Ah…the white nights, the glow that scatters the midnight sun’s illumination beyond the polar circles. But it only extends down to about 60 degrees latitude, below which the sun is too far below the horizon to light up the sky, and that will never change…or could it?
And no, I’m not talking about a change in the axial tilt of the Earth. For how long twilight extends for practical purposes depends not only on the glow of the air itself, but on the illumination of cloud decks as they are differentially illuminated by the setting sun. It’s well known that fog and low clouds are the first to succumb to Earth’s shadow, with higher cirrus clouds remaining illuminated for longer. Earth’s horizon “dips” as you move up in altitude, hence why higher clouds can glow noticeably against the twilit sky, and why rockets often produce what’s called “twilight phenomenon”.
Ordinarily this effect just permits you to see cirrus clouds high up stay white to yellow-to-red for longer than low clouds do, but still well within the realm of “normal” twilight. The highest cirrus clouds are perhaps 15 kilometers above you.
But there is a type of cloud that’s higher than the highest cirrus…much higher. These are the so-called noctilucent clouds, which, uniquely, form not in the troposphere or even the stratosphere, but in the mesosphere. The air up here is rarefied, low in pressure and perhaps a millionth of the humidity found in the deepest Sahara, but the rarity and dryness of mesospheric air can still be overcome by sheer cold: when temperatures drop below -120 Celsius (-184 Fahrenheit), as they do in the summer in the polar regions (the mesosphere is coldest in the summer, not the winter, owing to upwelling of cooler air from below), it becomes favorable for what little water there is to condense in the form of ice crystals.
The clouds that form are very thin, and so are not visible in the daytime, but when the twilight at the surface becomes deep enough, they glow noticeably against the darkness of the sky, because at their altitude, a full 75 to 85 kilometers above the surface, the sun shines brightly even while the surface is in nautical or even astronomical twilight. Noctilucent clouds indeed are seen at their best when the sun is 6 degrees to even 16 degrees below the horizon, being the last clouds to be visible after the sun sets and the first to become visible before the sun rises.
They’re not very common in real life, and the fact they only appear in summer over the polar regions and are too bright to be seen during the day means they’re usually only seen in deep twilight in the summer months between about 50 degrees and 70 degrees latitude.
These clouds are described as spectacular and otherworldly in their brightest manifestations, because they appear ghostly and nebular, with forms similar to cirrus but looking visibly higher up than a cloud would have any right to be. And furthermore, the dominant color tends to be an electric blue, owing to the low sun being filtered through heavier concentrations of ozone (these clouds form well above the ozone layer, in contrast to any other type of cloud on our planet).
In principle, a thick layer of these clouds could keep the sky aglow with cloud well below the latitudes white nights normally appear: the sky would be bright with electric-blue cloud even while the sun is 16 degrees below the horizon. In midsummer at midnight this extends not down to 60 degrees latitude, but as far down as 51 degrees.
In real life noctilucent clouds are relatively rare visitors, but in principle there’s no physical reason why they couldn’t be much thicker and more persistent. So it might be the case with the alternate-historical science-fiction universe I write my stories in. Already it’s a spacefaring civilization, with rocket launches being ubiquitous. The effect of this is the dumping of copious quantities of water vapor into the mesosphere, which (as we saw on a far smaller scale with the Space Shuttle in real life) enhances noctilucent cloud formation. So already by the end of the 20th century noctilucent clouds are likely more common in my world than they are in real life.
But where it gets really fancy is how there’s a climate change that occurs starting in the 2040s, a period of extreme weather associated with a record-strong and decade-long Western-Pacific-based La Niña, driven by man-made rapid drawdown of excess carbon dioxide and greenhouse gases, yes, but also the arrival of a rogue comet from beyond our solar system that pumps not only a solar superstorm on its perihelion passage, but also leaves behind a parade of meteoritic material that causes various exotic effects (in large measure because it’s enriched with nuggets of “up-down quark matter”).
But the exotic effect we’re looking at here is the enhancement of dust levels in the mesosphere, which also enhanced noctilucent cloud formation. Combined with the extra rocket traffic we could expect noctilucent clouds to be regular weather instead of rare visitors.
Where this scenario really takes off is if you suppose the mesosphere cools substantially; massive upwelling could theoretically take the mesopause below its critical temperature for cloud formation (-120 Celsius), not only in the polar regions in the summer but worldwide year-round. Is this the expected result of a global cooling trend? Perhaps not, but it is just the sort of weird weather shift that fits the climate change I envision in my world during this period.
The net effect? A global veil of noctilucent cloud that catches the light worldwide every night, though of course varying in thickness and shape…but always there, in stark contrast to today. Every night deep into the evening twilight you’d see those electric-blue swirls and streaks high up, glowing bright enough to see objects in silhouette against the sky, even well after it “should have” gotten dark by traditional standards. The stars and the Milky Way would easily be visible through gaps in this electric-blue glowing veil, leading to an otherworldly effect.
Where it gets really fancy is this selfsame upwelling could loft the mesosphere to higher altitudes than normal; instead of 80 kilometers, this part of the atmosphere could extend to 100 to 160 kilometers or even higher, with the effect being that the sunlight could be caught much deeper into the night. Instead of 16 degrees below the horizon being the limit of visibility, the clouds would glow when the sun was 20 degrees below the horizon, or perhaps even 25 degrees in extreme cases.
This would be notable because the limit of astronomical twilight is when the sun is 18 degrees below the horizon, so the books would need to be rewritten: twilight would now extend deeper into the night, to when the sun dips to 20 to 25 degrees below the horizon. At the extreme end, twilight could persist throughout the night in midsummer to as low as 42 degrees. That means white nights would now extend to as far south as Chicago.
Where it gets really fancy is that the polar regions are actually the brightest parts of our world; yes, the sun is only up above the horizon half of the time throughout the year, the same as everywhere else, but the sun’s path is more circular the higher you go in latitude, with the effect being the sun hugs the horizon closer. The net result is twilight lasts far longer the higher in latitude you go. Even at midwinter civil twilight is present at noon as high as 73 degrees north, and nautical twilight is seen as high as 79 degrees north. Astronomical twilight extends to as high as 85 degrees north at noon in midwinter, so true night is only present throughout the day in midwinter very near the poles themselves.
At the poles themselves, the sun never is more than 23.5 degrees above the horizon at midsummer, and is never more than 23.5 degrees below the horizon at midwinter (not coincidentally, the Earth’s tilt is also 23.5 degrees; without tilt the sun would circle the horizon at each pole forever, but the Earth’s tilt makes it corkscrew up and down throughout the year). With noctilucent clouds still being visible with the sun 25 degrees below the horizon, the net effect is this noctilucent twilight would encompass the polar regions throughout the day even in midwinter, with the effect being that it would never become fully dark at the poles. Or indeed at any location within 2 degrees of each pole, since the sun would never reach a nadir more than 25 degrees below the horizon even in midwinter.
In the more domestic latitudes, the effect of course is that true night is diminished greatly. In Chicago, of course at midsummer noctilucent cloud would still be visible even at midnight, but even in Los Angeles, at 34 degrees latitude, at the summer solstice the sun is only 32 degrees below the horizon at midnight. It only reaches -25 degrees at around 11 PM (Pacific Daylight Time), and it rises to -25 degrees again at 3 AM. So even at this southerly latitude the pitch-dark portion of summer nights would be rather short in the new regime. Visibility of noctilucent cloud would begin when the sun reached perhaps 6 degrees below the horizon, which is around 9 PM in the evening and 5 AM in the morning; that’s four hours combined of excellent visibility of Earth’s new electric-blue veil.
Somewhere like Longyearbyen would of course not see the clouds in the summer, since the sun is up all day, but during the winter visibility would be spectacular, even if not quite continuous. At midwinter’s day the sun would rise to the appropriate altitude at 5 AM and then peak at -12 degrees, still low enough to see the noctilucent clouds, at noon, and then descend to what’s now true night only at 7 PM. Which is 14 hours of continuous excellent visibility. Instead of being occasional visitors seen in the midlatitude summers, a global layer of high noctilucent cloud would become the all-dominant feature of the polar winter sky.
So small a change, so alien a result.
Even more alien results come courtesy of the climate change itself, as well as the science-fictional nature of the setting. Consider that the California coast is subjected to massive upwelling in my scenario, which means very chilly weather persists year-round, with gale-force winds and pea-soup fog. Skyscrapers dominate places like the Santa Monica Bay, meaning many residents would live above the marine layer. Dark-sky regulations impose downward-facing dim red lights throughout the cityscape. The net effect is that many denizens would see a red-lit roiling fogbank from horizon to horizon, the city below looking like embers during a rare clear window, with the sky above the fogbanks being illuminated by electric-blue noctilucent cloud, the dark-sky lighting ensuring that the sky is thick with stars and the Milky Way is easily visible in some detail even from the middle of the city.
Already that’s alien enough, though still recognizably “California, but much muchier”…but what about Chicago? Already seeing noctilucent clouds throughout the summer nights would be otherworldly on the shores of Lake Michigan, but consider that in my scenario of climate change the trade winds are enhanced globally, meaning, among other effects like the hurricane belts becoming cooler and drier, that the Atlantic world is dominated by the Saharan Dust Layer. The eastern United States has perennially dusty skies, especially in summer, which lead to spectacularly vivid sunrises and sunsets, as well as often dimmer sunlight during the day. In extreme cases the sun could be attenuated to appear as a dim disk, either a white or warm color or perhaps even a bluish color, as seen on Mars, depending on the exact scattering properties of the dust.
Weather patterns could take hold in the Midwest some years that lead to extreme droughts, perhaps with blowing dust at the surface as well, much like in the 2014 “Interstellar” movie or the Dust Bowl in real life. One idea I had was, in addition to the Saharan Air Layer dominating, the air masses from the Sonoran Desert starting to become more dominant, the net effect being a persistent heat wave but with extreme daily ranges. Think a summer where it doesn’t rain for months, but the upper atmosphere is dusty with vivid colors; somewhere like Nashville could set all-time record highs around 120 degrees Fahrenheit while dropping into the 50s at night, cooler than average. Somewhere like Chicago, further north, could be around 100 during the day but dip into the 30s at night. Combined with dry, still, dusty air, this is a recipe for thick layers of frost to form on more exposed surfaces like grassy fields and rooftops.
So the net effect is Chicagoans one Fourth of July seeing the sun as a dim blue disk high in the hazy sky as temperatures hit 100 degrees Fahrenheit; hot, but with the humidity so low that it’s pleasant to be outside, yet still warm enough in the sun to enjoy a swimming pool. But at night? The sun draws lower, there’s an extremely vivid and prolonged sunset, and then the noctilucent clouds light up the sky through midnight, as temperatures drop so low you can see your breath, and the grass in the parks starts to be covered in frost crystals under that continuous electric-blue glow from the sky above and the dim red lights from the cityscape itself, the skyline visible in silhouette the whole time. The sun rises in veritably Martian fashion over Lake Michigan, its crimson rays illuminating the frosty rooftops and balconies the next morning.
Very atmospheric, and very sci-fi, yet there’s nothing physically or even meteorologically implausible about it. Just turn a few dials, so to speak, and you can get a result that’s hard-sci-fi yet very alien at first glance: Fourth of July fireworks on a frosty white night…in Chicago. Further north the effects would be considerably more spectacular; think places like Minneapolis, or, better yet, Duluth or Sault Ste. Marie, which in my world have become much larger and more important cities than they ever did in real life. Makes me half-tempted to set one of my characters’ steel-mill-tinged double life I’m intending her to lead in Sault Ste. Marie instead of Birmingham (Nashville, alas, where she works, is too far south to see noctilucent clouds throughout the night even if their altitude rises to low Earth orbit; ugh).
But for now, both you and I (and perhaps her) can appreciate perhaps the most spectacular phenomenon that Earth’s atmosphere has to offer us in the twilight sky, and wonder not only what our world is, but what our world may become…