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| Green airglow shining through thin atmospheric clouds in the night sky near Swain, New York on August 15, 2026. Canon EOS Ra, EF 16–35mm f/2.8L II at 16 mm, f/2.8, ISO 40000, 25 seconds. |
On the night of August 15, 2026, near Swain in western New York, I was photographing the night sky with my Canon EOS Ra. The sky was dark enough that a noticeable green color from atmospheric airglow was visible across much of the photograph.
What caught my attention was not only the green sky, but also the thin atmospheric clouds crossing parts of the frame. Stars remain clearly visible through them, showing that these clouds were optically thin.
Most of the thin clouds lie outside the brightest part of the Milky Way and appear greenish against the strong green airglow background. Small portions that cross the Milky Way appear somewhat warmer or more reddish. The effect is subtle, but it raises an interesting question: why can thin clouds appear green at night?
Green Airglow High Above the Clouds
The green night airglow is produced mainly by atomic oxygen emitting at a wavelength of 557.7 nm. The strongest part of this emission comes from Earth's upper atmosphere at roughly 90–100 km altitude, far above ordinary weather and tropospheric clouds.
The atmospheric clouds in these photographs are much lower. Because they are thin enough for stars to remain clearly visible, a significant part of the green airglow coming from above and behind them can simply pass through the cloud layer.
The clouds can also scatter some of the surrounding light. However, scattering by cloud droplets and ice crystals is different from the Rayleigh scattering by molecules that makes the daytime sky blue. Cloud particles are much larger and their scattering is considerably less selective by wavelength, so a thin cloud can retain much of the color of the illumination and sky background around it.
Different from Light-Polluted Clouds
This appearance is very different from the clouds we commonly see near cities. Under a light-polluted sky, clouds can become white, yellow, or orange because they reflect artificial light coming upward from the ground. Thick clouds may become much brighter than the clear sky, while stars behind them disappear completely.
Here the situation is almost the opposite. The clouds are faint and transparent, stars remain visible through them, and much of the observed color appears to come from the natural night-sky background rather than from strong artificial illumination below.
There is another subtle detail in the main photograph. Where the background sky is dominated by green airglow, the transparent clouds appear greenish. Near some of the more reddish regions of the Milky Way, their color also seems to shift slightly toward warmer tones.
The Canon EOS Ra has enhanced sensitivity to red astronomical emission, particularly around the H-alpha wavelength at 656.3 nm, so reddish emission regions along the Milky Way can be recorded more strongly than with a standard digital camera.
Why This Effect Is Easy to Overlook
Greenish atmospheric clouds are not necessarily rare in night-sky photographs, but the effect is easy to overlook. In many astrophotographs the green color is simply described as airglow, while the clouds themselves are treated as an unrelated foreground or atmospheric feature.
The important distinction is that the clouds are not producing airglow. The green emission originates much higher in Earth's atmosphere, while ordinary weather clouds are far below it. When those clouds are optically thin, part of the green airglow remains visible through them, and the cloud-covered part of the sky can therefore appear greenish in the photograph.
This can easily be confused with several other effects: light pollution reflected from clouds, aurora, camera white-balance shifts, or simply aggressive color processing. The most useful clue is that stars remain visible through the clouds, showing that the cloud layer is transparent enough for the background sky emission to pass through.
For this reason, I think the effect is probably photographed more often than it is discussed. What makes this Swain observation especially useful is the combination of strong green airglow, a dark sky and thin atmospheric clouds, which makes the relationship between the airglow and the cloud color easier to see.
The Effect Was Visible Even in a 2.4-Second Exposure
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| A 2.4-second exposure showing green airglow and thin transparent atmospheric clouds with stars visible through them near Swain, New York on August 15, 2026. |
Earlier that night, I took this 2.4-second exposure. Despite the very short exposure time, the green background and greenish transparent clouds are already visible, although the Milky Way and faint cloud structure are less detailed than in the 25-second photograph.
I find this short exposure especially interesting because it shows that the green airglow was strong enough to be recorded in only 2.4 seconds. The later 25-second exposure collected much more light and reveals the airglow, Milky Way and thin cloud structure in considerably greater detail.
A Similar Example from Colorado
A very interesting comparison was photographed by Aaron Watson of Skies-Alive Photography near Paonia, Colorado, on May 21, 2025. His photograph, Emerald Green Airglow with Orange Clouds, shows strong green airglow together with clearly visible atmospheric clouds.
The photograph is especially useful for comparison because the clouds near the horizon are orange from artificial skyglow, while the atmosphere behind and above them shows strong green airglow. It demonstrates that airglow can still be visible even when some light pollution is present near the horizon.
My Swain photographs show a different case. The atmospheric clouds are much thinner and more transparent, with stars clearly visible through them. Most of these clouds appear greenish where they are seen against the green airglow background, while small portions crossing the more reddish Milky Way show slightly warmer tones.
Together, the two observations show that the appearance of nighttime clouds depends not only on the clouds themselves, but also on the light coming from different directions. Artificial light from below can make clouds orange or yellow, while natural airglow high above them can create a green background that remains visible through sufficiently thin clouds.
Airglow Is Not Aurora
A green night sky can easily be mistaken for aurora in photographs, but airglow is a different phenomenon. Aurora is produced mainly when energetic charged particles interact with Earth's upper atmosphere and is strongly connected with geomagnetic activity. Airglow, on the other hand, is present even during quiet geomagnetic conditions and is continuously produced by chemical and physical processes in the upper atmosphere.
The intensity of airglow can vary considerably from night to night and across different parts of the sky. Atmospheric tides, gravity waves, changes in temperature and density, and changes in the distribution of atomic oxygen around the mesopause can all affect the brightness and structure of the green 557.7 nm emission.
This is an important reminder that a genuinely dark night sky is not necessarily black. Under good conditions, a camera can reveal green, reddish and other faint emissions produced by Earth's own atmosphere.
Photography Settings
Main photograph:
Camera: Canon EOS Ra
Lens: Canon EF 16–35mm f/2.8L II USM
Focal length: 16 mm
Aperture: f/2.8
ISO: 40000
Exposure: 25 seconds
Date: August 15, 2026
Location: Near Swain, New York
Short test photograph:
Camera: Canon EOS Ra
Lens: Canon EF 16–35mm f/2.8L II USM
Exposure: 2.4 seconds
Date: August 15, 2026
Location: Near Swain, New York
A Dark Sky Reveals Its Own Color
The area near Swain has a very good dark sky for New York State. According to the Light Pollution Map, this location is approximately between Bortle Classes 3 and 4. However, light-pollution maps should be considered an estimate rather than an exact measurement of what an observer will experience on a particular night. Actual sky quality also depends on atmospheric transparency, humidity, aerosols, nearby lights, the time of night and even the surrounding terrain.
Local geography can sometimes create a significantly darker pocket than a large-scale map might suggest. I experienced a good example at Sand Beach in Acadia National Park, Maine. The beach is only about 290 yards long, but the combination of a dark open-ocean horizon and surrounding mountains that shield light from populated areas creates an unusually dark observing location. Only a few miles away, where that terrain shielding disappears, light pollution becomes more noticeable.
One of my own practical indicators of an excellent astrophotography sky is visible airglow. In the original Bortle scale, readily apparent airglow is one of the characteristics of a Class 1 sky. A camera, however, is much more sensitive than the human eye and can record airglow under skies that would not visually qualify as Bortle Class 1.
The strong green airglow near the zenith in these Swain photographs can therefore have several explanations working together. The site was genuinely dark, photo at ISO 40000 was sensitive enough to reveal very faint atmospheric emission, and the green airglow itself may simply have been stronger than average that night because airglow intensity naturally changes with conditions in the upper atmosphere.
For me, this is one reason airglow is such an interesting practical indicator of sky quality. A light-pollution map provides a useful prediction, but the photograph records what the sky was actually doing at that location and at that moment. When natural green or reddish atmospheric emission becomes clearly visible instead of being overwhelmed by artificial skyglow, I know I am photographing under a genuinely good dark sky.
These photographs also show why atmospheric clouds at night do not always have to be white, gray or black. Under a dark sky, a thin transparent cloud can reveal some of the color of the natural sky behind it. On this night near Swain, that color was unmistakably green.
Related Astrophotography Posts
- Meteor over the Milky Way – Swain, New York
- Cloudy Starry Night Sky
- Why the Equatorial Sky Glows More Red — Tanzania & Galápagos Astrophotography Experience
- Acadia National Park, Maine USA – Astrophotography
- Airglow Astrophotography – Bryce Canyon National Park – Sunset Point
- Arches National Park Astrophotography
- Sky Ladder – Galapagos Nightscape Astrophotography
