SE airplanes in clouds - near freezing level

[Kusi (us-ppl, sep, d.-ir)]

Those of you, who frequently fly SE airplanes without deicing
equipment, without turbo or second engine but use their IR Ticket to
poke around in water loaded clouds at or near freezing level:

What are your strategies to cope with the weather ?

Situation1: You climb through low clouds, but pass the freezing level
on top in clear sky.
After flying at -20°C for a while, you start your descend into the
water loaded clouds.
How quickly does the plane ice up? The airplane skin is relatively
small, so the exposed mass should be quickly warmed.
What are your personal experiences with the type of cloud? A heavy
loaded nimbostratus will have a different effect as a cirrus, for sure.
How do you preplan such a flight?

Situation2: While climbing through subcooled low clouds, what is the
most practical strategy? Climb further to reach real cold layers, which
are almost dry?
Quickly descend to reach warmer layers?
It depends?

[Thomas Borchert]

Sep,,

> Situation1: You climb through low clouds, but pass the freezing level
> on top in clear sky.
> After flying at -20°C for a while, you start your descend into the
> water loaded clouds.

If you fly a plane that will bring you to an altitude with -20 C after
flying through low clouds, you'll likely have de-icing equipment.

> Situation2: While climbing through subcooled low clouds, what is the
> most practical strategy? Climb further to reach real cold layers, which
> are almost dry?
> Quickly descend to reach warmer layers?
> It depends?
>

Well, your first strategy really depends on whether you can climb high
enough while you airplane will still climb. Often, that will not work.
Turbocharging helps, of course. So yes, the freezing level absolutely
needs to be above the MEA/MORA or whatever it is called where you fly. Or
it needs to be in the clear below the cloud base, with enough room to the
ground to fly safely.


--
Thomas Borchert (EDDH)

[Jim Carter[_1_]]

> -----Original Message-----

> From: Kusi (us-ppl, sep, d.-ir) ]

> Posted At: Friday, December 22, 2006 7:52 AM

> Posted To: rec.aviation.ifr

> Conversation: SE airplanes in clouds - near freezing level

> Subject: SE airplanes in clouds - near freezing level

>

....

> Situation1: You climb through low clouds, but pass the freezing level

> on top in clear sky.

> After flying at -20°C for a while, you start your descend into the

> water loaded clouds.

> How quickly does the plane ice up? The airplane skin is relatively

> small, so the exposed mass should be quickly warmed.

> What are your personal experiences with the type of cloud? A heavy

> loaded nimbostratus will have a different effect as a cirrus, for
sure.

> How do you preplan such a flight?

>



Having enjoyed this ride in a Commander 112 back in November '75 I can
relate that after flying in -15C at 12K on top of a solid deck from
Seattle to Bozeman we entered the side of an upslope cumulous there at
the ridge between Bozeman and Livingstone. The aircraft RAPIDLY built
enough ice to completely clog the engine air intake screen, take down
the long-wire ADF antenna and generally turn our world very bad, very
quickly. We switched to alternate intake but slung the ice off the prop
unevenly and induced some serious vibration. The instrument student in
the left seat executed a nice standard rate 180 but we lost enough
altitude that we never flew back out of the side of the buildup. We came
out of the bottom of the overcast in a little snow storm and were VERY
lucky to break out in a valley with a paved road in the bottom. Not much
damage to the aircraft except outboard of the fuel tanks where we hit
the bridge railing - the pastures were wet and snow covered.



We still had over 1 1/2" on the airframe after we came to a stop. Based
on the size of the airframe, I figure we picked up close to 1600# of ice
in less than 2 minutes.



After the aircraft was repaired, when we flew it back to Seattle we had
to let down again with a cold airframe through about 8000' of visible
moisture, but we had about a 1000' ceiling underneath. It seemed the
best way was to execute a modified ILS into BFI, so we hit the IAF
outbound at around 10K and flew the teardrop. Rolled out of the
procedure turn and hit the FAF almost perfect. Didn't pick up much ice
on the way down so what we did accumulate was gone before we touched
down.



How do you preplan such a flight?



If you don't have known ice equipment or don't know how to use it, and
you can't get over or around the highest tops enroute then I'd plan on
taking the bus or driving. But that's just my personal minimums now. If
all you have to do is penetrate a small layer during an approach then
depending on the probability of having to do a missed, and the aircraft
capability to execute the missed with a load of ice, and how big that
load of ice might be, and lots of other considerations, you might still
try it. The problem with this approach is that once you start you're
committed -- no pulling over to rethink the options. Personally, 30
years later, I have to admit that I like my ice in a glass and not stuck
to aluminum or plexi.

[Bob Gardner]

Jim Carter's experience is telling. When a cold-soaked airframe descends
into the clouds it is going to collect a lot of ice in a hurry. Where does
the "quickly warmed" idea come from?

Bob Gardner

"Kusi (us-ppl, sep, d.-ir)" > wrote in message
ups.com...
Those of you, who frequently fly SE airplanes without deicing
equipment, without turbo or second engine but use their IR Ticket to
poke around in water loaded clouds at or near freezing level:

What are your strategies to cope with the weather ?

Situation1: You climb through low clouds, but pass the freezing level
on top in clear sky.
After flying at -20°C for a while, you start your descend into the
water loaded clouds.
How quickly does the plane ice up? The airplane skin is relatively
small, so the exposed mass should be quickly warmed.
What are your personal experiences with the type of cloud? A heavy
loaded nimbostratus will have a different effect as a cirrus, for sure.
How do you preplan such a flight?

Situation2: While climbing through subcooled low clouds, what is the
most practical strategy? Climb further to reach real cold layers, which
are almost dry?
Quickly descend to reach warmer layers?
It depends?

[Robert M. Gary]

Kusi (us-ppl, sep, d.-ir) wrote:
> Those of you, who frequently fly SE airplanes without deicing
> equipment, without turbo or second engine but use their IR Ticket to
> poke around in water loaded clouds at or near freezing level:
>
> What are your strategies to cope with the weather ?
>
> Situation1: You climb through low clouds, but pass the freezing level
> on top in clear sky.
> After flying at -20°C for a while, you start your descend into the
> water loaded clouds.
> How quickly does the plane ice up? The airplane skin is relatively
> small, so the exposed mass should be quickly warmed.
> What are your personal experiences with the type of cloud? A heavy
> loaded nimbostratus will have a different effect as a cirrus, for sure.
> How do you preplan such a flight?
>
> Situation2: While climbing through subcooled low clouds, what is the
> most practical strategy? Climb further to reach real cold layers, which
> are almost dry?
> Quickly descend to reach warmer layers?
> It depends?

Personally, I'll decend through it to lower air but will not climb up
through it unless its an altitude climb (i.e. I'll climb from 8,000 to
12,000 but not on departure where I can't just decend back down). I'll
file enroute near the freezing level if I'm sure MEAs and ATC will be
flexible with altitudes.

Despite the "modern" FAA teaching method of ice avoidance (i.e. just
stay out), anyone who uses their IFR ticket is going to encouter ice at
some point. Just keep your options open, make sure you have warm air
below.

-Robert

-Robert

[Roy Smith]

"Bob Gardner" > wrote:
> Jim Carter's experience is telling. When a cold-soaked airframe descends
> into the clouds it is going to collect a lot of ice in a hurry. Where does
> the "quickly warmed" idea come from?

The thin sheet metal skin on the plane warms up fast. It's the fuel in the
tanks that hold the cold. So, obviously, the answer is to make sure you
don't have any fuel left when you start your descent :-)

[Stan Prevost]

"Bob Gardner" > wrote in message
. ..
> Jim Carter's experience is telling. When a cold-soaked airframe descends
> into the clouds it is going to collect a lot of ice in a hurry. Where does
> the "quickly warmed" idea come from?
>

It doesn't even have to get into the clouds. A cold soaked airplane
descending into humid air can accumulate a layer of frost, thickness of
which depends on time of exposure, humidity, and how cold the airplane was.
This is an exception to the common theory that visible moisture is required
for accumulation of airframe icing. It has happened to me in a Saratoga, I
landed with still 1/4 inch or so of ice under the wings, where the inboard
fuel tanks are located, and a little elsewhere. Never passed through a
cloud or any kind of visible moisture, had been flying a couple of hours at
16,500 or so, saw ice accumulating during descent through warmer air. The
thin skin without any thermal mass beneath it apparently warmed rapidly
enough to accumulate little and shed it, whereas the fuel tanks had enough
thermal mass to keep the ice frozen.

The recent FAA regional counsel letter about icing conditions included areas
of high humidity with near freezing temperature as known icing conditions.
It has widely been hooted down, but is not entirely wrong.

[Bob Gardner]

BTDT, in a Lear descending into Baton Rouge after a flight in the high 30
flight levels. Mucho ice on the bottoms of the wings.

Bob

"Stan Prevost" > wrote in message
...
>
> "Bob Gardner" > wrote in message
> . ..
>> Jim Carter's experience is telling. When a cold-soaked airframe descends
>> into the clouds it is going to collect a lot of ice in a hurry. Where
>> does the "quickly warmed" idea come from?
>>
>
> It doesn't even have to get into the clouds. A cold soaked airplane
> descending into humid air can accumulate a layer of frost, thickness of
> which depends on time of exposure, humidity, and how cold the airplane
> was. This is an exception to the common theory that visible moisture is
> required for accumulation of airframe icing. It has happened to me in a
> Saratoga, I landed with still 1/4 inch or so of ice under the wings, where
> the inboard fuel tanks are located, and a little elsewhere. Never passed
> through a cloud or any kind of visible moisture, had been flying a couple
> of hours at 16,500 or so, saw ice accumulating during descent through
> warmer air. The thin skin without any thermal mass beneath it apparently
> warmed rapidly enough to accumulate little and shed it, whereas the fuel
> tanks had enough thermal mass to keep the ice frozen.
>
> The recent FAA regional counsel letter about icing conditions included
> areas of high humidity with near freezing temperature as known icing
> conditions. It has widely been hooted down, but is not entirely wrong.
>
>
>

[Doug[_1_]]

The situation to avoid is where the only out you have is to descend
into unknown ceilings and a chance of icing. You have ice, can't climb
above it and can barely hold altitude. That is the ultimate nightmare.
You ice up, can't climb and are forced to descend into unknown
terrain/ceilings. Definitely DON'T do that.

The only situation that is deemed to be even slightly "ok" is to
descend from VMC down through a possible icing layer 1000' thick on
approach where you have weather report and know you can land, where
they are saying "light rime". That might be ok. People do it and get
away with it.

My experience with ice is you can't really predict it. If it's below
freezing and you are in a cloud or it's raining or snowing, you will
probably get it. And without a known ice airplane, you will NOT WANT TO
BE THERE!! Now if you have 7000' of VMC below you and the ground is 70
degrees F and you're up at 11,000' and you fly into a dark cloud and
get a LOAD of ice, you'll probably survive and live to fly another day.
>From that experience you take the knowledge that you don't want to do
THAT again. Personally I think it's too risky to get any ice in small
non-deiced GA airplanes. They just don't do well.

So don't go if you think there is ice. You will probably get it some
day and probably survive. But don't push your luck, because if you get
it, you are flying on luck, which is not a good idea at all.

[Jose[_1_]]

> The
> thin skin without any thermal mass beneath it apparently warmed rapidly
> enough to accumulate little and shed it, whereas the fuel tanks had enough
> thermal mass to keep the ice frozen.

So, suppose the fuel tanks were insulated from the skin by about half an
inch of (vented?) air... that should solve the problem, no? (albeit at
a weight cost)
--
"There are 3 secrets to the perfect landing. Unfortunately, nobody knows
what they are." - (mike).
for Email, make the obvious change in the address.

[Frank Stutzman]

Doug > wrote:
> You ice up, can't climb and are forced to descend into unknown
> terrain/ceilings. Definitely DON'T do that.

Given enough ice, you don't really have any other options.

--
Frank Stutzman
Bonanza N494B "Hula Girl"
Hood River, OR

[Mark Hansen]

On 12/22/06 15:49, Frank Stutzman wrote:
> Doug > wrote:
>> You ice up, can't climb and are forced to descend into unknown
>> terrain/ceilings. Definitely DON'T do that.
>
> Given enough ice, you don't really have any other options.
>

I read his comment as "Don't allow yourself to get into this
situation in the first place."

Of course, once you're there, there's no going back ;-\


--
Mark Hansen, PP-ASEL, Instrument Airplane
Cal Aggie Flying Farmers
Sacramento, CA

[Jim Carter[_1_]]

> -----Original Message-----
> From: Roy Smith ]
> Posted At: Friday, December 22, 2006 3:18 PM
> Posted To: rec.aviation.ifr
> Conversation: SE airplanes in clouds - near freezing level
> Subject: Re: SE airplanes in clouds - near freezing level
>
> "Bob Gardner" > wrote:
> > Jim Carter's experience is telling. When a cold-soaked airframe
descends
> > into the clouds it is going to collect a lot of ice in a hurry.
Where
> does
> > the "quickly warmed" idea come from?
>
> The thin sheet metal skin on the plane warms up fast. It's the fuel
in
> the
> tanks that hold the cold. So, obviously, the answer is to make sure
you
> don't have any fuel left when you start your descent :-)


Now there's a different approach to the problem -- can't say that I want
to try it though. Very unique answer Roy. ;-}

[Jim Carter[_1_]]

> -----Original Message-----
> From: Jose ]
> Posted At: Friday, December 22, 2006 4:22 PM
> Posted To: rec.aviation.ifr
> Conversation: SE airplanes in clouds - near freezing level
> Subject: Re: SE airplanes in clouds - near freezing level
>
> > The
> > thin skin without any thermal mass beneath it apparently warmed
rapidly
> > enough to accumulate little and shed it, whereas the fuel tanks had
> enough
> > thermal mass to keep the ice frozen.
>
> So, suppose the fuel tanks were insulated from the skin by about half
an
> inch of (vented?) air... that should solve the problem, no? (albeit
at
> a weight cost)
> --
> "There are 3 secrets to the perfect landing. Unfortunately, nobody
knows
> what they are." - (mike).
> for Email, make the obvious change in the address.

It might solve some of the problem, but in our case the upslope cumulus
was pretty unstable. The visible moisture had risen well above the
altitude (temp point) where it should have already turned to ice, so
when we stuck our aircraft into the moisture it quickly adhered to the
entire exterior.

I suppose if you were descending into warmer air the ice would not
accumulate anywhere except on pieces that took a while to warm up or in
areas of reduced pressure. On the other hand, if you are descending into
or flying through moisture that is still liquid but cooling rapidly you
are going to encourage cooling with your cold airframe -- and have a
souvenir to show for it.

[Stan Prevost]

"Jose" > wrote in message
. net...
>> The thin skin without any thermal mass beneath it apparently warmed
>> rapidly enough to accumulate little and shed it, whereas the fuel tanks
>> had enough thermal mass to keep the ice frozen.
>
> So, suppose the fuel tanks were insulated from the skin by about half an
> inch of (vented?) air... that should solve the problem, no? (albeit at a
> weight cost)

Probably. If it is indeed a problem.

[John R. Copeland]

"Jose" > wrote in message . net...
>> The
>> thin skin without any thermal mass beneath it apparently warmed rapidly
>> enough to accumulate little and shed it, whereas the fuel tanks had enough
>> thermal mass to keep the ice frozen.
>
> So, suppose the fuel tanks were insulated from the skin by about half an
> inch of (vented?) air... that should solve the problem, no? (albeit at
> a weight cost)
> --

How would the vented air behave differently from the free air?
Wouldn't the condensation freeze just as readily, inside the vented volume?

[Stan Prevost]

"Bob Gardner" > wrote in message
...
> BTDT, in a Lear descending into Baton Rouge after a flight in the high 30
> flight levels. Mucho ice on the bottoms of the wings.
>

Thanks for that little story, Bob. You are the only person I have ever
heard report the same kind of occurence. I have been accused of lying, even
though it seems obvious that it will occur.

[Jose[_1_]]

> How would the vented air behave differently from the free air?
> Wouldn't the condensation freeze just as readily, inside the vented volume?

Vented air would be at ambient temperature. Trapped air would probably
be at equilibrium with the fuel tank. If you are descending into warmer
air, it gives you an edge.

Jose
--
"There are 3 secrets to the perfect landing. Unfortunately, nobody knows
what they are." - (mike).
for Email, make the obvious change in the address.

[Stan Prevost]

"John R. Copeland" > wrote in message
...
"Jose" > wrote in message
. net...
>> The
>> thin skin without any thermal mass beneath it apparently warmed rapidly
>> enough to accumulate little and shed it, whereas the fuel tanks had
>> enough
>> thermal mass to keep the ice frozen.
>
> So, suppose the fuel tanks were insulated from the skin by about half an
> inch of (vented?) air... that should solve the problem, no? (albeit at
> a weight cost)
> --

How would the vented air behave differently from the free air?
Wouldn't the condensation freeze just as readily, inside the vented volume?


Vented air would probably add ice inside the wing, but would not disturb
the aerodynamics.

[John R. Copeland]

"Stan Prevost" > wrote in message ...
>
> "Bob Gardner" > wrote in message
> ...
>> BTDT, in a Lear descending into Baton Rouge after a flight in the high 30
>> flight levels. Mucho ice on the bottoms of the wings.
>>
>
> Thanks for that little story, Bob. You are the only person I have ever
> heard report the same kind of occurence. I have been accused of lying, even
> though it seems obvious that it will occur.
>

We pilots of Cessna tip-tank twins would support you too, Stan.
On ground after a high flight, the fuel level in the tip tanks is clearly evident.
Usually it's only condensation, but sometimes it can be clear ice.

[Jose[_1_]]

> Wouldn't the condensation freeze just as readily, inside the vented volume?

Hmmm... that's why I fly them rather than design them. :)

Jose
--
"There are 3 secrets to the perfect landing. Unfortunately, nobody knows
what they are." - (mike).
for Email, make the obvious change in the address.

[Bob Moore]

Stan Prevost wrote
> Thanks for that little story, Bob. You are the only person I have
> ever heard report the same kind of occurence. I have been accused of
> lying, even though it seems obvious that it will occur.

In the B-707, after an ocean crossing, we always landed with 1/4" of
frost on the bottom surface of the wings.

Bob Moore

[Stan Prevost]

Thanks, Bob and others, for your corroboration.


"Bob Moore" > wrote in message
46.128...
> Stan Prevost wrote
>> Thanks for that little story, Bob. You are the only person I have
>> ever heard report the same kind of occurence. I have been accused of
>> lying, even though it seems obvious that it will occur.
>
> In the B-707, after an ocean crossing, we always landed with 1/4" of
> frost on the bottom surface of the wings.
>
> Bob Moore

[John R. Copeland]

"Peter" > wrote in message ...
>
>
> Then we get onto the thorny subject of determining cloud tops in
> Europe.... but let's not confuse the Americans too much, with their
> superior weather services :)

I dunno about services, but perhaps America has superior *weather*.

[Robert M. Gary]

Doug wrote:
> The only situation that is deemed to be even slightly "ok" is to
> descend from VMC down through a possible icing layer 1000' thick on
> approach where you have weather report and know you can land, where
> they are saying "light rime". That might be ok. People do it and get
> away with it.

No, if the assigned altitude is still well above the MEA and warmer
air, its fine. Sometimes you have to tell ATC you can't decend to
12,000 but can take 8,000 if the MEA is only 3,000.

> My experience with ice is you can't really predict it. If it's below
> freezing and you are in a cloud or it's raining or snowing, you will
> probably get it. And without a known ice airplane, you will NOT WANT TO
> BE THERE!!

Yes, but once you start using your IFR ticket you'll find that you
either spend the winter on the ground or have the occasional ice
encounter. The trick is to keep outs available.

-Robert, CFII

[Robert M. Gary]

Mark Hansen wrote:
> On 12/22/06 15:49, Frank Stutzman wrote:
> > Doug > wrote:
> >> You ice up, can't climb and are forced to descend into unknown
> >> terrain/ceilings. Definitely DON'T do that.
> >
> > Given enough ice, you don't really have any other options.
> >
>
> I read his comment as "Don't allow yourself to get into this
> situation in the first place."
>
> Of course, once you're there, there's no going back ;-\

Personally, if I have warm MEA's below me I don't worry about it too
much. You just need to keep outs. Being able to decend into warm air is
an out. It varies greatly with the type of clouds you are in too.
Stratus clouds have large areas of ice but thin altitudes. CU clouds
tend to have verticle ice for thousands of feet but you pass through
them quickly. Mixed of course is the worse.
>From practicle point of view there are two types of instrument pilots.
Those that have encountered ice and those that keep their instrument
ticket at home.

-Robert

[Roger[_4_]]

On 26 Dec 2006 11:06:07 -0800, "Robert M. Gary" >
wrote:

>
>Doug wrote:
>> The only situation that is deemed to be even slightly "ok" is to
>> descend from VMC down through a possible icing layer 1000' thick on
>> approach where you have weather report and know you can land, where
>> they are saying "light rime". That might be ok. People do it and get
>> away with it.
>
>No, if the assigned altitude is still well above the MEA and warmer
>air, its fine. Sometimes you have to tell ATC you can't decend to
>12,000 but can take 8,000 if the MEA is only 3,000.
>
>> My experience with ice is you can't really predict it. If it's below
>> freezing and you are in a cloud or it's raining or snowing, you will
>> probably get it. And without a known ice airplane, you will NOT WANT TO
>> BE THERE!!
>
>Yes, but once you start using your IFR ticket you'll find that you
>either spend the winter on the ground or have the occasional ice
>encounter. The trick is to keep outs available.

IFR?

As a student (with instructor) we stayed strictly VMC, but still
brought a 150 back looking like a popsicle. Coming in to land it was
warm enough to start melting the ice. The stuff was coming off in
chunks that were flying back and hitting the tail. It sounded like a
trash can falling over.

Michigan and the Great Lakes in general are known for lake effect
storms. They can pop up suddenly and be isolated of cover wide areas.
This was about a week or so before my solo cross country.

As luck would have it, A whole bunch of lake effect snow storms popped
up between Cheboygan and Traverse City on the solo cross country. Even
with a detour to the south I still got bounced around a lot, but that
exposure to ice and lake effect storms really made the cross country
much more relaxed than it would have been otherwise.

That was an interesting cross country. I had to refuel twice.

You need those "outs" even as a VFR pilot. People generally think of
weather changing slowly, but even watching as closely as I do I've
been caught a couple of times. In many areas the weather can go from
great to IMC in a matter of minutes. If you know the conditions you
should have a good idea as to the shortest way out and what
frequencies to use to make sure.



>
>-Robert, CFII
Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

[Robert M. Gary]

Roger wrote:
> On 26 Dec 2006 11:06:07 -0800, "Robert M. Gary" >
> wrote:
> Michigan and the Great Lakes in general are known for lake effect
> storms. They can pop up suddenly and be isolated of cover wide areas.
> This was about a week or so before my solo cross country.

A very good reason to ask about weather from a local before flying in
any wx. I'm very familiar with California and the entire SW region and
Mexico. I can talk to you about summer TS management, Mexican monsoons,
etc but I can't tell you about Michigan wx. I would seek advice before
flying in that area. The WX in the Western part of the U.S. is
typically very predictable. My understanding is that is not the case
East of the rockies.
-Robert, CFII

[Brian]

One question I have is what is "warmer" air? If I climb to freezing
temps in the clouds (which I have done), is descending to an altitude
that is maybe 2C sufficient to prevent further icing? From my
experience, the answer appears to be that this is sufficient, but I'm
wondering if the surfaces are still cold enough to allow ice to build.

-Brian

Robert M. Gary wrote:

> No, if the assigned altitude is still well above the MEA and warmer
> air, its fine. Sometimes you have to tell ATC you can't decend to
> 12,000 but can take 8,000 if the MEA is only 3,000.

[Kusi (us-ppl, sep, d.-ir)]

Brian wrote:

> One question I have is what is "warmer" air? If I climb to freezing
> temps in the clouds (which I have done), is descending to an altitude
> that is maybe 2C sufficient to prevent further icing? From my
> experience, the answer appears to be that this is sufficient, but I'm
> wondering if the surfaces are still cold enough to allow ice to build.

It sure depends on the time beeing in the cold. A surface which has
collected ice but sees 1°C or 2°C "warm" air or water will definetly
melt after some time.
A certain delay might be caused (as we learned) with subcooled
fuel-(tanks) as the mass is much bigger. (So far lasts my "theory", as
I posted the topic to learn from the cloud pokers)

How about the "Freezing Fog" which the METARS show regularly since two
weeks over here in Europe ?
We have a 0°C .. 5°C surface temperature with a stable high pressure
system on top. Variable winds of 2kts and morning / evening fog, with a
lifting during the day to max. 500 feet in haze. On top (in 3000 feet)
we need sun cream!

How dangerous is freezing fog in such a small layer ??