I understand that clouds can be made out of water droplets 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/clouds 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?

Other sources, like the UK meteorological office say that water always remain in liquid state in fog, no matter how low the temperature is. Is that true? Is it true for clouds too? In that case there would be no such thing as "ice clouds".

One possibility is that ice or water clouds would give rise to different optical phenomena, like sun dogs that are produced by ice crystals, but usually in conditions were they are not dense enough to be considered fog, like in this image:

enter image description here

(Image from Wikipedia page for sun dogs)

So I wonder what's the experience of pilots, as they go through clouds?. When they do so, they know also the external air temperature, which is a plus. Do they see sun dogs or other optical phenomena that differentiate water from ice clouds?

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    $\begingroup$ There is also another phenomenon to distinguish: Water droplets can have a negative temperature and turn from water to ice only when they collide with the aircraft (supercooled water) $\endgroup$
    – mins
    Commented Feb 17, 2019 at 14:17
  • $\begingroup$ There may be reflective/refractive qualities of high thin clouds (halo around moon) which may be of some use. Modern jets also fly above most weather (notable exception thunderstorm cells, which can reach 55k feet). However, once in cloud visibility is very limited, and can easily drop to near zero. Visual, and radar, only shows what is reflecting back to you. What ever is behind that can be hidden. Clouds themselves change constantly not only from evaporation to condensation, but also again changing state from liquid to solid. These dynamic "weather makers" are difficult to predict. $\endgroup$ Commented Feb 24, 2019 at 14:19

2 Answers 2


Boeing provides some decent notes on this in their powerpoint which you can find in full here. Their sections on pilot reports and the summary section have some good notes for identifying ice crystals in flight/clouds/fog

  • Pilots have reported rain, sometimes ‘heavy rain’ on the windscreen, Sometimes at impossibly cold temperatures. This is believed to be the result of these small ice crystals melting on impact with the heated windscreen

  • No observations of significant airframe icing

  • Turning on landing lights at night and seeing reflective precipitation .It is likely they were seeing the larger particles even though they make up a smaller fraction of the population

  • Hearing a different sound from rain. Ice crystals hitting the airframe sounded different than rain or hail

  • Light to moderate turbulence

  • No significant airframe icing

  • TAT probe frozen
  • Ice detector not detecting ice (when installed)
  • Appearance of rain on the windshield

The same powerpoint also discusses that satellite imagery can be used to see ice crystals. A lot of planes even small stuff have the ability to receive feeds of this data which can be used to identify sections of ice crystals in flight.

According to various sources the threat is particularly dangerous due to the fact that there is no hard and vast way to visually identify the crystals and in many cases the effects are similar to other events and may lead the pilot to belive something else is happening.

For pilots we can only go by look, unless you are flying slow in a 172 you cant exactly pop the window open at altitude and feel the cloud. Simply put pilots can not reasonably tell if a cloud is vapor or crystal prior to flying into it and observing the above noted actions.

  • $\begingroup$ So the answer would be that both types of clouds can not be tell apart by visual observation? (and not even with aviation radars) I read the whole power point and that's what I understand. $\endgroup$ Commented Feb 23, 2019 at 16:23
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    $\begingroup$ @CamiloRada that would be my conclusion as well. It seems most of the incidents eluded investigators and the threat is fairly serious since there is no immediate way to visually identify the issue. $\endgroup$
    – Dave
    Commented Feb 23, 2019 at 16:33
  • $\begingroup$ @CamiloRada I have added my comment to the body, if you belive this answer is correct you should accept it. $\endgroup$
    – Dave
    Commented Feb 25, 2019 at 20:00
  • $\begingroup$ I gave the bounty to you but I won't accept it yet, because it still doesn't really answer the question, or give a solid answer like "no, they can't". Also, I would guess that not all ice clouds are of the kind that produced engine problems. $\endgroup$ Commented Feb 26, 2019 at 15:14
  • $\begingroup$ @CamiloRada Ill update the answer to reflect but i belive the simple answer is that they "no they can't". Generally answers like that are avoided here if the situation is a bit more case by case which this seems to be. $\endgroup$
    – Dave
    Commented Feb 26, 2019 at 15:53

The telltale sign is observation of ice buildup as supercooled water droplets strike and freeze onto leading edge surfaces. The probability of this happening increases as one approaches + 32 F (or 0 C), and decreases in colder air ( - 15 C). Freezing rain (rain falling from a warmer layer of air into a freezing layer of air) will also cause ice build up. Yes, it can rain 10 degrees below freezing.

In doing a pre-flight weather check, one can only determine probabilities, as weather (fronts, freeze lines, precipitation) cannot be pin pointed to the nearest mile and are always changing. However, certain weather patterns such as warm fronts overriding cold air, relative humidity, and near freezing temperatures do provide vital clues. One would build in a safety margin and avoid if at all possible.

Unfortunately, it is very difficult to tell exactly what is in a cloud until the ice actually starts to form. Vertical air currents could melt, supercool, and refreeze the same drop of water and a plane flying at the same altitude could experience a variety of "weather" as it passes through. Under these conditions constant observation is a must, as well as a "Plan B" to get out of there if icing compromises safety.

  • $\begingroup$ This seem to be to focus on freezing rain conditions, that as well as rain makes very obvious the state of the water in the cloud (water and supercooled water). However the question is about non-precipitating clouds. To distinguish if they are form by suspended ice crystals or water dropplets. $\endgroup$ Commented Feb 24, 2019 at 1:09
  • $\begingroup$ There is a fine line between "precipitating" and "nonprecipitating" clouds, governed by updraft forces. This is why rain can be a mist, or large hailstones can form. Certain conditions would increase the probability of one or the other, however, both could exist in the same cloud. From the pilots point of view, this makes it very difficult to determine. $\endgroup$ Commented Feb 24, 2019 at 1:57
  • $\begingroup$ One benefit of cruising at 35,000 feet is the air temperature is around - 55 Celsius, so yes, it's sun dogs or halos (ice). Lower altitudes (ascent or descent) can be more problematic. But altitude change does present an alternative to diversion to avoid icing conditions. $\endgroup$ Commented Feb 24, 2019 at 15:33

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