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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 (in the comments) and the fog wikipedia article 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.

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  • $\begingroup$ They have different optical properties and different thermodynamics. $\endgroup$ – BarocliniCplusplus Apr 6 '18 at 17:25
  • $\begingroup$ @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? $\endgroup$ – Camilo Rada Apr 6 '18 at 17:28
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    $\begingroup$ 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? $\endgroup$ – JeopardyTempest Apr 9 '18 at 1:12
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    $\begingroup$ Ice clouds are often recognizable from various halo phenomena. With a good flashlight, perhaps you might see something like this. $\endgroup$ – jvir Apr 9 '18 at 20:24
  • $\begingroup$ @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. $\endgroup$ – Camilo Rada Apr 9 '18 at 20:36
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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?", 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.

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  • $\begingroup$ 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. $\endgroup$ – Muze the good Troll. Mar 14 at 17:02
  • $\begingroup$ earthscience.stackexchange.com/questions/16466/… $\endgroup$ – Muze the good Troll. Mar 14 at 17:43
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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 Diamond dust is what frozen fog is called. https://saebloom.wordpress.com/2016/12/13/diamondsnow/

enter image description here 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://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://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

enter image description here 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.

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    $\begingroup$ Better, but that looks too thin to be considered fog. Also, the shape and size of the cristal most likely play a role too. $\endgroup$ – Camilo Rada Feb 17 at 1:25
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    $\begingroup$ @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. $\endgroup$ – Muze the good Troll. Feb 17 at 2:05
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    $\begingroup$ 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. $\endgroup$ – dplmmr Feb 17 at 3:06
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    $\begingroup$ @dplmmr You can uses a flash light to make a rainbow in the same way you can use a water hose and sprayer. $\endgroup$ – Muze the good Troll. Mar 14 at 4:03
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    $\begingroup$ @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. $\endgroup$ – dplmmr Mar 14 at 4:12

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