Rubescence

Sitting in a pleasantly textured seat, being driven in a bus across the south of England late last winter, I witnessed a phenomenon which I am going to call rubescence, from the Latin ruber or "red" since I can not remember any other word for it and the words "crepuscule" or "gloaming", though almost fitting, denote rather the twilight that occurs when the sun is only a bit below the line of the horizon. This phenomenon I saw consisted of skies awash in warm colours of dim rain hazed flames, apricot-skins, lemon oils, driven through by trays of clouds: dove-breast and seal-skin greys; warm colours I would not have expected during the day, but would have found natural only around dawn and the setting of the sun. There was a redness, a russet tone to this autumnal light for long hours at both ends of the day. Here, in this place and time, it looked at though they extend much further into the day than I would usually have seen further south. And indeed, in Prague the time during which the sky is rubescent is shorter and the light more white and brighter, less shaded and muffled during the day even in winter. In terms of feeling or atmosphere, of compositions of colors, local light and colour schemes seem to differ considerably between various geographic locations and seasons. There was a feeling as if light was coming trough frost or frosted glass, as if the lightscape, the arena where the lucency congealed was a rubbery new type of ever so slightly non-translucent glass, or a strong light-filled mist of smooth, though mutedly dramatic, cool flavour with a hint of almost passionate storminess; a surprising combination of lemon coloured bottle-glass and flat, powder-compact seagull-grays. Even though I was not so much further to the north, the difference in the type or qualities of the light or one could even say of the light-scape was noticeably, strikingly different to the environment of light of cities lying further south over the horizon, over the curved flank of the earth. I think the differences in qualities, colors, textures of skies and light in different places must probably depend on a number of factors - but in any case one real differentiating factor seems to be the angle at which light is received from the sun at the time and place in question.

I have noticed in the past that the one can witness a great variety of types of skies and related phenomena. The sky seems to have different qualities and parameters in different places - I saw this for example in Sweden. There the late summer sky seemed very upwardly deep, stacked with a scattering of fleshy-looking, looming clouds at a great multitude of levels. It was like looking upwards into a metallic, blue tunnel, or the what I imagine may be the tower-like architecture of a microchip. One pale violet cloud hovered, stacked above other lone clouds, going upwards into the gold-glowing blueness, hard and electric, between them. This was a markedly upwards-dimensioned flock of cloud; it was strange since I hadn't remembered the same phenomenon seen so strikingly: this phenomenon of stacking of similar clouds not only on one level, but on many layers. I came to the conclusion by and by that the sky, for all its apparent simplicity, has a structure which, though lapidary, is quite capable of a surprising variety of form; a shifting air-scape, cloud-encrusted, formed of fluid fluxes of air, crusts and floats of cloud. It strikes me as amazing that something we think of being simple, is actually, like most things, quite differentiated and complicated. Yes, even though it is is translucent or practically transparent, which only means it lets light through, that doesn't mean the air is not something real and material, and having definite qualities too. Also dawns and sunsets have different colors and the sun, in its setting and rising, is accompanied by different phenomena such as blushes of color and difference formations of cloud for reasons I don't quite understand.

In any case, this yellowness of the sky in England, these warm bands of color, like the orange smoky light in dying coals, which would fade away deep in the day, towards noon, and fade in again in the afternoon, got me questioning. I wondered why this phenomenon should exist and one reason I have found which may, at least partially (for phenomena can be complicated things) explain it quite nicely is Rayleigh scattering: "Rayleigh scattering of sunlight in the atmosphere causes diffuse sky radiation, which is the reason for the blue color of the sky and the yellow tone of the sun itself (Wikipedia: Rayleigh Scattering)".

What this involves, if I understand it that is, is the greater tendency of higher-energy wavelengths (in our case we perceive these as blue or violet or purplish colors) of light to scatter off the molecules that make up air. Light, presumably in the form of what we think of as photons, coming from the sun enters the atmosphere of Earth and gradually during its swift journey encounters a denser and denser medium, that is: more and more molecules of various gases. These gases, for some reason I don't yet quite understand, have the quality of scattering higher frequency light (in other words, in this case, this is blue light) in the visual range. I assume the other colours continue on their more or less direct journey through the atmosphere - this maybe why we see the sun as yellow. We see our star through a tunnel in the atmosphere and this tunnel is filled with yellow light, that is, white light containing many frequencies minus a portion of the higher frequencies, blue and violet ones, which are scattered to the sides into that which we call "the sky". In other words the photons, I imagine, come more or less directly from the sun, and depending on their energy and frequency, different things happen with them on their way through the atmosphere. Blue light scatters, that means it changes direction, and the photon gets ping-ponged on what I can assume to be a complex path through the atmosphere until penetrating down to the level where it can fall into our eyes and so be registered. Actually, since interactions of atoms with photons, as far as I can tell, involve absorbing the original photon and, as the case may be, shooting out another with a different or same frequency, I am not sure that it is always the original photon (at least in the sense of not having interacted (come in and out of an atom)) which originated in, or on the sun somewhere, that reaches our eye. We are in fact under a huge, vast sweep, a gentle cosmic waterfall, of cascading photons and that is why, it seems to me, the sky is filled with white or blue-white light. That is why there is light all around us and from all parts of the sky, why the sky glows: because the blue light gets spread or scatters in all directions through the gases of the air before they enter our eyes, if eventually they do that, since plenty of photons of course don't and I guess they are either absorbed by materials or reflected off them (and going out into space again.... that is what astronauts looking at our blue planet see), giving rise to an incredibly complex "soup" of photons, a three-dimensional photonic environment, photon-soaked, in which we move, and from which our eyes pick information about these photons and what they have been through on their way, what they, or those photons that stimulated their arising, have met; different maps and spreads and distributions of photons or light energy (or even we might say "luminous matter" if we consider photons or light to have a material aspect), which we perceive as the colors of all the things around us. If we didn't have an atmosphere there would probably be no color to the sky. Then it would be like being on the moon - the sun would look white in a black sky, light would fall directly towards the surface of the planet and the would be only be either bright light, and otherwise dark shadow. This is not the case here on earth: there are many gradations of shadow, there is a sort of light "in the air" which gets around things; one needs to shade off light very directly and well in order to get a very deep shadow in the daylight. However, judging as far as I can without any deeper or professional knowledge in the matter by photos of the moon's surface, the light-environment there is much more a matter of sharp contrast: in essence limited to highly defined zones of light or dark with no or little gradation in between. So on Earth, surrounded as we are by an environment which scatters light, it is as if we are on the surface of a dark marble - the earth - inside a layer of milky glass - the atmosphere - that scatters light around inside its volume. Rayleigh scattering can also, as I shortly mentioned before, be used to explain why the sun is yellowish: by the time its light reaches our eyes, most of the blue and violet photons are "leached out" of the stream of light coming in the direct line of sight from the sun's globe (from where it was about 8 minutes ago?). So what is left is the red, orange and yellow and green light, plus the remainder of other frequencies or photons at higher frequencies and so the image of the sun, in the form in which it reaches our eyes, appears yellowish. The same principle can be applied to explaining the red and orange colors of the sky and sun at dawn and sunset - because when we perceive the sun to be at the horizon, its light has to pass through more of the atmosphere ( the trajectory of the light we see coming closer to the sky's "flat" component, roughly parallel to earth's surface, rather than its vertical axis, which we look up when gazing at the top of the sky).

But what about the purple or violet clouds that typically hang around the line of the horizon in the direction of the setting sun? Why do they emit this color? This is something I have not manged to figure out any theory about.

However, even though realizing this, for some time I couldn't understand why length of the interval of pink or late light would differ as one went northwards. However when one draws up the situation of the Earth in relation to the Sun, an answer seems to present itself.

 In the above diagram, which shows the globe and two light trajectories coming from the sun, we have can imagine a person (a) standing on a point on the globe (the inner circle) surrounded by the atmosphere, whose outer boundary is marked symbolically by the outer circle . When the Sun is above him, or nearly so, at or around noon, the light has a shorter trajectory through the atmosphere, as shown by the vertical line (v), and when on the horizon, it has a long trajectory (s), as shown by the horizontal line. Analogously, we can imagine the person at (b) to be standing near the equator and the person at (a) to be farther north. It is imediately visible that at the northern position the light goes through a longer trajectory (s) through the atmosphere than the shorter one (e) sunlight takes to reach the eyes of the more southern observer (b).

Since in the northern hemisphere the angle to the sun is greater in winter than in more southern localities (and this is borne out by the fact that the diurnal arc of the sun, its journey through the sky during the day stays closer to the horizon in the winter the further north one goes), light coming from the sun looses most of its blue light (at least in the direction directly between the sun and the viewer) on the way. And there you have the solution: the sun is lower, the atmosphere at the level its light passes to the beholder is thicker, and so the light is redder or warmer, with less blue in the image of the sun (and theoretically more blue in the sky around I guess), when it arrives in our eyes after its comparatively longer journey in comparison to its summer journey, or indeed than it would take in southern skies. What makes the difference in this case, and why English light is redder than more southern light, is that the horizon there is at a greater angle towards the sun's ecliptic, that is, to the equatorial plane than cuts it symbolically into two hemispheres.

So in the picture below, you have a stylized earth where the night side is blue.

So in other words, if the theory I have in playing, hashed up is true, there is not only a band of  rubescence, that is, orange and red skies on the eastern and western leading lines of day, ie: what we would usually call dawn and sunset. The band of rubescence actually has a circular form and goes all around the cone of light coming from the sun. The earth being roughly spherical after all and so it is logical that its "section" of the light cone or, more accurately, light-sphere coming from the sun has a circular form. So it follows that on the northern and southern edges of this circle of illumination, which the sun casts upon the earth, there is also rubescence - in fact, that all around this ring there lies a band of rubescence.

Interestingly, and this is quite pivotal in the case we are discussing, since it brings with it a kind of geographical asymmetry, this ring comes closer to the equator on the northern side due to the tilt of the Earth, explaining quite well why yellow, rubescent skies should be so prevalent for so long during the day in England, which in fact is not so very northerly.

In the above picture, which is very much out of scale, so that the exaggeration may serve to make the situation clearer, we are seeing the earth from the direction the sun's light is streaming from. It is some time during the winter of the northern year. The line bisecting the circle horizontally is meant to be the equator. I think that the band of rubescence in the northern part of its circle will be wider, at least in winter, when the earth´s tilt brings the more northern points further away from the sun, because the tilt of the earth away from the sunlight cone's plane will be greater there. From space this band will probably not be visible, because it is not a reflective phenomenon, it is visible only from below, or maybe from the side. One needs to see it from the back, from our perspective, from behind the shield of the atmosphere, because it is formed in the atmosphere, for there the blue photons are leached out and spread into the sky away from their direct line from the sun.

Above, to give an example, are represented two cities, one an imaginary green dot, and one a red. The earth is spinning, the cities moving, they form lines in the diagram above, moving through zones of the illuminatory cone of the sun, that is: through night, crepuscule, rubescence and then day, or however one wants to characterize or categorize these changes and the same again in the evening. The more northern city goes through a thicker band of rubescence at both ends of the day and doesn't go through much of the central "daylight" circle at all,  moving only through its northerly zone, in fact, whereas the more southern city goes through a more moderate and slimmer ribbon of rubescence and a long region of daylight, coloured white in the above diagram.