<p>This doesn't aim to be a complete answer, but after putting a juicy bounty at Aviation SE on the question "<a href="https://aviation.stackexchange.com/q/60213/34312">Can pilots tell if a cloud or fog is made out of water droplets or ice crystals when flying through it?</a>", the answer seem to be: <strong>No, they can't</strong>. And all the things to look at to detect ice clouds doesn't seem to be useful if you don't have a plane at hand. There is nothing like "there is a special refraction pattern around the lights on the wings", or such.</p>

<p>So far it seem like ice and water clouds are indistinguishable.</p>

CC BY-SA 4.0
2019-03-14T17:02:47.627Z
You could do it from the ground using a white laser, telescope and angle. Maybe a special polarized lens to bring out the rainbow more.
Lately if you shine a flash light through frozen fog it should glitter but you may need to do it in the shade or dark enclosure to see it. It should look like diamond dust.
Muze
CC BY-SA 4.0
2019-03-14T17:43:54.673Z
https://earthscience.stackexchange.com/questions/16466/at-what-point-is-water-droplets-to-small-to-make-a-rainbow
Muze
CC BY-SA 4.0
2019-02-26T19:06:14.050Z
2019-02-27T23:01:30.947Z
2019-02-27T23:01:30.947Z
Camilo Rada
<p>Different size raindrops make different rainbows. Fog droplets make a colorless fog bow but frozen fog make unique colorful rainbows such as the one below.</p>

<p><a href="https://www.metoffice.gov.uk/learning/clouds/fog/freezing-fog" rel="nofollow noreferrer">https://www.metoffice.gov.uk/learning/clouds/fog/freezing-fog</a></p>

<p>The picture below is an ice fog rain bow.</p>

<p>So to conclude Ice equals color and distinct rainbow pattern when it comes to fog. </p>

<p><a href="https://i.sstatic.net/s8TLU.gif" rel="nofollow noreferrer"><img src="https://i.sstatic.net/s8TLU.gif" alt="enter image description here"></a>
Diamond dust is what frozen fog is called.
<a href="https://saebloom.wordpress.com/2016/12/13/diamondsnow/" rel="nofollow noreferrer">https://saebloom.wordpress.com/2016/12/13/diamondsnow/</a></p>

<p><a href="https://i.sstatic.net/A3XIz.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/A3XIz.jpg" alt="enter image description here"></a>
All the patterns are generated as sunlight (or moonlight) is reflected and refracted in flat six-sided water ice crystals in Earth's atmosphere.
<a href="https://apod.nasa.gov/apod/ap181221.html" rel="nofollow noreferrer">https://apod.nasa.gov/apod/ap181221.html</a></p>

<p>I found this picture of High clouds refracting light.
<a href="https://i.sstatic.net/OpO2q.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/OpO2q.jpg" alt="enter image description here"></a>
<a href="https://www.dkn.tv/tin-giai-tri/nhung-vang-hao-quang-ky-la-tren-bau-troi.html" rel="nofollow noreferrer">https://www.dkn.tv/tin-giai-tri/nhung-vang-hao-quang-ky-la-tren-bau-troi.html</a>
I could get my translator to work but just looking at the picture the sun is past the horizon of the camera but not the clouds and clouds that high is frozen creating a Fire Rainbow.</p>

<p><a href="https://i.sstatic.net/wAy2w.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/wAy2w.jpg" alt="enter image description here"></a>
<a href="https://physics.stackexchange.com/questions/289279/why-is-each-snow-flake-different">https://physics.stackexchange.com/questions/289279/why-is-each-snow-flake-different</a></p>

<p>To add: You could use a telescope and angled white laser or focused light to synthesis a bow to see if it refracts differently between ice/color or liquid/no color and the pattern it makes.</p>

<p>Added After Edit:</p>

<p><a href="http://contrailscience.com/aerodynamic-and-rainbow-contrails/" rel="nofollow noreferrer">http://contrailscience.com/aerodynamic-and-rainbow-contrails/</a></p>

<p>If the contrail has refraction or rainbow it is ice. The pilot can turn the plane to the angle needed to see the rainbow from the cockpit. </p>

<p><a href="https://i.sstatic.net/VnkEh.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/VnkEh.jpg" alt="enter image description here"></a></p>

<p><a href="https://i.sstatic.net/BtFPX.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/BtFPX.jpg" alt="enter image description here"></a>
Rain bow at night with spot light. <a href="https://atoptics.wordpress.com/2017/02/05/binary-double-rainbows/" rel="nofollow noreferrer">https://atoptics.wordpress.com/2017/02/05/binary-double-rainbows/</a></p>

<p>If this was ice or fog rather than rain the rainbow will be different in pattern and color. I am interested to try this with the whitest of light or maybe a white laser which does exist.</p>

<p>Last edit: I believe the sunlight is refracted inside the ice and is warmed because of that there must be a equilibrium between the cold air and ice to stay frozen during the day but at night the ice fog could be warmer. 
I have not considered if the friction between the air and ice as a factor. </p>

CC BY-SA 4.0
2019-02-16T08:22:11.297Z
Let me know if this is better?
Muze
CC BY-SA 4.0
2019-02-17T01:25:04.337Z
Better, but that looks too thin to be considered fog. Also, the shape and size of the cristal most likely play a role too.
Camilo Rada
CC BY-SA 4.0
2019-02-17T02:05:01.910Z
@CamiloRada unlike a water drop the refracted light will be affected similar to a glass prism and have more angles to cast refracted light. I do notice that the rainbows in the picture are nothing like any other rainbow and looks below 40 Fahrenheit. I ll find out where this is soon.
Muze
CC BY-SA 4.0
2019-02-17T03:06:49.210Z
This answer is good in describing the characteristics of liquid and ice particles in refracting/scattering solar radiation. However, seeing these often requires fairly high visibility or a particular unobstructed viewpoint relative to the sun.
It's often the case that the phase state of thicker fog is ambiguous to an observer from within the fog, particularly if the sun is obscured - knowledge of the air temperature or secondary indicators (eg, supercooled water freezing onto exposed surfaces) may be required for identification. Expanding the answer in this area would make it more applicable.
dplmmr
CC BY-SA 4.0
2019-03-14T04:03:22.203Z
@dplmmr  You can uses a flash light to make a rainbow in the same way you can use a water hose and sprayer.
Muze
CC BY-SA 4.0
2019-03-14T04:12:46.037Z
@Muze That's pretty neat, I've never heard of that, I hadn't realized the light source would be powerful enough to show anything. That would be useful in this context.
dplmmr
CC BY-SA 4.0
2019-03-14T04:36:11.313Z
@dplmmr I added a picture.
Muze
CC BY-SA 4.0
2019-03-14T17:16:09.790Z
@Muze the problem is that this answer fail to demonstrate that these optical effects happen on every ice fog. Unlike water droplets, ice crystals can take many different shapes and all the phenomena you show might rely on the existence of a very particular type of crystal. It also fail in demonstrating that these phenomena can be reproduced with artificial light sources.
At this point I think it might be easier to find a signature of liquid fog, and assume that anything without that signature is ice.
Camilo Rada
CC BY-SA 4.0
2019-03-14T17:18:28.070Z
@CamiloRada yes but will still sparkle
Muze
CC BY-SA 4.0
2019-03-14T17:19:41.840Z
@CamiloRada I added that to the answer. Just need a flash light or sun.
Muze
CC BY-SA 4.0
2019-03-14T17:27:44.177Z
But how do you know liquid fog doesn't produce rainbows? The most common rainbows are produced by rain, and those are definitely liquid water droplets.
And by the way I think white lasers doesn't exist.
This are all very interesting pictures, but without an independent way to prove that in all cases they arose from ice crystals, they are of no help to prove anything. Specially if there is no a strong argument to say that similar phenomena can't happen for liquid water fog.
Camilo Rada
CC BY-SA 4.0
2019-03-14T17:35:58.260Z
The fog droplets are too small to refract into a rainbow but ice crystals are wider than fog and thinner. When fog freezes it widens.
Muze
CC BY-SA 4.0
2018-04-11T02:44:00.903Z
2019-03-14T17:33:22.457Z
2019-03-14T17:33:22.457Z
Muze
<p>I understand that clouds can be made out of water droplets and/or ice crystals. And that fog is nothing else than low-lying clouds.</p>

<p>However, I've experienced fog in a wide ranges of temperatures. I like mountaineering and you often get fog high in the mountains. I've been lucky enough to visit the high mountains of Antarctica and I've also experienced fog there at very low temperatures.</p>

<p>This <a href="https://earthscience.stackexchange.com/a/5383/11908">answer</a> (in the comments) and the <a href="https://en.wikipedia.org/wiki/Fog" rel="noreferrer">fog wikipedia article</a> mention that ice fog happens only at extremely low temperatures (-35 or -40 C). But I think I've experienced fog at those temperatures too. Some times, with the right light I've seen the air filled with tiny crystals, but I wouldn't call that fog, because it is very thin.</p>

<p>Then, as far as I can tell, all thick fogs feel and look the same. No matter if they are made out of ice crystals or water droplets. <strong>Is that right?</strong> <strong>Or is there a way to differentiate between ice/water fogs?</strong></p>

<p>Or maybe contrary to what I think, I've never experienced frozen fog.<br />
But: would I notice the difference if I were to ever experience it? How would they feel/look different?</p>

<p>Finally, if somebody also knows why water droplets can stay liquid in clouds down to -30°C, it would be great to learn about that too.</p>

CC BY-SA 4.0
2019-02-17T06:55:57.480Z
@gansub What do you mean? Differentiating both kinds of fog might be possible by remote sensing, but I'm not so sure, and it is not the kind of answer I'm looking for. I look for an in-situ way to differentiate both types of fog (if a thick ice fog really exists).
Camilo Rada
CC BY-SA 4.0
2019-02-27T19:53:07.730Z
it is strange if nobody have tested what happens if one freezes fog.i can not say i have ever observed frozen fog even if i live in an area where fog and frost are common.
trond hansen
CC BY-SA 4.0
2019-02-28T21:03:25.793Z
@trondhansen True. I was surprised to find that distinguish fog/clouds made of water or ice is so difficult, but up to some point it does makes sense.
Camilo Rada
CC BY-SA 4.0
2019-03-14T16:50:11.440Z
https://space.stackexchange.com/questions/34801/how-is-atmospheric-temperature-measured-from-a-satellite
Muze
CC BY-SA 3.0
2018-04-06T17:25:45.340Z
They have different optical properties and different thermodynamics.
BarocliniCplusplus
CC BY-SA 3.0
2018-04-06T17:28:14.307Z
@BarocliniCplusplus Perhaps they can display different optical phenomena, but when you are immersed on them even at night with a flashlight they look the same. Am I wrong? Is there a key property that could be tested for?
Camilo Rada
CC BY-SA 3.0
2018-04-09T01:12:10.643Z
One of those questions that this Floridian can't answer well from experience! And another subdiscipline I don't know as well as some. But I'd offer a guess that it looks basically the same either way... much as I, at least, don't notice any difference in clouds below the freezing level vs above. But then I haven't flown enough to know what they look like close up either! If I remember right, it only supercools if there's no condensation nuclei for it to use. Near the ground, I'd think there'd be no shortage of CCN, so would offer a weak educated guess that most below -10°C would be ice fog?
JeopardyTempest
CC BY-SA 3.0
2018-04-09T20:24:30.623Z
Ice clouds are often recognizable from various halo phenomena. With a good flashlight, perhaps you might see something like [this](https://atoptics.wordpress.com/2016/01/18/two-days-of-diamond-dust-halos-in-bozi-dar-cz-nov-27th28th-2015/).
jvir
CC BY-SA 3.0
2018-04-09T20:36:30.400Z
@jvir That could be one approach. From pictures like that, I'm some times inclined to think that frozen fog is might not ever become as dense as normal fog.
Camilo Rada
<meteorology><clouds><ice>
How to tell if fog is made out of water droplets or ice crystals?
Camilo Rada
# How to tell if fog is made out of water droplets or ice crystals?

(Asked by: username_0 on 2018-04-06)

I understand that clouds can be made out of water droplets and/or ice crystals. And that fog is nothing else than low-lying clouds.

However, I've experienced fog in a wide ranges of temperatures. I like mountaineering and you often get fog high in the mountains. I've been lucky enough to visit the high mountains of Antarctica and I've also experienced fog there at very low temperatures.

This [answer](https://earthscience.stackexchange.com/a/5383/11908) (in the comments) and the [fog wikipedia article](https://en.wikipedia.org/wiki/Fog) mention that ice fog happens only at extremely low temperatures (-35 or -40 C). But I think I've experienced fog at those temperatures too. Some times, with the right light I've seen the air filled with tiny crystals, but I wouldn't call that fog, because it is very thin.

Then, as far as I can tell, all thick fogs feel and look the same. No matter if they are made out of ice crystals or water droplets. **Is that right?** **Or is there a way to differentiate between ice/water fogs?**

Or maybe contrary to what I think, I've never experienced frozen fog.  
But: would I notice the difference if I were to ever experience it? How would they feel/look different?

Finally, if somebody also knows why water droplets can stay liquid in clouds down to -30°C, it would be great to learn about that too.

**Tags:** meteorology, clouds, ice

> **Comments:**
> **username_3**: They have different optical properties and different thermodynamics.  
> **username_0**: @username_3 Perhaps they can display different optical phenomena, but when you are immersed on them even at night with a flashlight they look the same. Am I wrong? Is there a key property that could be tested for?  
> **username_4**: One of those questions that this Floridian can't answer well from experience! And another subdiscipline I don't know as well as some. But I'd offer a guess that it looks basically the same either way... much as I, at least, don't notice any difference in clouds below the freezing level vs above. But then I haven't flown enough to know what they look like close up either! If I remember right, it only supercools if there's no condensation nuclei for it to use. Near the ground, I'd think there'd be no shortage of CCN, so would offer a weak educated guess that most below -10°C would be ice fog?  
> **username_5**: Ice clouds are often recognizable from various halo phenomena. With a good flashlight, perhaps you might see something like [this](https://atoptics.wordpress.com/2016/01/18/two-days-of-diamond-dust-halos-in-bozi-dar-cz-nov-27th28th-2015/).  
> **username_0**: @username_5 That could be one approach. From pictures like that, I'm some times inclined to think that frozen fog is might not ever become as dense as normal fog.  
> **username_0**: @gansub What do you mean? Differentiating both kinds of fog might be possible by remote sensing, but I'm not so sure, and it is not the kind of answer I'm looking for. I look for an in-situ way to differentiate both types of fog (if a thick ice fog really exists).  
> **username_1**: it is strange if nobody have tested what happens if one freezes fog.i can not say i have ever observed frozen fog even if i live in an area where fog and frost are common.  
> **username_0**: @username_1 True. I was surprised to find that distinguish fog/clouds made of water or ice is so difficult, but up to some point it does makes sense.  
> **username_2**: https://space.stackexchange.com/questions/34801/how-is-atmospheric-temperature-measured-from-a-satellite  

---

## 2 answers

### Answer by username_0 on 2019-02-26. Score: 4

This doesn't aim to be a complete answer, but after putting a juicy bounty at Aviation SE on the question "[Can pilots tell if a cloud or fog is made out of water droplets or ice crystals when flying through it?](https://aviation.stackexchange.com/q/60213/34312)", the answer seem to be: **No, they can't**. And all the things to look at to detect ice clouds doesn't seem to be useful if you don't have a plane at hand. There is nothing like "there is a special refraction pattern around the lights on the wings", or such.

So far it seem like ice and water clouds are indistinguishable.

> **Comments:**
> **username_2**: You could do it from the ground using a white laser, telescope and angle. Maybe a special polarized lens to bring out the rainbow more. Lately if you shine a flash light through frozen fog it should glitter but you may need to do it in the shade or dark enclosure to see it. It should look like diamond dust.  
> **username_2**: https://earthscience.stackexchange.com/questions/16466/at-what-point-is-water-droplets-to-small-to-make-a-rainbow  

---

### Answer by username_2 on 2018-04-11. Score: 2

Different size raindrops make different rainbows. Fog droplets make a colorless fog bow but frozen fog make unique colorful rainbows such as the one below.

<https://www.metoffice.gov.uk/learning/clouds/fog/freezing-fog>

The picture below is an ice fog rain bow.

So to conclude Ice equals color and distinct rainbow pattern when it comes to fog.

[![enter image description here](https://i.sstatic.net/s8TLU.gif)](https://i.sstatic.net/s8TLU.gif)
Diamond dust is what frozen fog is called.
<https://saebloom.wordpress.com/2016/12/13/diamondsnow/>

[![enter image description here](https://i.sstatic.net/A3XIz.jpg)](https://i.sstatic.net/A3XIz.jpg)
All the patterns are generated as sunlight (or moonlight) is reflected and refracted in flat six-sided water ice crystals in Earth's atmosphere.
<https://apod.nasa.gov/apod/ap181221.html>

I found this picture of High clouds refracting light.
[![enter image description here](https://i.sstatic.net/OpO2q.jpg)](https://i.sstatic.net/OpO2q.jpg)
<https://www.dkn.tv/tin-giai-tri/nhung-vang-hao-quang-ky-la-tren-bau-troi.html>
I could get my translator to work but just looking at the picture the sun is past the horizon of the camera but not the clouds and clouds that high is frozen creating a Fire Rainbow.

[![enter image description here](https://i.sstatic.net/wAy2w.jpg)](https://i.sstatic.net/wAy2w.jpg)
<https://physics.stackexchange.com/questions/289279/why-is-each-snow-flake-different>

To add: You could use a telescope and angled white laser or focused light to synthesis a bow to see if it refracts differently between ice/color or liquid/no color and the pattern it makes.

Added After Edit:

<http://contrailscience.com/aerodynamic-and-rainbow-contrails/>

If the contrail has refraction or rainbow it is ice. The pilot can turn the plane to the angle needed to see the rainbow from the cockpit.

[![enter image description here](https://i.sstatic.net/VnkEh.jpg)](https://i.sstatic.net/VnkEh.jpg)

[![enter image description here](https://i.sstatic.net/BtFPX.jpg)](https://i.sstatic.net/BtFPX.jpg)
Rain bow at night with spot light. <https://atoptics.wordpress.com/2017/02/05/binary-double-rainbows/>

If this was ice or fog rather than rain the rainbow will be different in pattern and color. I am interested to try this with the whitest of light or maybe a white laser which does exist.

Last edit: I believe the sunlight is refracted inside the ice and is warmed because of that there must be a equilibrium between the cold air and ice to stay frozen during the day but at night the ice fog could be warmer.
I have not considered if the friction between the air and ice as a factor.

> **Comments:**
> **username_2**: Let me know if this is better?  
> **username_0**: Better, but that looks too thin to be considered fog. Also, the shape and size of the cristal most likely play a role too.  
> **username_2**: @username_0 unlike a water drop the refracted light will be affected similar to a glass prism and have more angles to cast refracted light. I do notice that the rainbows in the picture are nothing like any other rainbow and looks below 40 Fahrenheit. I ll find out where this is soon.  
> **username_6**: This answer is good in describing the characteristics of liquid and ice particles in refracting/scattering solar radiation. However, seeing these often requires fairly high visibility or a particular unobstructed viewpoint relative to the sun. It's often the case that the phase state of thicker fog is ambiguous to an observer from within the fog, particularly if the sun is obscured - knowledge of the air temperature or secondary indicators (eg, supercooled water freezing onto exposed surfaces) may be required for identification. Expanding the answer in this area would make it more applicable.  
> **username_2**: @username_6  You can uses a flash light to make a rainbow in the same way you can use a water hose and sprayer.  
> **username_6**: @username_2 That's pretty neat, I've never heard of that, I hadn't realized the light source would be powerful enough to show anything. That would be useful in this context.  
> **username_2**: @username_6 I added a picture.  
> **username_0**: @username_2 the problem is that this answer fail to demonstrate that these optical effects happen on every ice fog. Unlike water droplets, ice crystals can take many different shapes and all the phenomena you show might rely on the existence of a very particular type of crystal. It also fail in demonstrating that these phenomena can be reproduced with artificial light sources. At this point I think it might be easier to find a signature of liquid fog, and assume that anything without that signature is ice.  
> **username_2**: @username_0 yes but will still sparkle  
> **username_2**: @username_0 I added that to the answer. Just need a flash light or sun.  
> **username_0**: But how do you know liquid fog doesn't produce rainbows? The most common rainbows are produced by rain, and those are definitely liquid water droplets. And by the way I think white lasers doesn't exist. This are all very interesting pictures, but without an independent way to prove that in all cases they arose from ice crystals, they are of no help to prove anything. Specially if there is no a strong argument to say that similar phenomena can't happen for liquid water fog.  
> **username_2**: The fog droplets are too small to refract into a rainbow but ice crystals are wider than fog and thinner. When fog freezes it widens.  

---

