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)?



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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


mtn970 t1_j2l7ac3 wrote

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.


dcgrey t1_j2mgn76 wrote

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.


[deleted] t1_j2n6iye wrote



RVA_RVA t1_j2or55q wrote

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.


dcgrey t1_j2owanp wrote

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."


MadMagilla5113 t1_j2p5eml wrote

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.


91901bbaa13d40128f7d t1_j2wopog wrote

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.


NoMalarkyZone t1_j2oz3fc wrote

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.


[deleted] t1_j2n6ea5 wrote



0ne_Winged_Angel t1_j2nchg5 wrote

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.


PlainTrain t1_j2njf97 wrote

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.


warriorscot t1_j2p7zjt wrote

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.


some_random_noob t1_j2oeize wrote

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.


TheGrandExquisitor t1_j2ohf84 wrote

Except in the winter when you get those cold days right after a Nor'easter and can see all the way Boston to Providence...


funguyshroom t1_j2pgypr wrote

Doesn't 120° cause heavy shimmering regardless of humidity?


JustaCucumber t1_j35nnk2 wrote

Is this why the sky in places like Montana looks so much bigger than it does out East?


kenlubin t1_j2lpfi8 wrote

Would cold, low humidity air also allow sound to travel more clearly (and more loudly)?


Omsk_Camill t1_j2m0v87 wrote

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:


RLutz t1_j2n1kdw wrote

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


Headytexel t1_j2mw39d wrote

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.


funnyman95 t1_j2lu5om wrote

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.


Tartalacame t1_j2n63bx wrote

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?


binaryblade t1_j2nek5j wrote

Tempeture gradients can conduct sound along the ground. Importantly, snow being a good sound absorber tents to quiet everything down.


DaRealSherlock t1_j2lqfcs wrote

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.


bigbluedoor t1_j2mqjf4 wrote

also pollen! in much of the world the warmer months have air quality reduction from tree and plant pollen in the air.


Another_Penguin t1_j2nqzow wrote

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.


WellMakeItSomehow t1_j2lrid4 wrote

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?


citylikeAMradio t1_j2lup6q wrote

No, you're thinking of absolute humidity.

Lower (higher) temperature for the same amount of water vapor results in higher (lower) relative humidity.


WellMakeItSomehow t1_j2lxsj9 wrote

Right, if it gets colder, that doesn't mean there's less water in the air. That makes sense, thank you.


Sirkelsag t1_j2nssqc wrote

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


LawOfSmallerNumbers t1_j2m52fx wrote

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:

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.


kilotesla t1_j2mcqmg wrote

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.


LawOfSmallerNumbers t1_j2ni7j3 wrote

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.


jkmhawk t1_j2pbbow wrote

Depending on the relative altitudes you could observe something through more atmosphere on earth than looking up at stars.


kilotesla t1_j2mg39m wrote

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. 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.


RandomThrowaway410 t1_j2lxziq wrote

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


redpandaeater t1_j2lz1hn wrote

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.


windchaser__ t1_j2mee8w wrote

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.


kickstand t1_j2mvgbo wrote

Also, the sun is lower in the sky during winter, and sidelighting tends to look “crisper” than overhead lighting (like noon in summer).


fatamSC2 t1_j2niqql wrote

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).


JustMeHere8888 t1_j2o3fvc wrote

So that’s why the stars look clearer on a cold night! Thanks for the explanation.


arebee20 t1_j2oiqoo wrote

Is this the same reason why when you look out in the distance in hot environments the air looks “wavy”?


AerodynamicMarrow t1_j2oz73b wrote

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.

  • Cold conditions often follows a cold front and the rain from this front 'cleans' the atmosphere of aerosols, increasing visibility.
  • Several processes in the Earth system, particularly trees and other vegetation, produce more aerosol as it gets warmer (reducing visibility on a warmer day).
  • Warm air promotes atmospheric mixing and convection, which reduces visibility a bit.
  • On cold days, the wind may be blowing in cleaner air from a different direction, avoiding sources of aerosol in nearby regions.

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!


morphballganon t1_j2klbzg wrote

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.


daveescaped t1_j2kxr2b wrote

How is that different than saying, “dry air is more clear”? And wouldn’t that be so regardless of temperature?


Dont____Panic t1_j2l2s7u wrote

Summer sun also creates convection currents that distort light.

So both dry and cool (which often go together) make clearer views.


solarish t1_j2lfwao wrote

More specifically, it is the increased variance in refractivity (due to convection) that distorts light.


cstheory t1_j2l2vmw wrote

What about those waves you see from heat. I bet that effect is involved too


IlexAquifolia t1_j2lgu5e wrote

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.


Arquen_Marille t1_j2kme6v wrote

Can’t it also be less pollen in the air?


Weird_Element t1_j2kphh2 wrote

That can be in rurla areas, in urban landscape it can be less smog, as rainfall drags it to the ground.


DumbLikeColumbo t1_j2myaq0 wrote

Is this why I’ve noticed the winter sunsets are more beautiful than usual?


cryptotope t1_j2kzpez wrote

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.


tylerchu t1_j2lpick wrote

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.


astro-sobrien t1_j2mb4t4 wrote

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”)


Alis451 t1_j2m1v1a wrote

> 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.


Ebikeboi t1_j2oh0vv wrote

Sounds to me like you're seeing heat haze in the summer, very common down here In Australia.


scroti_mcboogerballs t1_j2l46b1 wrote

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.


ChillyBearGrylls t1_j2litq6 wrote


AliMcGraw t1_j2m3ihy wrote

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.


December292022 t1_j2ljd7f wrote

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.


dmsfx t1_j2neras wrote

In addition to temperature, humidity & other atmospheric effects that others have mentioned, there is some perceptual trickery going on.

  1. the low angle of the sun casts long shadows all day throughout winter. In a place like Colorado those shadows make it easier to see topographic details that would be uniformly lit during the day. Those shadows also change substantially throughout the day as the sun crosses the sky, and you’re more likely to pay attention to the changing landscape.
  2. The low angle of the sun also extends the length of golden hour. You’re probably familiar with that all-day twilight feeling in the winter. Just as it does during sunrise and sunset, sun’s light refracting through the atmosphere during winter leaves more long red & green wavelengths. The yellow-orange color is the additive complement of the blue hues you typically see on distant mountains. in addition to the value contrast from the shadows, this also creates a color contrast. In art we call this successive contrast where contrasting colors next to each other make both appear more intense.
  3. the presence of snow can also setup situations of simultaneous contrast, where 3 values can look like 4. Snow remains longer on the north side of the mountains where it’s shaded from the sun. You end up with a situation where the light patches in the otherwise shaded areas of the mountains appear lighter than they are and make the shadow appear darker.

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)


Willing-Ant-3765 t1_j2nr9lv wrote

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.


Prestigious_Carpet29 t1_j2ups0a wrote

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".


Starbucks__Coffey t1_j5ptytd wrote

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.


[deleted] t1_j2kzphw wrote