Evidence of ripples in the atmosphere, resulting from a storm over Bangladesh, could be seen in imagery from space and on the ground in April.
Airglow has been known for many years, mainly among astronomers, but research of airglow from space has increased since the launch of satellites such as Suomi-NPP.
More information and detailed analysis of the feature can be found in the In Depth section.
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Recently, studies of airglow have gained in importance, not only for pure scientific reasons, but also due to the obvious link between airglow appearance and some of the ground-based or tropospheric, possibly severe, phenomena.
The launch of the Suomi-NPP satellite in November 2011, with its low-light sensitive Day-Night Band (DNB), has greatly aided research of airglow from space.
On moonless nights, DNB is capable of detecting airglow, including its various wave-like patterns.
Waves in airglow can be generated by various mechanisms — gravity waves being the most important among these.
On 27 April, Jeff Dai took several incredible photos of what he called 'ripples in the sky' — concentric waves in airglow, covering the nocturnal sky above the Tibetan Plateau of China, the Himalayas and lowlands north of the Gulf of Bengal.
He attributed the origin of these concentric wave-like patterns in airglow to gravity waves generated by violent storms above Bangladesh.
The storms, which generated the gravity waves interacting with the airglow, as seen in Figure 1, can be seen in the Metop-A AVHRR band 4 black and white image (Figure 2) and the brightness temperature colour-enhanced version (Figure 3).
AVHRR with brightness temperature overlaid
The temperature of an overshooting top in centre of this storm reached about 175 K (-98 °C) — evidence of strong updrafts inside the storm.
The airglow photo, captured at 15:56 UTC, was taken at almost the same time as the next satellite, Metop-B, passed over the region, at 15:50 UTC.
AVHRR with brightness temperature overlaid
Examination of the clouds on the Metop-B AVHRR band 4 black and white image (Figure 4) and the colour-enhanced version (Figure 5), did not show any trace of wave-like patterns which might possibly correspond to the airglow waves.
No traces of the waves could be seen on the Metop-B Night Microphysics RGB (27 April, 15:50 UTC) image, either.
However, looking at imagery captured by the Suomi-NPP satellite's Day-Night Band three-and-a-half hours later (Figure 6), the rippled airglow was visible.
It could be seen spreading out in concentric waves up to about 900 km away from their centre above Bangladesh.
It was also possible to distinguish another set of airglow waves above the Gulf of Bengal, oriented in a north-south direction, the source of which is uncertain.
Besides the airglow itself, clouds and snow-covered mountains could also be seen, illuminated either by airglow, the light of the stars, or city lights.
Clouds can be distinguished from the airglow in Figures 7, 8 and 9.
All these three images depict exactly the same region as the DNB image above, enabling very easy comparison of the airglow appearance with clouds.
This comparison confirms that the waves were only present in the airglow, not the clouds.
In the DNB image, city lights can be seen in both cloud-free and cloud-covered areas.
Several short lines there (oriented approx. in E-W direction) represent storm flashes, saturating several nearby sensor pixels.
In all the satellite images the red dot in each of the images shows the location from which Jeff Dai took his pictures.
Figure 7: Suomi-NPP VIIRS M16, 27 April 19:35 UTC. Full Resolution
Figure 8: M15 colour-enhanced brightness temperature, 27 April 19:35 UTC. Full Resolution
Figure 9: Suomi-NPP Night Microphysics RGB, 27 April 19:35 UTC. Full Resolution
Convectively-generated mesospheric airglow waves over Texas (CIMSS Blog)
Severe Weather in the Mesosphere (VIIRS Imagery and Visualization Team Blog)
Sprite, Air Glow Gravity Waves, Stars, Storms (Space Weather)
Sky on Fire! video (WeatherVideoHD.TV)
Expanding Hydroxyl Rings Above a Thunderstorm (YouTube)
Gravity Waves May 20 2012 (and Red Sprites) (YouTube)
Mesospheric Gravity Waves and Sprites over Western Kansas June 05 2014 0617 UT to 0840 UT (Vimeo)
Description of airglow (Atmospheric Optics)
Airglow Formation (Atmospheric Optics)
Description and images of airglow (University at Albany)
What is Night Glow/Airglow? (Aurora Night Glow Blog)
Passive remote sensing of the earth's atmosphere in the optical spectral range (PDF)