Frost formation

[pelmet]

It was -5 yesterday as I sat in my window seat of a narrowbody jet overlooking the wing waiting for the pushback. It was late at night for the 1 hour flight and a few minutes prior to pushback, I wondered if we would have to de-ice. I looked at the wing and did not see any frost. Good. Then I said to myself, it must be cloudy outside because if it was clear skies, the wing would be covered in frost. It sounds strange and maybe I don't know what I am talking about but it has been my observation over the years that for some reason, frost doesn't seem to form when it is cloudy but does on a clear night. Has anyone else ever noticed this?

Out came the cellphone and I checked the NavCanada website and guess what? The Metar was saying SKC which meant that we should be getting frost if what I have been saying is true. I looked up at the dark sky but saw no stars. But maybe that was because of the bright lights of the airport ramp. A metar from an airport about 30 miles away said 260SCT. I figured that there must be a layer of cloud up high. So I watched as we climbed out(no deicing was done as none was needed) but was still able to see the ground when we levelled off at FL250, according to the seat screen. But I only ever saw one or two stars, perhaps because of the interior lighting of the aircraft. Maybe there was a cirrus layer but there was no moon to help me out. We descended and landed in the early morning with the metar at the departure airport still showing SKC.

I didn't get around to following up until about 10am when I looked at a webcam for all areas down near the departure airport and there was quite a bit of cirrus cloud, so perhaps it was there 8 hours earlier as well.

So, am I right that cloud seems to prevent frost formation and if so, why. And even just a layer of cirrus cloud? What has been your observation over the years?

[goingnowherefast]

The ground, airplanes, cars, etc. all radiate their heat out into space at night. Overcast clouds reflect some of that radiant heat back down to the surface. Clear nights are always cooler and cool down much faster than overcast nights. That moisture has to go somewhere, so it sticks to stuff as frost or dew depending on temperature.

[Mr. re]

That’s a good question. I’ve found myself trying to predict if it was going to be a frosty morning or not the next day. I have heard the same thing about Sky clear over night and you can expect frost as oppose to no frost when it’s cloudy. Personally, I think there’s a bit more to it than just that. Ie. Temperature and especially the moisture content of the air mass itself.
The frost formed on a clear night is formed when the heat gives up its stored heat energy during the day from the sun and condenses and freezes on the cold skin of the airframe. I’m sure there’s some knowledgeable aviators that can explain all this in simpler way. It’s nice when there’s a light dusting of snow over night, it’s like putting wing covers on, no frost underneath. Quick brush and go flying. At lease that’s my experience in the North.

[BTD]

As far as airline ops are concerned, you can and should utilize an active frost chart. If the dew point and OAT are within the reference spread based on what the actual dew point is, then active frost and you should plan on a spray. If not then check the wing to make sure and you will be okay.

[BTD]

https://www.tc.gc.ca/eng/civilaviation/ ... m#toc-1267

12.7.1.2

Edited to add for clarity. This is reference and provides guidance. Aircraft skin temp can be different from OAT depending on what it has done recently fuel temps etc.

[ant_321]

https://www.tc.gc.ca/eng/civilaviation/ ... m#toc-1267

12.7.1.2

Edited to add for clarity. This is reference and provides guidance. Aircraft skin temp can be different from OAT depending on what it has done recently fuel temps etc.

Is that something people actually look at? I’ve been in the airline world for 8 years and have never seen anyone use it. I’m not asking to be an ass, just curious.

[BTD]

I discuss it during line indoc. Yes I do look at it. However, those references are included in company provided documentation in an app we have.

But often when we use our ACARS system for hold over times and it will often say “no precipitation detected” “check for active frost conditions” and during line training guys will look out the window and say “looks good” no active frost. That’s usually where it comes up.

[Eric Janson]

It is even possible to get frost on the wings after taking-off.

I've only seen it happen a few times as you need very specific conditions.

What you need is a temperature inversion where you are taking a cold soaked wing into a warmer, moister airmass.

I have watched the wing surface change from reflective bare metal to opaque as a very thin layer of frost forms and then back to bare metal as the frost burns off. It may be more difficult to spot on a painted surface.

[BTD]

It is even possible to get frost on the wings after taking-off.

I've only seen it happen a few times as you need very specific conditions.

What you need is a temperature inversion where you are taking a cold soaked wing into a warmer, moister airmass.

I have watched the wing surface change from reflective bare metal to opaque as a very thin layer of frost forms and then back to bare metal as the frost burns off. It may be more difficult to spot on a painted surface.

The situation I’m talking about is on the ground.

[pelmet]

The ground, airplanes, cars, etc. all radiate their heat out into space at night. Overcast clouds reflect some of that radiant heat back down to the surface. Clear nights are always cooler and cool down much faster than overcast nights. That moisture has to go somewhere, so it sticks to stuff as frost or dew depending on temperature.

Doesn't make sense to me. If it is minus 5 it is minus 5 whether it is cloudy or clear. Why is there no frost on the cloudy -5 night. I find it difficult to believe that heat is being reflected down to prevent frost on the cloudy -5 night unless the temperature increased to above freezing.

[digits_]

The ground, airplanes, cars, etc. all radiate their heat out into space at night. Overcast clouds reflect some of that radiant heat back down to the surface. Clear nights are always cooler and cool down much faster than overcast nights. That moisture has to go somewhere, so it sticks to stuff as frost or dew depending on temperature.

Doesn't make sense to me. If it is minus 5 it is minus 5 whether it is cloudy or clear. Why is there no frost on the cloudy -5 night. I find it difficult to believe that heat is being reflected down to prevent frost on the cloudy -5 night unless the temperature increased to above freezing.

A quick change in temperature will create condensation. If that freezes, you will get frost. On a clear night, the temperature will drop quicker, giving a bigger difference in temperature in a shorter time. The moisture in the air doesn't change temperature as quickly, so it condensates.

On a cloudy night, the temperature and moisture in the air will cool down at a more gentle rate. You won't have a big temperature differential, so the moisture in the air won't condensate.

As far as I know, the weather mechanisms that create fog, create frost as well, just depends if it happens in the air or on a surface.

[Ki-ll]

The ground, airplanes, cars, etc. all radiate their heat out into space at night. Overcast clouds reflect some of that radiant heat back down to the surface. Clear nights are always cooler and cool down much faster than overcast nights. That moisture has to go somewhere, so it sticks to stuff as frost or dew depending on temperature.

Doesn't make sense to me. If it is minus 5 it is minus 5 whether it is cloudy or clear. Why is there no frost on the cloudy -5 night. I find it difficult to believe that heat is being reflected down to prevent frost on the cloudy -5 night unless the temperature increased to above freezing.

If you really want the answer, you’ll have to look in meteorological literature. A quick glance on Google mentions that a clear 5 degree night will not be a cloudy 5 degree night when all other conditions are equal, it’ll be warmer, even with AS or CI clouds, even without overcast. Ever noticed that frost doesn’t form on the grass under a tree?
https://www.tandfonline.com/doi/pdf/10. ... .v3i3.8634
https://www.tandfonline.com/doi/full/10 ... 18.1450589

[fish4life]

You also don’t get frost when it’s really cold like -35 and below because the air has no moisture then

[rxl]

You also don’t get frost when it’s really cold like -35 and below because the air has no moisture then

You would think, but that’s not necessarily true. Have you ever seen a METAR published on a really cold day with a temperature but no dew point?
I’ve had heavy frost accumulation on the aircraft a number of times with temps in the -35 to -40C range. Really have to pay attention to the fluid concentration and LOUT on those days. It seems that if there is a wind blowing at say 10 or more knots, then frost formation is rarely an issue.