# Calling out to all scientists! \[Conditions to perceive a yellow sky\] **URL:** \ **Category:** Factual Questions\ **Created:** [April 30, 2015, 8:20pm UTC](https://boards.straightdope.com/t/calling-out-to-all-scientists-conditions-to-perceive-a-yellow-sky/718924 "2015-04-30T20:20:03Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![BelahTympaniez](https://avatars.discourse-cdn.com/v4/letter/b/9fc348/32.png) [@BelahTympaniez](https://boards.straightdope.com/u/BelahTympaniez)\ **Post date:** [April 30, 2015, 8:20pm UTC](https://boards.straightdope.com/t/calling-out-to-all-scientists-conditions-to-perceive-a-yellow-sky/718924/1 "2015-04-30T20:20:03Z")
I was wondering if anyone can answer my question: I was wondering what conditions would be needed to perceive a yellow sky and how this sky would change colour as the sun(s) set. Its for a book I’m writing which is not set on Earth (as you may be able to guess) ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 30, 2015, 8:29pm UTC](https://boards.straightdope.com/t/calling-out-to-all-scientists-conditions-to-perceive-a-yellow-sky/718924/2 "2015-04-30T20:29:34Z")
The simplest method would be to just have a bunch of yellow particulates suspended in the air. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [April 30, 2015, 8:35pm UTC](https://boards.straightdope.com/t/calling-out-to-all-scientists-conditions-to-perceive-a-yellow-sky/718924/3 "2015-04-30T20:35:56Z")
Title edited to give an idea of the subject (although I’m not sure that “perceive” is the word you want). Colibri General Questions Moderator ---
**Author:** ![Fear\_Itself](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fear_itself/32/19637_2.png) [@Fear\_Itself](https://boards.straightdope.com/u/Fear_Itself)\ **Post date:** [April 30, 2015, 8:54pm UTC](https://boards.straightdope.com/t/calling-out-to-all-scientists-conditions-to-perceive-a-yellow-sky/718924/4 "2015-04-30T20:54:32Z")
A planet with a fluorine atmosphere would have a yellow sky. [http://www.xenology.info/Xeno/5.4.2.htm](http://www.xenology.info/Xeno/5.4.2.htm) ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [April 30, 2015, 8:59pm UTC](https://boards.straightdope.com/t/calling-out-to-all-scientists-conditions-to-perceive-a-yellow-sky/718924/5 "2015-04-30T20:59:22Z")
I looked into this a few years ago; as far as I can determine, a planet with a clear, transparent particulate-free atmosphere with a density of 40 x Earth’s atmosphere would be yellow at the zenith, but whiteish around the horizon. Sunsets would be dark red, I think. Here’s a diagram I copied from Atmospheric Optics by Craig F Bohren, which shows the shift in sky colour with increasing density. [![](https://www.orionsarm.com/im_store/skycolor3.PNG) ](https://www.orionsarm.com/im_store/skycolor3.PNG) more details on this page (partly my own work, written for a SF scenario) with a number of references at the bottom that will give more details [http://www.orionsarm.com/xcms.php?r=oa-page&page=gen\_skyonalienworlds](http://www.orionsarm.com/xcms.php?r=oa-page&page=gen_skyonalienworlds) ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [April 30, 2015, 9:01pm UTC](https://boards.straightdope.com/t/calling-out-to-all-scientists-conditions-to-perceive-a-yellow-sky/718924/6 "2015-04-30T21:01:45Z")
Suspended particles needn’t be yellow themselves – they just have to preferentially scatter yellow light. This is, perversely, just the opposite of the situation we have here in normal Earth skies, where Rayleigh scattering preferentially scatters blue light (which is, ultimately, why the sun is perceived as yellow – but I don’t want to dredge that up again right now). You can get this circumstance where chemical compounds are present that absorb or scatter differently than the air molecules, dust particles, etc. are present in place of or in addition to what’s in the air now. Under those circumstances you can have Blue and Green Moons and Suns. Look up stuff on Blue Moons and what causes them, or have a look at van de Hulst’s _Light Scattering by Small Particles_. I know that one circumstance that can lead to this is after forest fires, when there’s a lot of plant residue – ash, fine sap, pollen, ertc. – in the air. Also, after volcanic eruptions and meteor falls you can get a lot of dust in the atmosphere that will give you spectacular sunsets, including yellow skies near sunset. Look up “Noctilucent Clouds” After the Tunguska Meteorite, the noctilucent cloiuds lit up the European night sky so brightly that people could read by their light well into the night. > **[Noctilucent cloud](https://en.wikipedia.org/wiki/Noctilucent_cloud)** > > Noctilucent clouds, or night shining clouds, are tenuous cloud-like phenomena in the upper atmosphere of Earth. When viewed from space, they are called polar mesospheric clouds (PMCs), detectable as a diffuse scattering layer of water ice crystals near the summer polar mesopause. They consist of ice crystals and from the ground are only visible during astronomical twilight. Noctilucent roughly means "night shining" in Latin. They are most often observed during the summer months from latitudes b... I’d also add that the sky over Mars is colored, mostly by iron oxide (I believe), giving it an orange to yellow cast. [https://images.search.yahoo.com/search/images;\_ylt=AwrBT8qBl0JVat0AR1lXNyoA;\_ylu=X3oDMTExZzQ0YmY1BGNvbG8DYmYxBHBvcwMxBHZ0aWQDVUlDMV8xBHNlYwNzYw--?p=Mars+Sky&fr=yfp-t-252](https://images.search.yahoo.com/search/images;_ylt=AwrBT8qBl0JVat0AR1lXNyoA;_ylu=X3oDMTExZzQ0YmY1BGNvbG8DYmYxBHBvcwMxBHZ0aWQDVUlDMV8xBHNlYwNzYw--?p=Mars+Sky&fr=yfp-t-252) So you could get a yellow sky by the addition of various dusts, chemical droplets, or other substances in the atmosphere, which need not be yellow themselves in bulk, but which preferentially scatter and/or absorb light to give you the color. Or you could have a lot of dust in the sky that “amplifies” the yellow twilight sky to be more brilliant and last longer. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [May 1, 2015, 1:57am UTC](https://boards.straightdope.com/t/calling-out-to-all-scientists-conditions-to-perceive-a-yellow-sky/718924/7 "2015-05-01T01:57:36Z")
It’s worth mentioning that in some rainy/overcast conditions the sky can take on a yellowish hue. And once in near-tornado conditions I saw an aquamarine sky. ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [May 1, 2015, 7:19am UTC](https://boards.straightdope.com/t/calling-out-to-all-scientists-conditions-to-perceive-a-yellow-sky/718924/8 "2015-05-01T07:19:34Z")
That’s right. Recently I noticed a particular raincloud over the Yorkshire Wolds; below the cloud the sky was a pale straw-yellow, and above it the most beautiful blue. This is due to Mie scattering, I expect. However I can’t suggest a way to reproduce that colour in a cloudless sky on an alien planet using raindrops, so I think you’d have to imagine some other particulate component.