds are formed by ice crystals approximately 83 kilometers above the Earth, in the so called mesosphere atmospheric layer. This is about four times higher than the highest clouds we normally see. Because the air is so thin in this part of the atmosphere, the temperature needs to dip below -120°C before the noctilucent clouds can be formed. This can only happen during the northern hemisphere summer. While all the hot air in the bottom part of the atmosphere expands, it cools the upper layers allowing a drop in temperature.
For the clouds to form, they also need some kind of particles for the water to condense on to. In the mesosphere, the main source of these particles is from meteors. Thin particles from incoming meteors are released when they burn up in the Earth’s atmosphere.
OCEAN OF ICE
When the sunlight is reflected at a certain angle from the ice particles it makes the noctilucent clouds glow in the dark. The clouds can only be observed around the summer solstice when the sun is located between 6° and 18° elevation below the horizon. One to two hours after sunset the sky is dark enough for the clouds to become visible.
The light that is reflected from the noctilucent clouds passes through the ozone layer before it gets to our eyes. Ozone absorbs red light and allows blue light to pass through, which is why the noctilucent clouds take on a striking blue colour.
Even though the air may look still in the mesosphere the noctilucent clouds show us how the atmosphere flows at the edge of space. Noctilucent clouds form many different shapes which often makes them look like an ocean of ice in the sky.
INDICATOR OF HUMAN-CAUSED CLIMATE CHANGE
The noctilucent clouds were first time observed in 1885, two years after a massive eruption of the Indonesian volcano Krakatoa. All the ash and water vapor that were spewed into the atmosphere created the noctilucent clouds, mesmerizing sunsets and other effects around the world. The impacts of the volcano disappeared but the new clouds did not. In fact, the noctilucent clouds have slowly increased over the last century. In the late 1800s the clouds were likely only visible on special summer nights once every few decades. Today they are normally visible a few times each summer.
Now, a new study explains why – more noctilucent clouds are forming because of human-driven climate change. It might be the clearest signal of global change tied to the greenhouse gas build up.
Methane emissions have increased water vapor concentrations in the mesosphere by about 40 percent since the late 1800s, which has more than doubled the amount of ice that forms in the mesosphere. Methane produces water vapor through chemical reactions in the middle atmosphere.
The noctilucent clouds might be both immensely beautiful and intriguing - but their increasing strength and appearances on the night sky is alarming.
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Photographer and Editor: Casper Rolsted
CasperRolsted.com
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NaturalSilence.org - Learn about natural silence
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Music: Peter Nanasi
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