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What set of physical parameters (if any) would enable a planet to have warmer nights than days? For example: Peak temperature near midnight and minimum temperature near noon, with an distribution of temperatures between similar to that seen on Earth (but the other way round, obviously). In this situation 'night' and 'day' are defined by the amount of human visible light. Day has more visible light than night.

Preferably this effect should be planetwide, but if that isn't possible then regional instances of this effect would be useful.

Assume that the planet hasn't got to deal with seasons (unless seasons are required for your answer), and use whatever atmospheric composition/interplanetary layout you like. If you need to have days of 800 degrees in order to have nights of 1000 degrees that's fine, the only thing that's important here is that the planet have warmer nights than days.

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I don't understand your last sentence. Do you mean Days are Cooler than nights or the opposite ? – Kii Jan 15 at 11:52
    
@Kii maybe he means that he only cares about yearly average temperature. – Christian Irwan Jan 15 at 11:53
    
@Kii: It seems I missed a 0. Edited. – Joe Bloggs Jan 15 at 11:56
    
Do the temperature must come from proximity to a sun or can it come from the planet itself ? (I'm thinking about plate tectonics, and volcanos right now). – Kii Jan 15 at 11:58
    
@Kii: Wherever you like. – Joe Bloggs Jan 15 at 12:48

12 Answers 12

up vote 12 down vote accepted

First Answer
The planet is the moon of a large brown dwarf (Glorfindel B for the sake of this answer), which is just this side of the fusion threshold.

The brown dwarf mostly radiates in the infrared spectrum, so a lot of heat, but not much light. It's orbit is outside of the Goldilocks zone on the cold side of the primary star (Glorfindel A), and so Glorfindel A doesn't provide much heat, but it does provide light.

Because of the speed of rotation and the length of it's orbit, during the day when Glorfindel A is providing light, you are generally facing away from Glorfindel B, which provides the heat.

Technically it would be more accurate to call it noon when Glorfindel B is highest in the sky, but defining day as when it's brightest out would meet the requirements of the question.

Second Answer:
Everyone lives underground because the surface is to hot to live on. During the daylight hours there is a lot of sunlight, and so a lot of solar energy to use for things like cooling.
At night power is more restricted, and so certain things have to be dialed back, and this includes the fans of the cooling system. This isn't as big of a deal since even dialed back it's not having to work as hard at night.
However, at night the heat stored in the soil radiates into the tunnels and builds up, and without the cooling system running temperatures do begin to rise. During the day the cooling system is run at full blast, which brings the temperatures back down.
Fortunately the nights are short, so the heat never gets that bad, and they never saw the need to turn the cooling up at night.

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I've always known I am in fact a double star ... – Glorfindel Jan 15 at 19:50
    
Your second answer made me laugh. It's an entirely logical twist on the question. +1! – Joe Bloggs Jan 15 at 20:41
    
@JoeBloggs Something I've always wanted to put into a story is the idea that cold water might be harder to get than hot water, just so I could use a phrase like "if so and so didn't hurry and get out of the shower there wasn't going to be any cold water left for me". This was one of the few situations where I could see that happening. This is from never having enough hot water in the mornings growing up. – AndyD273 Jan 15 at 20:47
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@AndyD273 that makes sense. I posted my own answer with a couple of other attempts to solve this worldbuilding.stackexchange.com/a/33471/7130 – Nathaniel Jan 16 at 2:30
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@AndyD273 I suppose another way might be to have the planet tidally locked in a 2:3 ratio like Mercury. Then at least it would follow a pattern on a short time scale: there would be one cold day, then a day that's hot in the morning and cold in the evening, then one that's cold in the morning and hot in the evening, then it repeats. (At least I think that's right.) – Nathaniel Jan 16 at 2:33

Day is light, Night without.

Day is when the surface of the planet is exposed to its sun. On the contrary, night is when the surface of the planet is not exposed to its sun.

2 Moons, 1 always between the Planet and the Sun, 1 at the opposite side of the Planet

In a similar way to Solar eclipses, it should be possible to design a planet which possesses a moon that is permanently provoking Solar eclipses. For this, the moon needs to be always between its planet and the planet's Sun. Depending on the size of the moon and your planet, this is a way to reduce temperature on some part of the surface exposed. Days are now cooler (and a bit darker) !

Now, create another moon positioned at the opposite. This moon must be facing the dark side of the planet. Make the Sun microwaves bounce on this moon to go on your planet. Depending on the rays, the reflection, and others factors, it should be feasible to produce some heating. (Didn't see hard-science tag :D ) Voilà ! Nights should be hotter, and a little brighter.

2 geostationary Moons at opposite sides of the Planet

EDIT : Instead of making the moon always positioned between the Planet and the Sun, we can make them geostationary (i.e : they orbit around the planet at the same speed the planet rotates). Each day, the Sun will rise and then one of the moon will hide it and the other one reverb the light of the Sun for the opposite side. This way, Days and Nights could theoretically respect your conditions BUT it would work only for some parts of the planet. Not all the surface of the planet.

Conclusion, it may be possible but very unlikely. Terra-forming this might be a way to make this happen. I'm not sure about heating the dark side of the planet via reflection of the rays by the moon. If composed by some kind of natural (or artificial) mirrors it might work.

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HAH. I was literally about to start writing this exact same thing! There's a question Here on the physics stack exchange on the heating effect from the moon (1/10000) of the heat from the sun apparently). Technology increasing reflection would certainly work, but you could also have a case where the power from the sun (or proximity) increases the sun's power (meaning the world would be uninhabitable without its shield) – I Stanley Jan 15 at 12:25
    
"In a similar way to Solar eclipses, it should be possible to design a planet which possesses a moon that is permanently provoking Solar eclipses." - no, it is not possible, it would be unstable. – Radovan Garabík Jan 15 at 12:57
    
@RadovanGarabík : Wouldn't adding a moon of the same mass at the opposite of the planet fix that problem ? – Kii Jan 15 at 13:01
    
@Kii No, it would make the situation doubly unstable. – Radovan Garabík Jan 15 at 13:07
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@Jay Now the question becomes: is it possible for the moon of a gas giant to have the properties desired... – Draco18s Jan 15 at 17:30

For it to be colder while the planet surface is exposed to more energy input from the sun, most of that energy needs to be either intercepted or absorbed during the day and then released at night.

Both scenarios I came up with would make the most sense if the planet used to be colder and/or had extreme temperature variations, prompting some advanced species to engineer a climate control system. A million years later, the planet is more moderate, but the systems still work to the old specs, essentially overcompensating to the point where the day is colder than the night.

Intercepting the energy

Intercepting the sun's energy seems feasible from a physics perspective, but still a purely artificial advanced science thing. The system would consist of a ring of large composite panels in geostationary orbit that let through 60-80% of visible light and absorb all other radiation during the day, then rotate their sun-facing side towards the planet at sunset and radiate all that excess energy to the planet.

The reason it can't let all visible light through is that annoyingly, life evolves to see exactly in the spectrum that the sun's output is highest in, and assuming some energy is lost from the panels by radiation, they could never warm the planet enough during the night. Probably the panels need more than simple black body radiation at night as well, some kind of process to transport all the heat to the planet-facing surface. With this, the planet would have a nice warm reddish glowing ring around it at night, while the sunshine does not feel warm at all during the day.

Absorbing the energy

Purely absorbing the heat/energy during the day, even at 100% efficiency with perfect timing for the release at night, would require most of the planet to be covered in whatever plant/chemical soup/device does this. Even at 50% of the surface, it would only even out the temperature over the day/night cycle.

So what is needed is essentially planet-wide airconditioning, except that the evaporator is not in the air outside, but in underground water. At night, the cycle is reversed and the water is cooled while the surface component is heated, as in a reversible heat pump.

I'm going to go out on a ledge and assume you weren't thinking of skyscraper sized airconditioning towers dotting the planet, so let's at least try to make it look natural.

What I could imagine is forests of bio-engineered trees that grow tubular roots full of refrigerant deep into the earth until they hit water. During the day they are in cooling mode, generating electricity in their leaves from the sunlight to power the heat pump as well as charging some chemical energy storage (sugar?). At night, that chemical process powers the reverse cycle.

Seas may contain kelp forests that do the same.

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The planet is actually a moon in orbit around a large gas giant, almost a brown dwarf, that emits a lot of infrared but negligible visible light. The gas giant in turn orbits a regular bright star at a very great distance. The only visible light comes from the star, but is dim because of the distance, and contributes next to nothing to the planet's heat budget. The planet is of course tidally locked to the gas giant, and has an eccentric orbit, so that for one side of the planet its orbit is closer to the gas giant, and thus warmer, when it's facing away from the star (and so it's night). Which side of the planet it is will change over time, but if it's sufficiently far from the star it will take centuries or even millennia.

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That would have to be a hideously hot (and radiating by that point) gas giant unless the star is a fair way off, but I like the answer! – Joe Bloggs Jan 15 at 20:38
    
@JoeBloggs hot gas giants do exist, and are quite common in the Universe. (AKA brown dwarfs - there isn't much difference really) – Nathaniel Jan 16 at 1:56
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Unfortunately, from an orbital mechanics point of view you can't really have a planet that both has an eccentric orbit and is tidally locked. Orbital eccentricities cause much greater tidal forces than rotation, and hence they get dissipated much more rapidly by tidal friction. So the orbit becomes circular very quickly, and then only slowly does the rotation become synchronised. – Nathaniel Jan 16 at 1:58
    
@Nathaniel Damn, I'm sure you're correct. – Mike Scott Jan 16 at 6:55

I guess it is possible if we assume that heat provided by the sun is far inferior than heat provided by some "night only" source. Make the planet far from it's star is a good start, then we have to deal with this "night only heat source"

First candidates: volcano If you have a lot of volcanic activity on one side of the planet and none on the other you might get warmer nights. The thing is, it will be difficult to explain how volcano move so they are always on the night side of the planet, well, make your planet tidally locked, no night/day rotation and the night side is warmer than the day side.

however assuming you want an actual night/day rotation and do not mind about your planet actually orbiting another planet (being a moon in fact)

Second candidate: volcano and tidal heating

http://www.astro.washington.edu/users/smith/Astro150/Tutorials/TidalHeat/

Disclaimer: I don't know which orbit/rotation speed/distance from the other planet your "planet" will need to actually have warmer nights but hey that's a way to heat up one side of the planet without affecting the other which can move with the planet and that has not been proposed yet.

It'll probably need a specific configuration to actually get this one sided eating synchronized with the night/day rotation, even more specific than the one you need to get any tidal heating at all. The concept is the same as tides except with... rocks. if you make your "planet" orbit a giant gas planet in a configuration that allows tidal heating, this planet is going to have a huge volcanic activity. You might get peaks of this activity when your moon is facing the planet at the nearest point of its orbit creating this "one planet side warmer than the other but absolutely not related to sunlight", then by some extreme coincidence get this effect exactly opposed to the night/day rotation

note: it might be incompatible but that is definitely too much math for me if someone wants to explore on this, feel free to do so.

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Good link on tidal heating! – Joe Bloggs Jan 15 at 20:39
    
Just a little thing that is worth mentioning about tidal heating, If tidal forces are strong enough to heat a planet, they are extremely likely to cause huge daily tsunami. Just imagine a planet like earth with 70% of its surface made of water, pull an entire ocean 100m high and then drop it, now pick some popcorn an enjoy the apocalypse. It means that you'll also need to make your planet with almost no liquid on it's surface, underground oceans might be ok, lakes might be too, but planet wide oceans are definitely not going to allow any kind of land creature to survive. – Alexandre THOUVENIN Jan 15 at 23:44
    
That is... awesome. – Joe Bloggs Jan 18 at 14:01

Here's my attempt to build such a planet:

  1. The planet orbits a relatively cold star. In particular, the star's emission maximum is in the infrared spectrum, and only a very low amount of energy goes into visible light. That way, the heating of the planet is primarly provided by the infrared part of the spectrum.

  2. The planet's atmosphere has a very high concentration of infrared absorbing gases (greenhouse gases, like CO2). Therefore while the light reaches the ground directly (but generates relatively little heat down there), the infrared radiation is absorbed higher up, and the heat must diffuse downwards (note that this generates an inversion weather, which is quite stable; no convection to help you). Diffusion is relatively slow, therefore the heat needs some time to reach the ground.

  3. The planet rotates relatively fast (i.e. days and nights are relatively short), so that by the time the heat reaches the ground, the day is already over (and the upper layers of the atmosphere start cooling down by radiating their heat to space; the cooling of the upper layer will cause the next lower part to cool down as well (convection will happen in the already-cooling zone, so that process goes faster than the transport of the day heat), and therefore before the sun rises again, the air on the ground will have cooled down. At this point the cycle starts anew.

Note that I don't really know if this could actually work, but it at least seems plausible to me.

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I may have a third candidate for those warn nights. Here it is:

Third candidate: greenhouse gases, photocatalysis and molecule capture

Here I explored an alternative energy source to the star so sunlight and heat can be totally decorrelated but what if the star WAS the energy source but it energy is released into the air only during the night? To do that you'll first need a rather black planet with an high thermic potential so a lot of solar energy gets captured and then released as infrared

https://en.wikipedia.org/wiki/Greenhouse_effect

There is a natural well know phenomenon called the greenhouse effect which make a part of this re-emitted energy return to the ground. Now what if this greenhouse effect was only active during the night? Well since this effect is really low during the day, heat does not stay in the atmosphere even if the ground stores some of it making the day rather cold. If this effect gets really strong during the night, most of the energy stored in the ground is emitted into atmosphere and stays in it making the night warmer that the day

https://en.wikipedia.org/wiki/Sulfur_hexafluoride

Now the question is how to achieve such a thing? First you'll need an extremely potent an durable greenhouse gas, Sulfur hexafluoride is extremely good at that making low concentration able to retain a crap load of heat. Then you'll need some way to deactivate this molecule during daytime. The best way to deactivate a molecule is to make it transform into something else. That's where we summon photocatalysis. There are chemical reaction that only happens when exposed to light but we still have a problem: greenhouse gases are usually extremely inert and making them react with something else is extremely difficult and some photons are not going to change that

https://en.wikipedia.org/wiki/Endohedral_fullerene

Now let me summon my last card: molecules that can trap other smaller molecules. There are some molecules out there with an empty cavity capable of trapping smaller molecules. Fullerene is made of carbon an looks like nanotube except more... spherical. Those molecule can be photocatalysed to trap our potent greenhouse gas when exited by high energy photons from the star and unable to keep them in after leaving their light-induced excitation state. Moreover the fact that Sulfur hexafluoride is EXTREMELY electronegative make it susceptible to some EXTREMELY electropositive traps, and electropositivity changes are in the range of things a photon might be able to do to a molecule

and voila you got a greenhouse effect only active during the night making your planet's atmosphere warmer when the sun does not light it.

so basically: take the earth, remove greenhouse gases, add one that does just as much with 20000 times less molecules, add a photo-reactive molecule trapping the first one when exposed to UV in stoichiometric proportions.

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trust me, I'm a computer scientist and definitely the best expert to talk about chemistry – Alexandre THOUVENIN Jan 16 at 1:45

The most obvious, and perhaps the most satisfying way to do this is to give the planet two suns - one that provides light and one that provides heat. The problem is, how do you get one sun to appear only when the other does not?

Some other answers have suggested that the planet could orbit close to a brown dwarf (or hot gas giant, which is essentially the same thing), while the brown dwarf itself orbits a much more distant blue giant star. The brown dwarf would loom large in the sky as a dull red disk, warming the planet like a giant electric heater but not providing much light, whereas the blue giant, being so far away, would appear as a small but very very bright point of bluish white light, providing not much in the way of heat. (It actually gives out a lot more heat than the brown dwarf, but not so much of that reaches the planet since it's so far away. A substantial amount of light does reach the planet because blue giants are really, really hot and therefore give out an insane amount of light in comparison to the huge amount of heat they also give out.)

The problem is that as the planet orbits the brown dwarf the location of the two stars in the sky will change. Sometimes the planet will be between its two stars, so that one rises as the other sets, and then the days will be cold and the nights warm. But after the planet has progressed halfway around its orbit of the brown dwarf, the two stars will be near each other in the sky, and they'll rise and set at more or less the same time, just like on Tattooine. (Except that one star appears much bigger than the other in the sky.) Sometimes the 'day' star might even get eclipsed by the 'night' one. Depending on the orbital period of the planet around its brown dwarf, these seasons might cycle every week or so, or it might be a few years.

It's worth noting also that as well as giving out its own dull red light, the brown dwarf will reflect light from the blue giant, so it will have phases like Earth's moon, being 'full' when the stars are on opposite sides of the sky, and waning to a crescent and then a 'new sun' as they approach the same side. The unlit part will appear red and the lit part might be white, or it might be coloured, depending on what chemical processes are occurring in the upper layers of its atmosphere. (See the gas giants in our Solar system for examples of what this colouring might be like.)

This is all quite interesting, but what if you want the nights to always be hotter than the days, and not just sometimes? I can think of a couple of ways that might work, depending on what you want.

One way is to make the seasons change really slowly. If you make the "night" sun a red giant rather than a brown dwarf then you can put the planet really far away from it, because red giants are really really big and put out a huge amount of heat. Being far away means a slow orbital period. If the Sun became a red giant then the planets in the outer Solar system would receive enough heat to have liquid water, so you could make the orbital period a few hundred years. By giving the star a larger mass you could make it longer still. In this situation, the nights will be hotter than the days, but it would have been the other way around a few hundred years ago, and people might have legends about that.

The other way is to put your planet at what's called the "L1 Lagrange point" between the two stars. That's the point where their gravity exactly cancels out, so the planet can sit between them (but closer to the smaller one) without orbiting at all. This sounds ideal, except that the L1 point is a so-called "unstable equilibrium." Staying in that point is a bit like balancing on the top of a really tall poll - it doesn't take a lot of energy, but unless you keep shifting your weight in just the right way you'll fall down and end up somewhere else. This means your planet won't stay in that position by itself, but depending on what propulsion technology your world has, it might be possible to keep it there technologically. If you're looking for a high-tech solution, this might be it.

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The planet would naturally be so hot that when colonised a solar shield was put in on the day side and a mirror on the night side to allow control of the amount of light and heat the planet received.

Technically the day* would be cold and dark and the night** light and warm.

*as defined by the face of the planet towards the star
**as defined by the face of the planet away from the star


Exothermic nocturnal creatures. They rest during the day because they would overheat in direct sunlight and come out at night but give off so much heat that the air is warmer than it would be in daylight.

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I defined the day/night requirements in the question as based on the amount of visible light, but if the terraforming reflectors only worked in the Infrared (for whatever reason) then this might be viable. – Joe Bloggs Jan 15 at 12:53
    
You did, but I quietly ignored that for practical reasons – Separatrix Jan 15 at 13:37

As the sun is the source of both light and heat, and presumably by definition "day" is the portion of the rotation cycle when a place is receiving light from the sun, it is difficult to see how it could receive heat from the sun at some other time.

The only way that I see that it would be possible is if the planet receives heat from some other source, and somehow this other source only operates at night.

There could be some source of heat beneath the surface, volcanic activity or some such. But what would cause this to only operate at night?

Maybe some biological activity: creatures that are only active at night, and that give off heat when they are active, and they give off lots of heat. But for plants or animals to give off more heat than the sun, well, there's nothing comparable on Earth. Maybe it's possible. Of course the energy would have to come from somewhere, but I guess they could absorb heat from the sun during the day, and then release it at night.

You could imagine beings with sufficiently advanced technology doing this deliberately. They build some gigantic heating units sufficient to heat half a planet, and they only turn them on at night.

Maybe the planet has a moon whose orbit places it always on the opposite side from the sun, and which has major volcanic activity or some other process emitting so much heat that it warms the planet? I'd have to work out the physics, but I think for the moon to stay opposite the sun it would have to be very far from the planet, and so the amount of heat produced to warm the planet would have to be huge. At that point, wouldn't it be glowing, and thus the night would be as bright as the day? You'd have to come up with some process that would produce lots of heat but no visible light, or where something blocks the visible light but not the heat, without that "something" eventually getting so hot that it itself radiates light.

I guess for a science fiction story, even if something like what I've suggested above is not really physically possible, you could do some hand waving and pretend it's possible. You can always postule some physical or biological process unknown to 21st century human science.

  • Addendum *

I just calculated the orbit for a moon to always be on the opposite side of the sun. That would presumably mean that the moon's orbital period is equal to the planet's year. For the Earth, the radius for a given orbital period is (4.0e14 * (p/(2*pi))^2)^(1/3), where "p" is the orbital period in seconds and 4.0e14 is the Earth's mass times the gravitational constant. Plugging in p=60*60*24.365.25 I get r=7.0e9m or about 7 million kilometers.

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The orbit you're looking for is called the L2 Lagrange point. Unfortunately it's not a stable orbit - you can keep something there artificially, but a natural object will not stay in that orbit. – Nathaniel Jan 16 at 2:44
    
@Nathaniel Well, I didn't mean Lagrange points. I meant a moon with an orbit whose radius is large enough that the period of it's orbit is equal to its primary's year. See the calculation in my addendum. I don't see any reason why this wouldn't be stable. Now that you mention it, L2 might fit the requirements also. My understanding was that the Lagrange points ARE stable, I'm not sure what you mean there. – Jay Jan 18 at 5:29
    
"a moon with an orbit whose radius is large enough that the period of it's orbit is equal to its primary's year" is the L2 Lagrange point, that's its definition. It is not a stable orbit, as explained in the link in my comment above. (You may be thinking of the L4 and L5 Lagrange points, which are stable, but L1, L2 and L3 are not.) – Nathaniel Jan 18 at 7:31

What defines day? What defines night? Day is when there is a bunch of visible light on the planet. Night is when there is not a lot of visible light.

So, let's shift the spectrum up, and put a ring around the planet. Normally, as mentioned, life evolves to see this peak range. However, let's go with a species that evolved from a nocturnal animal. These animals evolved to see in the night, based on the ringshine. We'll make this work with a very slow spinning planet, that's very large and far away from the star.

Over time, instead of adapting to see in the day as the species evolved, the planet's species evolved to see more from the ringshine spectrum (which dominated a larger portion of the day and was more consistent) and less from the sunlight - until the star looks like a faint moon and the ring dominates during the active period - the day. To anyone visiting, the day (when the star is out) is warm and the night is cold, but to the native species the night (when the star is out) is warm and the day is cold.

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I like this solution from an evolutionary perspective, but I did stipulate human visible light in the question. – Joe Bloggs Jan 15 at 20:35
    
@JoeBloggs I missed this the first time, but it doesn't invalidate the premise. A class B star (or even an O star, assuming life can evolve that quick) would still be visible, but it has a higher spectrum. It just would be faint because it is far-off. The reflection? Very visible and generating quite a bit of heat. – Mark Jan 15 at 21:05

Rather than a planetary approach, I've decided to go with a biological one.

The world is covered with a specific, alien plant. This plant stores a huge oasis of water deep below the ground. During the day, the plant works like a Peltier device, using energy from the sun to pull heat from the environment and dump it into the reservoir at its roots. The tree-sized plant has a tall canopy of broad leaves, to gather as much energy from the sun as possible, and below that rings of thin, fin-shaped leaves, for absorbing heat. Other plants may do the same, in the ocean or elsewhere, though through roughly the same process.

During the day, the trees create artificially cold areas around themselves, lowering the temperature by several degrees below ambient. The larger the forest, the lower the temperatures. At twilight, as the energy provided to the tree through light is only enough to maintain balance, the temperature would quickly rise to that of the actual atmospheric temperature; once nightfall occurs, the trees will have no incoming energy to hold onto their heat, and as they vent their heat, the temperature will rise still farther.

The warmest point of the day would be the early morning, an hour or so before dawn; the coolest would be around noon, when the tree has the most energy to work with.

Presumably, the trees do this for some evolutionary reason, attracting birds or something. Or maybe it's peer pressure - they just do it to be cool.

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