I took this photo. Above the horizon there is a thick layer that looks like a cloud, but it surely isn't. Is this a Fata Morgana? What do you think?
EDIT: I think the more appropriate name is superior mirage, not Fata Morgana.
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1$\begingroup$ +1 for teaching me that such an observation with this name exists! I think the negatives are from people who did not click the link. $\endgroup$– anna vJun 20, 2013 at 19:02
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2$\begingroup$ I don't think it is. It looks just like some clouds - isolated clouds very far from the horizon and some uniform cloudiness near the horizon. The picture is way too sharp for a fata morgana. The layers near the horizon on your image are just too regular. Fata morgana requires more complex, position-dependent reversal caused by the heated air near the horizon. It doesn't seem to be present on your picture. A fata morgana worth the name must show some illusion of a localized object of a shape that isn't there at all. Like a fake beautiful lake that thirsty explorer with camels see in Sahara. $\endgroup$– Luboš MotlJun 20, 2013 at 20:05
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$\begingroup$ (A search on Google Images shows that it certainly isn't unique. For anyone who might wonder.) $\endgroup$– ŘídícíJun 20, 2013 at 21:00
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$\begingroup$ How's that this layer of clouds have such a uniform height above the horizon? I would expect that when clouds appear more dense near the horizon they'll go from sparse to dense in a more gradual transition than seen here. I see a lot of similarity in the uniform height of the layer to that seen in superior mirages, like here. $\endgroup$– Andrey SJun 21, 2013 at 0:06
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$\begingroup$ To me, it actually looks like the kind of fog you sometimes see off the California coast on a summer day. Where and when was this picture taken? $\endgroup$– Peter ShorJun 21, 2013 at 1:10
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1 Answer
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My reply does not claim to be the correct answer. Rather, because it intrigues me, it may start a discussion as it does contain speculations, too. Nor I am a native English speaker.
From observation of the picture, the sun is below the horizon. The horizontal line we call horizon depends on the level of elevation we stand on. If the picture were taken on a celestial object without a significant atmosphere (like the moon of the earth), the picture likely would display the sky of night with light emitting objects (stars) or light reflecting ones like satellites. However, our earth possess an humid atmosphere. On droplets of water in higher layers of the atmosphere (vide supra), light emitted by the sun is subject of diffraction. Beside the blue sky, this offers us phenomena called twilight/gloaming/crepuscule, etc. Returning to the picture, diffraction deviates the pathway of the light, eventually providing some remnant light passing towards the observers direction.
Yet: why there is a less bright layer just on top of the sea's surface and below the partially open sky? At least some clouds are depicted in the foreground of the picture. This is likely to happen in the early evening of a sunny day; warmer sea water still evaporates into the already less warmer layers of air, yielding clouds. Light, diffracted and diffracted again in these clouds eventually does not get its way to the observer's perspective, especially at this angle of observation. For the observer's point of view, this appears like absorption of light, and thus darkens this layer. If you were underneath these ``dark clouds at the horizon'' not-necessarily would be pitch-dark.
Does this dark line possess a particular name? Beside dusk? Likely this were an answer for a trained physicist. On the other hand, I would not call it a Fata Morgana. This qualification is based on the absence of any object appearing (at least in some part) partially mirrored upside-down. On the picture, however, I do not spot a boat, nor a bridge, nor a lighthouse nor an other object indicating to me the requirements met to actually yield a Fata Morgana.
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1$\begingroup$ Maybe Fata Morgana is a wrong name for this phenomena. I think that superior mirage is more adequate. If this dark layer is because of light diffraction by clouds as you suggested, why its boundary is so sharp and at such a uniform height above the horizon? I would expect the change in the brightness to be more gradual. $\endgroup$– Andrey SJun 21, 2013 at 0:04
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1$\begingroup$ Hi Buttonwood, and welcome to Physics Stack Exchange! This is a Q&A site, not a forum, so we ask that you do not post things whose purpose is to start a discussion. Rather, when you post an answer, it should actually answer the question. (and preferably correctly!) Of course, your last paragraph here actually does answer the question, so I think the disclaimer in your first paragraph is unnecessary. $\endgroup$– David ZJun 21, 2013 at 4:29
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1$\begingroup$ I want to add a parallel bit of trivia. The day in Moslem countries centuries ago was divided into twelve hours ( the night too) regardless of the season. This means that daylight hours in summer were longer whereas in winter shorter than 60 minutes. Now the relevance with the question comes on how the beginning and end of day was defined . It was defined by the appearance of the demarcation of blue and red in the desert evening and morning sky, which one sees in the photo, above the dark cloud line. $\endgroup$– anna vJun 21, 2013 at 8:09