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Freezing fog formed at Truckee once after I landed to go skiing. It was a thin layer with sunlight above and it was magical walking across the ramp with the little ice crystals sparking all about.

If the air is supersaturated, the RH can be >100%. This calculator gets about the same answer, but I don’t know if it’s correct in this case. https://bmcnoldy.rsmas.miami.edu/Humidity.html

Good question for @Scott Dennstaedt.

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All but the most expensive relative humidity sensors are inaccurate whenever the relative humidity is above 90 to 95%.  Get close to freezing and all bets are off.   Freezing fog will freeze on the sensor and its shield.   

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Happens in the flight levels on rare occasion also. Clouds made up of ice crystals. Have to be careful with induction icing if you are in it too long. Time to switch the alternate air door if the MP drops. That’s why the door was made automatic, because pilots would soldier on not realizing what was happening.

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5 minutes ago, 1964-M20E said:

I learned of freezing fog when I was in Idaho Falls.  Very spectacular to see in the sunlight.  Looks like sparkling diamonds falling gently to the ground.

Freezing fog also looks nice from above, when you're on approach and stop descending because of it . . . . All misty and white like fog, plus it's very sparkly. Just don't fly into it! In my case, the freezing fog filled only the valley with the airport in it, was ~300' thick and went firmly to the ground. We went elsewhere for breakfast that otherwise-beautiful morning.

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1 hour ago, jlunseth said:

Happens in the flight levels on rare occasion also. Clouds made up of ice crystals. Have to be careful with induction icing if you are in it too long. Time to switch the alternate air door if the MP drops. That’s why the door was made automatic, because pilots would soldier on not realizing what was happening.

Had this happen to me at 22,000 over Pueblo/Colorado Springs late one night. Yes induction icing can happen to a 231. Lost power and was unable to maintain altitude. I was heading to Centennial (APA) and was essentially on the ILS to 35R. Center tried to direct me towards DEN and I turned them down. Ask for and got my decent into APA 50 miles out. Ice melted and I had a normal approach once low enough.

Later read a story in Flying about a 231 that had induction icing crossing the North Atlantic. He had to stay under 500’ over the ocean to keep the ice melted. 
Induction ice in very rare conditions is a quirk built in to the 231. If conditions are right it will ice. All you can do is avoid those conditions by changing altitude. Alternate air does not help.

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Alternate air has always worked for me. Routes the intake air to come from inside the cowling. I have had to do it about a half dozen times, usually ice crystal clouds in the flight levels but once or twice down low in snow. I always manually pull the knob even if the auto mechanism has activated, just tosure.

Same conditions here in MN today:

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12 hours ago, 0TreeLemur said:

All but the most expensive relative humidity sensors are inaccurate whenever the relative humidity is above 90 to 95%.  Get close to freezing and all bets are off.   Freezing fog will freeze on the sensor and its shield.   

Sling psychrometers are not too expensive and are pretty accurate, even at high humidity, but don't lend themselves well to automation.  ;)

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2 hours ago, EricJ said:

Sling psychrometers are not too expensive and are pretty accurate, even at high humidity, but don't lend themselves well to automation.  ;)

exactly.  The FAA would need to invest in infrared gas analyzers with heated optics for that.  :mellow:

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If the scientific impossibility is having moisture actually exist below the freezing point of water...

This is the cool thing about MS...

You will find that steady state is often assumed... but, in reality... there are affects of time and space....that din’t get accounted for in the real world...

In this case, they are measuring the OAT... not the temp of the water... or it’s purity...

In the case of really pure water... it’s freezing point initiation... is often an airplane flying through it... This is a phenomena of the water droplets being super cooled...

Super cooled water droplets are ready to freeze, instantaneously, when given the opportunity... your plane is that opportunity, that the fog has been waiting for... (a phenomena related to crystallization)

The amazing part... we actually get useful weather info delivered to the cockpit in time to use it... :)

Pp thoughts only, not a weather guru...

Best regards,

-a-

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I saw one morning in England the cool effects of ice fog on the ground. It had froze on all the spiderwebs in the fields and made the webs very easy to see. The scary part was the number of webs. Had to be in the millions.  Made me not want to ever go camping without a tent over there. 

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4 hours ago, Will.iam said:

I saw one morning in England the cool effects of ice fog on the ground. It had froze on all the spiderwebs in the fields and made the webs very easy to see. The scary part was the number of webs. Had to be in the millions.  Made me not want to ever go camping without a tent over there. 

You do realize that New England isn't a frog mecca, and that Texas has just as many?

I hope that I am through with such things as snowfall and freezing fog, but I still expect the occasional ice storm every few years--you know, rainfall when the droplet temp is below freezing, hits the ground / houses / trees / etc. and immediately freezes and builds? On a ten-minute walk home once, my umbrella formed a solid layer of ice, made it hard to close--right here in Lower Alabama.

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10 hours ago, carusoam said:

If the scientific impossibility is having moisture actually exist below the freezing point of water...

This is the cool thing about MS...

You will find that steady state is often assumed... but, in reality... there are affects of time and space....that din’t get accounted for in the real world...

In this case, they are measuring the OAT... not the temp of the water... or it’s purity...

In the case of really pure water... it’s freezing point initiation... is often an airplane flying through it... This is a phenomena of the water droplets being super cooled...

Super cooled water droplets are ready to freeze, instantaneously, when given the opportunity... your plane is that opportunity, that the fog has been waiting for... (a phenomena related to crystallization)

The amazing part... we actually get useful weather info delivered to the cockpit in time to use it... :)

Pp thoughts only, not a weather guru...

Best regards,

-a-

Generally, water needs a nucleus to freeze around, a speck of dust, or the wing of your airplane, either will do. Ice will freeze without a nucleus, but at a lower temperature, that's why there are ice clouds in the flight levels. I am not an expert on this, ask Scott D., but I think it is around -35 dF. That is the reason your plane gets slammed with ice, flying through a cloud of SLD (Supercooled Large Droplets).

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On 1/6/2021 at 10:53 AM, jlunseth said:

Generally, water needs a nucleus to freeze around, a speck of dust, or the wing of your airplane, either will do. Ice will freeze without a nucleus, but at a lower temperature, that's why there are ice clouds in the flight levels. I am not an expert on this, ask Scott D., but I think it is around -35 dF. That is the reason your plane gets slammed with ice, flying through a cloud of SLD (Supercooled Large Droplets).

Yes, a cloud drop needs an ice nuclei to freeze.  That's called heterogeneous nucleation. The nuclei needs to look remarkably similar to an ice crystal, that's why they are relatively rare compared to condensation nuclei.  So ice crystals make the best ice nuclei, of course. You will get homogeneous freezing once the temps drop to below -40°C. But your airplane makes a great ice nuclei. :)

Most non-convective clouds are glaciated at a temperature below -25°C.

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