This tweet from the European Space Agency (ESA)'s Spanish account show a striking photo of a round, flat, "space-ship-shaped" cloud in an otherwise completely clear sky. The stars are out so I assume this is at night, possibly with a Moon.

Could this be real? If so, what kind of cloud is it? and how can it appear out in the open?

enter image description here

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    $\begingroup$ For people in the northern hemisphere, the stars to the right of the cloud is what the constellation Orion looks like from the southern hemisphere. $\endgroup$ – Fred Jan 10 '19 at 8:28
  • $\begingroup$ @uhoh - earthscience.stackexchange.com/questions/8375/… $\endgroup$ – gansub Jan 10 '19 at 8:29
  • $\begingroup$ @gansub hadn't even noticed the mountain, "...and how can it appear out in the open?" I know funny things happen at the tops of mountains but this (at first) seemed to come out of nowhere. Thanks! $\endgroup$ – uhoh Jan 10 '19 at 8:45
  • $\begingroup$ Yes indeed, that's another reason I may have been distracted. $\endgroup$ – uhoh Jan 10 '19 at 8:48
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    $\begingroup$ @gansub Good additional info here and in your answer - I was worried about getting too specific without a good feel for height in this photo, as well as the wind direction and topography outside what's shown here. $\endgroup$ – dplmmr Jan 29 '19 at 16:13

The image shown is a lenticular cloud, a type of cloud often observed around significant topographical features such as the mountain peaks shown in the photo. Depending on the local atmospheric moisture, they can occur on their own (as in this case), apart from other cloud features.

All cloud growth depends on the condensation of water resulting from air being cooled as it is lifted vertically by some forcing mechanism. However, these clouds are an exception to more typical cloud growth, which depends on the atmosphere having a tendency (or instability) for air to continue rising - and thereby condensing more liquid water - once forced upward.

Following the initial source cited above, lenticular clouds form in environments that are stable (i.e., not conducive) to these vertical motions, so upward motion is entirely driven by air forced over an obstacle - in this case, mountain terrain. From a horizontal view, the vertical motion forced by this obstacle commonly takes the form of a persistent wave pattern: orographic waves image source

This occurs downwind of the terrain feature, with clouds forming over the portion of the wave featuring upwards vertical motion. This becomes extremely evident when viewing time-lapse videos of this phenomenon.

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    $\begingroup$ I was so fascinated by the cloud and distracted by the stars that didn't really notice the mountain in the photo until reading your answer. Yep, there it is! Thanks! $\endgroup$ – uhoh Jan 10 '19 at 5:13
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    $\begingroup$ It's my favorite type of cloud, and a good example of how clouds are often indicators of some other mechanism at play, even subtle terrain features! $\endgroup$ – dplmmr Jan 10 '19 at 5:17
  • $\begingroup$ A type of Orographic clouds that develop in response to the forced lifting of air by the earth's topography. $\endgroup$ – Keith McClary Jan 10 '19 at 22:42

I just want to add couple more points that are not covered in the excellent answer above by dplmmr. It is the fact that lenticular clouds do not form in the trough part of the wave because that corresponds to air that is warming but they do form in the crest of waves as air parcels condense. How does one obtain the trough and crest of the Mountain wave that produced the lenticular cloud? Well you use a cloud resolving model-Cloud resolving models(these models are typically non hydrostatic) and then plot the streamlines.

The flat bottom of the lenticular cloud is as a consequence of level of condensation and and the curved top is as a consequence of the wave crest. These lenticular clouds are stationary wave clouds.

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    $\begingroup$ oh that is an extremely cool paper! I am going to read it and track down how the simulation is done as well. Thank you! $\endgroup$ – uhoh Jan 29 '19 at 7:56
  • $\begingroup$ I wonder if "the only thing" I have to do now is learn how to do those humidity soundings, and I can predict when this will happen over me? $\endgroup$ – uhoh Jan 29 '19 at 7:59
  • $\begingroup$ Well I was asking there about using on-line data, but in my comment here I was joking about measuring reflections of a laser or radio beam myself to probe for water over my home. Thanks, I will read that too! $\endgroup$ – uhoh Jan 29 '19 at 8:04
  • $\begingroup$ Currently I live around here but much of Taiwan is very mountainous, and there is of course plenty of humidity. But I don't think there are these isolated volcanos like Japan's Mt. Fuji. $\endgroup$ – uhoh Jan 29 '19 at 8:07
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    $\begingroup$ I'll head south instead where the air is clean and the skies at dark. There's also an experiment on my bucket list that I need to do ;-) $\endgroup$ – uhoh Jan 29 '19 at 8:14

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