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My brother noticed something very odd last night: it was amazingly light out when he stepped outside for a smoke even though it was between 3 AM and 3:30 AM EDST and we are near Toronto Ontario Canada, not in the Far North. He is outside at that time most nights and is no stranger to being up very very early at all times of the year. He has always lived here and is 63 years old, not an inexperienced child. He said the amount of light in the sky was equivalent to the amount of light you'd see in the mid-afternoon of an overcast day. He'd never seen anything like it.

Since I have a lifelong love of science fiction, he asked me if I had any idea why this might have been; he thought there might be some natural phenomenon that might explain it. I asked if he had used some kind of intoxicant but he assured me he was completely sober.

I'm at a loss to think of any plausible source for his experience but I'm not a scientist of either the professional or amateur kind so I thought I'd ask here if there is some rare atmospheric condition that might explain it.

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    $\begingroup$ Why not the nearly full Moon? $\endgroup$
    – Mike G
    Mar 28 '21 at 3:38
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    $\begingroup$ Share a picture! $\endgroup$
    – obe
    Mar 28 '21 at 16:49
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    $\begingroup$ Human vision can handle a huge range of brightness conditions; stepping outside from a dark home could make the outside seem bright, like when you step outside during the day on a cloudy day (from a room well-lit by windows). So it's hard to estimate in absolute terms how bright it is outside, and you could be off by an order of magnitude (maybe? I just made that figure up). This makes @uhoh's hypothesis about reflected light pollution more plausible. $\endgroup$
    – Peter Cordes
    Mar 28 '21 at 17:51
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    $\begingroup$ Snow on the ground, low clouds and street lights really brighten things up. $\endgroup$
    – Wayfaring Stranger
    Mar 28 '21 at 21:36
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    $\begingroup$ Here's a weird coincidence (nothing more): I live in rural New Zealand (little light pollution and no snow) and commented to my wife as I went to bed last night (about 11pm) how unusually light it was outside. It was overcast: the moon must have been particularly bright because while I couldn't see where the moon was, the whole sky was lit up like it was early dusk. So my vote is for the (near) full moon. Edit: See my comment below about the "Super full moon" $\endgroup$
    – pcdev
    Mar 28 '21 at 23:48
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I come from a similar latitude and enjoyed the night skies and two things that can add to "the amount of light in the sky" that I can think of. The first is the near-full moon as mentioned above, and the second is unusually low clouds. Being near Toronto and outlying communities, there is a great deal of light produced, and some goes up. On nights when the clouds or even semi-transparent haze are present and unusually low, they can reflect light pollution more often than normal. Add to that the light from the Moon and it should be enough.

The reason I mention latitude is that when the ground is covered in white, reflective snow, the light pollution is even worse; lights projected preferentially downward are more strongly reflected back upwards. I don't know if there's any snow on the ground still there; I haven't seen snow in way too many years.

Since you didn't mention that your brother noted the full Moon, I'm guessing it was cloudy and so they didn't see it. So I'm guessing that instead they saw clouds or haze lit by it and/or by local light pollution.

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    $\begingroup$ According to timeanddate.com it was mostly cloudy on the morning in question and had been above freezing all week. $\endgroup$
    – Mike G
    Mar 28 '21 at 8:41
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    $\begingroup$ timeanddate.com also reveals that it was very close to a full moon; it was "waxing gibbous 99%". I spoke to my brother this morning and he accepts this as a plausible explanation but I suppose I'm a little skeptical. We're actually in Kitchener which is roughly an hour west of Toronto but Toronto is very large and Mississauga,Brampton, Halton Hills, Guelph and Cambridge leave few gaps between here and Toronto. However, we've also lived through many many full moons in those years and he'd never seen it quite so light before at that time of night. $\endgroup$
    – Henry
    Mar 28 '21 at 14:10
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    $\begingroup$ @Henry It's always good to be skeptical. This is a plausible explanation based on available information. What happens with clouds, fog, and other forms of water droplets in the air is quite varied; maybe the triple-coincidence of an unusual weather event, along with a full moon, along with your brother being outside at the right time to notice is simply rare. Let's see if others post additional or alternative answers. Welcome to Astronomy SE! $\endgroup$
    – uhoh
    Mar 28 '21 at 14:41
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    $\begingroup$ It's unlikely at the moment with no mass events, but I've found it light enough to cycle quickly by the light pollution from a sports field over 2km away with low overcast cloud (no snow, rain in fact). I reckon that's brighter than a full moon, though I was probably more dark-adapted near the stadium than when I've cycled by moonlight (don't worry, I had lights, just covered the front one with my hand). Normal urban light pollution used to be obvious from its colour, but with the switch from sodium to LED lighting this isn't so obvious $\endgroup$
    – Chris H
    Mar 28 '21 at 18:13
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    $\begingroup$ @PeterMortensen question was posted March 27 22:34 Toronto (EDST) time and says "last night" and "3 AM" which I take to be March 27 03:00. From in-the-sky.org/skymap.php I get 40° above the horizon in the WSW. i.stack.imgur.com/vdrv7.png $\endgroup$
    – uhoh
    Mar 28 '21 at 23:56
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I decided to add my comment as an answer, as I think it is relevant:

I live in rural New Zealand (little light pollution and no snow) and commented to my wife as I went to bed last night (about 11pm) how unusually light it was outside. It was overcast: the moon must have been particularly bright because while I couldn't see where the moon was, the whole sky was lit up like it was early dusk.

Seeing this question immediately piqued my interest, and looking into it a bit more timeanddate.com reveals that there has been a recent "Super full moon", which can appear "up to 30% brighter" than a micromoon: https://www.timeanddate.com/astronomy/moon/super-full-moon.html

So while snow and cloud cover probably helped, I think it was the effect of a closer full moon.

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    $\begingroup$ Oh that's interesting! The Moon's distance varies and when a full moon happens near periapsis it will be significantly brighter. It was more than a day before the exact full moon so I don't know if opposition surge contributed as well. $\endgroup$
    – uhoh
    Mar 29 '21 at 0:10
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    $\begingroup$ So I've just asked How long does lunar opposition surge last? Are there measurements of the full Moon getting suddenly brighter? $\endgroup$
    – uhoh
    Mar 29 '21 at 0:28
  • $\begingroup$ searching recent news for "supermoon suez canal" produces mildly interesting results $\endgroup$
    – uhoh
    Mar 30 '21 at 14:26
  • $\begingroup$ Additional anecdote: Same in Chicago with plenty of light pollution, past few nights the moon caught my eye looking unusually bright (as seen from my couch, so it's not like I just happened to be outside at the right time). $\endgroup$
    – Izkata
    Mar 30 '21 at 15:42
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Was there a certain color to the light? When I notice it being especially bright out at night where I live I can tell it's because A) there's snow cover to reflect the light falling on the ground and B) low clouds to reflect the light back to the ground. The light has a yellow/orange hue because most of the street lamps in my area are sodium.

Fresh snow has an albedo (amount light it reflects) of about 80%. Clouds vary widely based on drop size, liquid/ice content, and thickness. This can be more than 90%. So on an exceptional night, about 72% of the light will be reflected from the ground to the clouds and back to the ground.

In that case, 72% of that light repeats the cycle. Well, less because some of the light is reflected sideways eventually out to areas outside the city. If the clouds are low this won't matter as much since less distance is covered by the same reflection angle.

If we go with 72% and keep iterating then each cycle, after 10 cycles we end up the clouds reflecting back 243.77% of the light generated by the street lamps (or other things like headlights and lights from windows) meaning on average the ground is illuminated by about 3.5 times as many street lights of the clouds and atmosphere didn't reflect any light back. Without the reflection from the sky normally there is roughly 100% and that is concentrated near the streets. With the reflections there is an extra 240% that is spread more evenly to illuminate the tops of trees, roofs, back yards, and other areas that don't normally receive that much illumination at night.

Using this very simplified model, eventually about 71.5% of the light gets absorbed by the "ground" (including going into your eyeball) and the rest, about 28.5% of it makes it into space.

I found an article that might be worth looking at with what I'm sure are better formulas.

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overcast + full moon + light pollution = late winter afternoon sky

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    $\begingroup$ Could you please add more details so people who are not familiar with this subject can understand it more? $\endgroup$
    – fasterthanlight
    Mar 29 '21 at 13:51
  • $\begingroup$ "Afternoon" vs. 3 AM? $\endgroup$
    – Chris H
    Mar 29 '21 at 14:06
  • $\begingroup$ @ChrisH I think Anim8D means "overcast + full moon + light pollution can look like late winter afternoon sky" $\endgroup$
    – Jim421616
    Apr 1 '21 at 20:49
  • $\begingroup$ @Jim421616 possibly, but it's not clear enough to tell $\endgroup$
    – Chris H
    Apr 1 '21 at 21:07
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Tonight in Central Florida 12:30 A.M. December 13th 2021, the 'Nocturnal Sun' night (as they are called) is happening now. On all sides the sky is very bright. The entire sky is cloud covered and light, but the sides on each side are very bright, like dawn or sunset. It is amazing.

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    $\begingroup$ Welcome to Stack Exchange! I've added some supporting sources for the term 'Nocturnal Sun'. If they agree with what you're explaining that's great. If not (and I am not sure they do!), please feel free to find some better ones. Stack Exchange answers should strive to support assertions of fact with authoritative sources whenever possible. Thanks! $\endgroup$
    – uhoh
    Dec 13 '21 at 10:16
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    $\begingroup$ Central Florida never experiences a "nocturnal Sun": not during the summer and definitely not near the winter solstice (Dec 21). Central Florida does experience a lot of light pollution which will reflect off of clouds and make the sky 'bright' at all hours of the night. $\endgroup$
    – JohnHoltz
    Dec 13 '21 at 17:39

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