Submitted by colorado_hick t3_100y631 in askscience
I live in rural Colorado. It appears to me that during the winter time when I look at the mountains in the distance the edges seem to be more crisply defined then the same views in the summer time, especially on really cold days. I do not know if this is because the colors are different? Or if it is something psychosomatic? Or does the higher heat on warm days lead to some sort of optical distortion (similar to a low-grade mirage effect)?
This is the key u/colorado_hick. Whether it’s 120* in AZ or -20* in CO, the western US has clarity from low humidity. Growing up in New England, the humidity obscures a lot of detail at a distance.
Though it's also considered an aesthetic feature, the "breathing" of Appalachian-range mountains in the morning as the sun heats overnight dew and obscures the view with rising fog at each ridge. While many find it beautiful, I've met a couple western-U.S. natives who found it almost claustrophobic.
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East coaster here. I went out to the west coast to hike the PCT some years ago. I felt uneasy the entire time. Low humidity, strong wind, and completely exposed was a mixture we don't have over here. I'm used to the safety of the forest.
I loved it out there, I've been back a bunch of times to hike. I just have never been able to shake the uneasy feeling if being so exposed.
It's definitely unsettling to see a mountain in the distance that's an hour drive away, as one would in parts of Colorado. Imagine getting off a boat in Portland, Maine, and having someone say "And if you glance to the west there, those are the White Mountains over in New Hampshire."
Lol, I live in the Seattle Area, you don’t have to glance in order to see mountains. You have the Cascades to the East and The Olympics to the West and they’re both more than an hour away.
Take the victoria clipper north sometime and stand on the rear deck watching Mount Rainier. As you leave Seattle, you see the city recede into a dot, but the mountain behind it doesn't appear to get any smaller. It's wild.
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Do the Knife Edge trail up Mt Katahdin in Maine for a throwback.
We did section J a few years back, Katahdin has areas that are similar up knife edge due to its prominence.
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Until nature doubled down on the restricted visibility and made the Blue Ridge mountains. It’s about the only thing that comes close in terms of spectacular.
But otherwise you’re absolutely right. I think growing up in KY is part of why things like the Grand Canyon or the ocean didn't register as mind blowing as they really are. I could never see a 10 mile distance, and even if I could, anything as clear as the GC was would’ve been much closer. It wasn’t till I did an internship in NM that it really sank in. Going east on 40 from Albuquerque, theres a point near Sedillo where you exit the Sandias and can see the road stretched out in front of you till the next hill. I guessed it was about a mile and a half away, but it was actually four and a quarter.
I was on Amtrak’s Empire Builder westbound in Montana. Off in the distance, I could see the mountains of Glacier National Park. I thought they were maybe 30 miles away. They were 150.
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The weirdos, but what can you expect when they walk around in a place where you are clearly at risk of falling off the Earth.
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the north east is run on crappy servers so we need water vapor to reduce draw distances or we get frame lag, the south west has much better servers so they get the full draw distance without the need for performance improvements granted by the fog.
Servers are the same hardware, just more players mean that there's less performance per per user.
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Except in the winter when you get those cold days right after a Nor'easter and can see all the way Boston to Providence...
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Doesn't 120° cause heavy shimmering regardless of humidity?
Is this why the sky in places like Montana looks so much bigger than it does out East?
Would cold, low humidity air also allow sound to travel more clearly (and more loudly)?
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Up to a point. It's not that the air needs to be cold, it's that it needs to be cooler near the ground than above it. Sound travels faster in the hotter air, and sound waves curve and refract from the zone of higher speed into the zone of lower speed.
So in the night, when the air gets colder, the sound from up above tends to curve towards the ground and propagate farther. During the day is the other way round. More to read: https://blog.thermaxxjackets.com/refraction-of-sound-waves-acoustic-shadows-explained
I hope this gets more scientific discussion, but I'm very sure this effect happens. When I lived in MN if I'd go outside on the absolute coldest days, like -30F sort of days, the one thing that always struck me was how loud everything seemed, from the crunching of snow under my feet, or even neighbors at the end of the block talking in a normal speaking voice and yet still being able to hear what they were saying.
Edit: Did some Googling, and apparently the explanation about refraction is correct? Essentially the ground air is extremely cold but there's a layer of warmer air above which effectively ends up bouncing the sound waves back towards the ground allowing them to carry much farther than normal
If it snows, snow is a pretty effective sound absorber and can make sounds sound clearer due to the lack of reflections on surfaces covered in snow. It wouldn’t be louder though, it would be softer.
If you mean without snow, I have no clue.
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I would imagine yes but that’s because humid air is less dense. The more dense the air the faster waves can travel, so dry may equal farther distance sound can be heard.
I think this is a common misconception. Cooler (denser) air molecules move more slowly than warmer air molecules, and that effect negates the 'packedness' that typically results in denser=faster sound. For ideal gases, denser=slower sound.
http://hyperphysics.phy-astr.gsu.edu/hbase/Sound/souspe3.html
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Would be the opposite, no? Sounds travel faster and further underwater than in air.
Would be logic that high humidity air allows sounds to travel further too, no?
Tempeture gradients can conduct sound along the ground. Importantly, snow being a good sound absorber tents to quiet everything down.
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I agree with this, but there might be more to it. Hot air has a different refraction index due to the faster movement of particles in it. This is just something to add on.
also pollen! in much of the world the warmer months have air quality reduction from tree and plant pollen in the air.
Scintillation causes blurring of images over distance. The mechanism is that rising bubbles of warmer air act like lenses. This kind of lensing is what causes stars to twinkle, and also the "heat shimmer" you might see above a hot roof or hot pavement.
Scintillation is generally weakest in the morning when the ground is cold.
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Isn't the RH already independent of the temperature? So 50% RH at 20 C would yield the same bext as 50% RH at -10 C. Do the lower temperatures also imply a lower RH?
No, you're thinking of absolute humidity.
Lower (higher) temperature for the same amount of water vapor results in higher (lower) relative humidity.
Right, if it gets colder, that doesn't mean there's less water in the air. That makes sense, thank you.
Sounds like you got it backwards. If it gets colder, it means there will be less water/humidity in the air. Colder air will be "dryer" then warmer air.Its why your skin and lips tend to dry out when your outside in the winter, if that helps your imagination.
Edit.. oh right just noticed the comment below, heh
As noted above, water content within the air is the dominant effect. Colder air holds less water, so in general, winter atmospheres have less water and are more transparent.
The variable usually used to capture this (i.e., modeling the effect of water on atmospheric clarity) is water column content, measured in grams per square centimeter. This is the mass of water in a column of atmosphere 1cm by 1cm, from the Earth surface up to the top of atmosphere. It typically ranges from 0 to 5 g/cm2, at least for non-cloudy atmospheres.
At 5 g/cm2, your naked eye will notice haze and muted colors. The colors are muted because some spectrally distinct light from distant objects is being intercepted by water and scattered as white light. E.g., a fraction of the greenish light reflected from a tree is absorbed by water and re-radiated as panchromatic light, before finally reaching your eye. So you see 90% photons from the tree, and 10% photons scattered off water between your eye and the tree. And of course this scattering causes blurred edges as well.
This variable (column water content) can be measured by scientific instruments in several ways, fairly directly, by its impact on the spectrum of sunlight. NASA has several satellites that make this measurement regularly, sometimes just to calibrate out the effect of atmospheric water on other measurements.
Here’s a nice map of water content, animated monthly: https://earthobservatory.nasa.gov/global-maps/MYDAL2_M_SKY_WV
You can see the decease in atmospheric water in Colorado in the winter months, when the atmosphere there is dominated by cool, arctic air; and then increasing in summer as you get air from the tropics. (As the narrative explains.)
You can also notice stronger seasonal effects over land than over oceans, partly due to thermal regulation by the ocean’s water mass. Again, this makes mid-Continent areas like Colorado have a stronger seasonal “swing” than, say, coastal California.
It sounds like you're talking about a parameter that would describe the visibility of stars, etc., which you have to look through the whole atmosphere to see, whereas OP is looking horizontally, just through the lower atmosphere. The humidity would matter for both, but specifically counting the total in a vertical column would be less relevant for the horizontal view.
That’s true, the path integral of water viewpoint is what you would ideally want, especially if you were trying to explain just how far you could see on a given day, or why the top of the mountain is usually clearer than the foothills below (more water and dust at lower altitudes).
In fact, because you do care about scattered light coming from anywhere, you can’t just look at the line between your eye and the mountain. You need to know the whole spatial distribution, including stratification (e.g., rainbows). Indeed, some of those photons are scattering multiple times before they enter the light path from your eye to that green tree (double rainbows). Radiative transfer modeling is hard!
But column H2O is a well measured quantity for which there are off the shelf data that do show and explain the seasonal effect that OP mentioned (as the map shows). Basically, the gross, hundreds-of-kilometer monthly-average H2O that’s in that map is the constant factor out front of the light-path integral that is the “right thing” to use.
Depending on the relative altitudes you could observe something through more atmosphere on earth than looking up at stars.
The references both to colder air holding less water vapor, and relative humidity, might be confusing. The water vapor, when it stays in the form of water vapor, has minimal impacts on visibility. A larger impact, captured in the equations above, occurs when the water vapor combines with pollution in the air, specifically ammonium nitrate and ammonium sulfate in those equations. Those are hygroscopic salts that will become hydrated if the relative humidity is high enough, forming larger particulates that will scatter light more, especially if they start to clump together as they can do. Whether they become hydrated is controlled by the relative humidity, not the absolute humidity.
For more on this, see the EPA's guidebook on air quality impacts on visibility, written with an emphasis on views in national parks. https://www.epa.gov/sites/default/files/2016-07/documents/introvis.pdf Section 4 is the most directly relevant, particularly starting on p. 22 (p. 31 of the pdf).
The relative humidity in Colorado tends to be low in both summer and winter, and also varies greatly with time of day, so I'm not sure to what extent humidity explains what OP is seeing.
Edit: the monthly average RH in Limon CO (on the plains, east of the mountains) is a little bit higher in the winter than in the summer, but the swing in relative humidity over the course of a single day is often much larger than that seasonal variation.
Can you link a Wikipedia article or similarly informative page where I can read more about this? Or give me a term I can Google? This is fascinating
I put this in another comment as well, but to provide a direct answer to your request, here's a link to the EPA's guidebook on air quality impacts on visibility, written with an emphasis on views in national parks. https://www.epa.gov/sites/default/files/2016-07/documents/introvis.pdf
Does the refractive index of the air also play a role at all? I wouldn't imagine it would be too noticeable just looking at mountains, but if looking out over the horizon I would imagine it might.
It's less the refractive index, and more variations in the refractive index, which makes the view less crisp.
Think of how air shimmers over a hot road. The heat rising off the road causes differences in the refractive index, which causes the shimmer.
Over long distances, with a lot of these together, the view would be fuzzier.
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I live in the north. Is this why the stars are amazing at -40?
Also, the sun is lower in the sky during winter, and sidelighting tends to look “crisper” than overhead lighting (like noon in summer).
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Really it's kinda common sense. You can't see as well in fog, or in any other high humidity situation (sauna, steamy shower, etc.) so even without knowing the actual science/formulas it's pretty easy to extrapolate that the less humid places will have more visibility (although not dissing the formulas at all, respect for knowing the exact science).
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So that’s why the stars look clearer on a cold night! Thanks for the explanation.
Is this the same reason why when you look out in the distance in hot environments the air looks “wavy”?
This is an important point, but as a commenter down below has noted, this explanation alone actually produces a result in the wrong direction - better visibility when it is warm!
The haze you see is scattering of light, primarily from atmospheric aerosols. Things like sea salt, desert dust, sulphate droplets from burning fossil fuels. These scatter light as haze and reduce the visibility (some examples here).
As the relative humidity (RH) increases, these aerosol particles take up water and swell, making them larger. These larger particles are even better at scattering light, making the atmosphere hazier as the relative humidity increases (even if you have the same number of aerosol particles).
You can actually see this effect near clouds (where the humidity is very high) - (Koren et al, 2007)
However, if you keep the absolute (or 'specific') humidity the same (i.e. keep the same number of water molecules), the relative humidity increases as it gets colder. This means that following the explanation above, the visibility actually gets worse in colder weather (all else being equal)!
The full reason for clearer skies when it is colder comes from a variety of effects.
These factors may not be true in all places. Some of the biggests and most damaging smogs in London were in winter, as local sources of aerosol particles (from buring fuel for heat) increased significantly in cold conditions.
Obviously clouds have an effect too, but that would be a much longer post!
The temperature has an effect on what happens to the water vapor in the air. At cold temps, water condenses on surfaces ("morning dew"). Colder yet, it freezes. So, yes, warmer temps often coincide with greater amounts of water vapor in the air.
How is that different than saying, “dry air is more clear”? And wouldn’t that be so regardless of temperature?
Summer sun also creates convection currents that distort light.
So both dry and cool (which often go together) make clearer views.
What about those waves you see from heat. I bet that effect is involved too
Dry air is clearer regardless of temperature, but warm air holds moisture more easily, so cold air is more likely to be dry and clear.
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Can’t it also be less pollen in the air?
That can be in rurla areas, in urban landscape it can be less smog, as rainfall drags it to the ground.
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Is this why I’ve noticed the winter sunsets are more beautiful than usual?
There are (at least) a couple of different factors at work that can affect how clearly you can see a distant object.
The first is 'stuff' in the air. Dust, pollen, water (or ice). These can all change with the weather (and the seasons). The behavior of humidity - the amount that air can carry, and whether it condenses out - depends strongly on temperature.
The second is the air itself. Non-uniform air temperatures between you and those distant mountains affect the density of the intervening air, and result in weak, transient lensing effects. As you noted, it's the same phenomenon that results in mirages or heat shimmer--or which causes stars to 'twinkle' at night. Greater variations (and sharper gradients) in air temperature, coupled with greater turbulence in the air, make for worse seeing.
I thought the twinkle was because stars have such a small solid angle, any fluctuation in their output is perceived as a huge difference visually. And since fluctuations always happen there’s always a change in what we see thus the twinkle.
The twinkling of stars (technical term is scintillation) we perceive isn’t due to any intrinsic change in the star’s output*. It’s due to turbulent regions of our own atmosphere distorting the light from the star. This means that effectively light from the star is scattered into or out of our detector/eye and so we see the brightness of the star changing.
There’s some more complex stuff with the light wavefronts being distorted and interference patterns but I’m too rusty on it to remember and explain it, plus the simplified description above is enough (it got me through a Masters thesis on the topic!).
Edit: Also forgot to add, the reason we see stars twinkle while the planets in our solar system don’t. This is because stars are so far away that they’re point sources, whereas the planets are near enough that we can resolve them as disks on the sky with our eyes (it may not look that way but we do see them as more than single point sources with the naked eye). So while the light from the planets (well, reflected by them) is distorted in the same way as described above, it’s effectively being distorted across the extent of the disk from our point of view, so we don’t perceive any change in the brightness.
(*the intrinsic brightness of stars does vary but, on the timescale of twinkling, this variation is way too small to perceive. On longer timescales you can get variations that are large enough in magnitude that you could technically notice it, but again this isn’t what we’d call “twinkling”)
> any fluctuation in their output
the heat shimmer is what causes that fluctuation. there are other causes like gas pockets and dust, but the heat differences is the most common.
Sounds to me like you're seeing heat haze in the summer, very common down here In Australia.
As other have explained, it's the moisture in the air is what creates a haze. So, yes colder temps do generate a more visually "crisp" look. You'll also notice that sounds carries much further, this is also because there is less moisture in the air to slow down the sound.
No one else has included this but - when the plants are active, they will be emitting terpenes into the air, a natural source of volatile hydrocarbons that produce haze.
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People have done good explanations of why the colder air is "more clear," but you can also check with your friendly neighborhood amateur astronomer, who will tell you that cold, dry nights are the clearest for stargazing. And that, sadly for us stargazers, the best nights for stars are often the ones where your fingers get too cold to operate the scope. :)
Where I live, when it's around 20-35*, there's always a lot of moisture in the air, and it's usually cloudy. When it's cold, crisp, and clear as a bell, it's usually below 10 degrees; the conditions are AMAZING for stargazing, but my poor fingers freeze within half an hour.
Clear Dark Sky is a Canadian government forecast service that gives a stargazing forecast for North America .. they give good and clear explanations on atmospheric transparency and "seeing." I expect if you peek at the astronomy forecast,you will find your view of the mountains is the most crisp on days when transparency is high. https://www.cleardarksky.com/c/Ottawakey.html
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It's possible that the appearance of the mountain edges being more crisply defined in the wintertime could be due to a phenomenon called atmospheric refraction. This is when light is bent as it passes through different layers of the atmosphere, which can cause objects to appear differently to the observer.
During the winter, the air is generally colder and drier, which can lead to clearer visibility. This is because there is less water vapor in the air to scatter light and cause the "haze" effect that can sometimes be seen on warmer, more humid days.
Additionally, the angle of the sun in the sky can also affect the way that objects appear. In the winter, the sun is lower in the sky, which can create longer shadows and more contrast between light and dark areas, making objects appear sharper and more defined.
It's also possible that the perception of the mountain edges being more crisply defined could be due to a psychological effect, as you mentioned. Our brains can play tricks on us and sometimes interpret things in different ways based on our surroundings and expectations.
In addition to temperature, humidity & other atmospheric effects that others have mentioned, there is some perceptual trickery going on.
Source (even though they used red and green for their example of contrasting colors when red and cyan would be more appropriate in an additive system) https://colorusage.arc.nasa.gov/Simult_and_succ_cont.php
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Absolutely. Heat from the ground causes the horizon to look wavy. I’m sure everyone has seen this on a hot summer day. When the ground is cold there is not as much heat rising so things in the distant look more HD.
Plenty of other good answers.
In addition to the comments about the air effectively being "clearer" at low temperatures etc, there is a known visual or psychovisual phenomena where higher-contrast images or scenes are perceived as being sharper (this is exploited by people trying to sell you new TVs etc).
"Mucky" air will decrease the contrast (as well as perhaps physically blurring) which will make the scene "pop" less. As others have said, the angle of the sun can also dramatically affect the scene contrast.
Related to this, I find general urban street scenes "pop" in the sunshine shortly after rain - the rain clears the air, washes away dust, and if surfaces are wet and shiny the contrast is much higher - it can look "hyper-real".
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As a fello colorodoan, and fiber optics engineer I'm qualified to help add to the other answers,
Most of the other comments are accurate about pollen, humidity, etc. A few things left out is the marijuana haze from increased outdoor consumption, an increase in air pollution due to increased vehicle usage, the mirage effect is also more prevalent in Colorado during summer because the relative temperature between the mountain peaks and the plains is relatively similar during winter (A LOT of factors in this), and if you're on the eastern plains looking at the mountains in colorado the contrast plus angle of the sun in winter makes the mountains brighter and stand out more for a lot longer in the day. During the summer if youre up early enough on a cold day in summer the mountains will stand out more than at like noon.
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uber_snotling t1_j2kteqn wrote
Humidity is the key thing here. Colder air holds less water vapor.
There is an equation that describes visibility and the Western US always has much longer view-lengths than the Eastern US because of the relative water vapor content (and to a lesser extent, sulfates from coal are way way down in the whole US).
Too much detail here, but RH = relative humidity and the other stuff is pollutants in particulate matter in the air.
bext ≈ 3 × f(RH) × [Ammonium Sulfate] + 3 × f(RH) × [Ammonium Nitrate] +
4 × [Organic Mass] + 10 × [Elemental Carbon] + 1 × [Fine Soil] + 0.6 × [Coarse Mass]
+ Rayleigh scattering