Blue hour is a part of nautical twilight when sky is very blue. This blueness appears because of Chappuis absorption of reddish light in stratospheric ozone.

At later stages of nautical twilight, near the horizon around the solar azimuth there's a glowing blob of colors changing from reddish-orange to pale yellow to white (which is an interplay between Chappuis absorption, Rayleigh inscattering, and extinction) and fading to background blue (which exists due to multiple scattering), like in the following photo:

Ladoga photo

The photo above was taken from 60.3°N 32.6°E, on the shore of the Ladoga lake on 2019-04-14 at 21:47 UTC+3. (Ignore the W and NW markers, they are the remainders of my manual reprojection from fisheye to perspective, I forgot to remove them when saving.) So far so good, seems to make sense and also to match a simulation via CalcMySky in Stellarium more or less after increasing exposure of the screenshot:

Stellarium screenshot

Now, when I watch sunsets from Mauritius (20.4°S 57.4°E), both by naked eyes and using my camera, the nautical twilight always appears to be pinkish. The sky around the zenith does have some hints of blue, but generally everything, especially the bright blob near the horizon, is nothing like the photo from Ladoga or the simulation in Stellarium. Here's how this pink twilight looks, captured on 2023-05-30 at 18:25 UTC+4:

Photo from Mauritius

Note that this is not what's called afterglow (whose exact reasons are looked for in this question), the afterglow has already faded out by the time I watch this "nautical pinkness".

I first suspected that this might be due to the difference in ozone column above Ladoga (~400 DU) vs Mauritius (~270 DU), but a simulation tells me that the difference in colors would be only in saturation of blue, retaining the same qualitative picture.

So I wonder, what is the reason for such a drastic difference in twilight colors between the continent around Ladoga and the ocean around Mauritius? I'm interested in a detailed explanation, so that I could make a new model of atmosphere and reproduce this pinkness.

  • 1
    $\begingroup$ Don't know exactly, but first guess is of course aerosols... pink sunsets are often associated with volcanic eruptions (see e.g. eartheclipse.com/science/geography/…). Maybe that's an explanation as the post you linked states you already found there's sulphur aersols? $\endgroup$ Commented Jun 4, 2023 at 22:51
  • $\begingroup$ @CharlesTucker3: It may also be salt aerosols from the ocean. $\endgroup$
    – Fred
    Commented Jun 5, 2023 at 1:44
  • $\begingroup$ It would be interesting to see the corresponding view from Stellarium for Mauritius. The significant difference in latitude between Mauritius & Lake Ladoga might be factor. $\endgroup$
    – Fred
    Commented Jun 5, 2023 at 1:47
  • $\begingroup$ @CharlesTucker3 I assume that the sulfur aerosols indeed come from volcanoes; besides, the afterglow that happens due to them exists in both locations and generally seems to be a rather global effect. But the pinkness here is very different: first, it should be noted that the whitish glow in the photo from Ladoga should be the light scattered once in the very high layers of atmosphere, higher than about 60km. $\endgroup$
    – Ruslan
    Commented Jun 5, 2023 at 8:19
  • $\begingroup$ @CharlesTucker3 I know this because if I set simulation cutoff at this altitude, this glow is cut off and loses its blueness at the top of the blob. And if the reason were tropospheric, then I'd expect a much redder horizon due to the extreme path lengths at such shallow angles. $\endgroup$
    – Ruslan
    Commented Jun 5, 2023 at 8:20


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