Icing yesterday over WV - Can cloud tops be colder than surrounding clear air?

[Bill Watson]

Flying at night, I picked up some rime ice as I punched through some cloud tops despite that fact that the surrounding clear air was above freezing. Can clouds be 1 or 2 degrees colder than the surrounding air? ( I generally thought not). Has this ever happened to you?

I was making an early morning flight from Pittsburgh to Durham NC, running from the approaching cold front and several days of tough, cold weather. The flight was planned to start at first light with a low cruise in above freezing conditions, followed by a climb into increasingly warm air as I moved away from the front. Timing of the flight versus the frontal passage was critical.

Conditions were scattered to broken at 4:00 am so I decided to takeoff around 5:00am, climb up to 8 or 9k where I expected clear conditions over the gnarly Appalachians , and fly home in the dark.

I took off into blustering pre-frontal warm air, climbed through scattered clouds and seemed to be above all the moisture at 6k on my way to 9k. Outside air temperature at 9k was 34F. Then I hit several turbulent clouds tops. I was in and out in less than a minute each time but after the 4th or 5th something made me shine my flashlight out on the wings and I had a good accumulation of rime ice! It was limited to outboard of the full fuel tanks.

I was confused because the clear air was still measuring 34F. I never did check it while in a cloud top because of the short duration and light turbulence... and of course I was flying blind so I couldn't see them coming, though I could see stars and some ground lights.

I took a climb to 10k where the temps were still above freezing but I had completely escaped the cloud tops at that point.

Other notes:
My temperature gauge as proved to be very accurate in other recent ice encounters.
There were light mountain wave conditions - the up and down air made detection of any performance loss tricky.

 

[Bill]

Flying at night, I picked up some rime ice as I punched through some cloud tops despite that fact that the surrounding clear air was above freezing. Can clouds be 1 or 2 degrees colder than the surrounding air? ( I generally thought not). Has this ever happened to you?

A few years back, flying near Rome, GA. There was an airmet for ice not far to the north, and I was flying in clear air on an IFR plan. The OAT was about 7C when I was in the clear air, then when into a cloud. The temps quickly dropped, and then it started raining, so I quickly looked at the OAT gauge. Crap, -1C. I looked at the wings and I was getting SLD icing on the wings, banged an immediate 180, then told the controller what I was doing. She wasn't happy with my 180, and I told her I'd be happy to declare if that made things easier for her. She said no, just report clear of clouds and then resume course. I diverted to RMG to wait out the airmet, and by the time I got to the airport environment, the ice had melted off.

 

[DGlaeser]

Due to the energy involved in changes of state, clouds can definitely be colder than the surrounding air. The tops of clouds are a well known place to pick up ice. Been there, done that a number of times.
Sounds like your warm(er) fuel prevented icing in that area of the wing. Nice, but tough to count on
It can also happen the other way - supercooled water droplets can exist as liquid in temps below freezing, and then freeze as they hit a passing airframe.
Icing is tricky business. When in doubt, assume you will get it, and have a 'golden out' available!!

 

[Bill Watson]

Due to the energy involved in changes of state, clouds can definitely be colder than the surrounding air. The tops of clouds are a well known place to pick up ice. Been there, done that a number of times.

It can also happen the other way - supercooled water droplets can exist as liquid in temps below freezing, and then freeze as they hit a passing airframe.
Icing is tricky business. When in doubt, assume you will get it, and have a 'golden out' available!!

Yes, I've been going through the 'change of state' physics in my head and in the literature. I can make a case for colder air in cloud adjacent to warmer clear air but everything I read says that temperature equilibrium is reached almost instantaneously. Still looking....

My understanding is that icy clouds typically contain below freezing vapor (as well as ice) and that is what ices us up. I think the range for icy clouds is from 0C to about -40c. Below that it is all ice and doesn't ice planes up. Above -40c it's a mix and we are subject to getting iced. Above 0C, no icing.

 

[jeremyk13]

With respect to vapor-cooling only: As you move through the cloud, you're continuously forcing a phase change in a "new" section of the cloud by adding energy to that section. So even if the temperature equalizes quickly enough for the section you just went through, you've already added more vapor-cooling.

This is ignoring any other cause of temperature shifts (rapid compression and expansion of gas, i.e. J-T Effect, transitioning between mediums effecting the rate of convective cooling, etc).

 

[Everskyward]

Can clouds be 1 or 2 degrees colder than the surrounding air? ( I generally thought not). Has this ever happened to you?

Yes, definitely. In fact, when descending into a cloud layer I pretty much count on in. Others can explain the physics behind it, but I have witnessed it many times.

 

[coloradobluesky]

Supercooled droplets of water is water that is liquid below freezing (as low as -20C). When they impact an air frame they turn from liquid water into ice. Water CAN exist as a liquid when it is below freezing. This supercooled droplets of water can float around in air that is above freezing (32F - 40F is ideal).

The experiment they reference to understand it is to put a vial of water in a freezer (that is very still, cant have the compressor in it vibrating) and cool the water to below freezing say -10C. It CAN stay liquid. Hit the side of the vial with a butter knife and watch in amazement as it all suddenly turns to ice.