Flying in snow
Joined: Aug 2000
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From: UK
I wrote:
to which crab replied:
The question of whether snow is an indicator of reduced supercooled water provoked some research on my part, because I really wasn't sure of the cloud physics of glaciation when I wrote the words above. The point about certainty remains a good one -- there will rarely be a guarantee when it comes to icing. Nevertheless, I thought this was worth sharing. The papers I cite are available as full text online, links at the bottom.
The Bergeron Process is one of many competing processes going on in the cloud. The cloud microphysics models tend to consider water in six states: water vapour, cloud water, cloud ice, hail/graupel, rain and snow. The processes that cause transition between these states are complex -- Lin et al consider between 20 and 30 of them, of which the Bergeron process is just one. If I'm counting correctly, there are 9 responsible for snow formation. In Lin's model example, the Bergeron process turns out to be significant, but not the most significant process in snow production -- simple accretion of cloud ice and cloud water by the snow is the most effective in making more of it.
Reisin et al have a more sophisticated model involving nucleation, though the phases considered are essentially the same. They run the model for a number of different types of cloud e.g. maritime and continental, with different densities (low and high respectively) of cloud condensation nuclei. What is noticeable about their results is that in every case where snow is produced, the cloud water concentration has fallen to a small fraction (say 10% of its maximum) by the time significant snow is present. It's worth noting that the simulations are for cumulus cloud with a relatively high base (about 4000 ft and 4 degC), so the model is not simulating the clouds we see as producing snow on the ground. Nevertheless, the microphysical processes are substantially the same. In every case considered, the cloud is substantially glaciated (water has turned to ice) before snow is produced.
Zawadski and Szyrmer suggest ways of predicting supercooled water content (SWC) from radar reflectivity. That paper is interesting for the assertion (based on another Zawadski paper) it makes in the introduction:
"In a study of the development of microphysics in an
Atlantic storm, Zawadzki et al. (1993a, henceforth to
be referred to as ZOL) showed that during the devel-
opment of a precipitating system, the SCW appears as
a transient phenomenon during the uplift of initially
clear air. Once precipitation develops within the super-
cooled cloud and snow grows at the expense of the
liquid, SCW vanishes rapidly. It is also possible for
SCW to coexist at equilibrium with snow if the vertical
air motion is strong enough so that the rate of the gen-
eration of water vapor excess overcomes the rate at
which snow grows by deposition."
Much of the rest of the paper is about working out the critical vertical velocity that permits SCW to coexist with snow. I'd interpret it as saying that some pretty substantial updrafts are required to make that possible. The more snow, the more difficult it is for SCW to exist.
Then finally, there's the paper that describes the NWS/FAA Current Icing Potential (CIP) model. CIP is a real-world prediction of icing potential, and is quite sophisticated. You can see the output here. The paper describes how data like numerical model output, satellite, METARs, radar and PIREPs are combined to reach the prediction of icing potential.
"In a similar situation in which only snow is reported
at the surface, ice crystals are clearly present beneath
and within the lowest cloud layer. These crystals scav-
enge SLW through riming and may completely glaciate
the cloud (Geresdi et al. 2005). In such cases, CIP de-
creases the maximum possible icing potential somewhat
by including a snow factor in the equation (see Table 2).
When the snow is associated with widespread radar
echoes of greater than 18 dBZ, there is likely to be an
abundance of large ice crystals aloft, implying more
riming, and the icing potential is further lowered. As
more of the grid box is filled with snow echoes, this
factor becomes stronger, further decreasing the poten-
tial for icing."
All that leads me to the same conclusion: seeing snow falling from a cloud significantly reduces the likelihood that the cloud will offer a significant icing hazard. Whether "significantly reduces" is good enough in the circumstances is a rather different debate!
Bulk Parameterization of the Snow Field in a Cloud Model
Journal of Applied Meteorology
Volume 22, Issue 6 (June 1983)
Yuh-Lang Lin, Richard D. Farley, and Harold D. Orville
Diagnostic of Supercooled Clouds from Single-Doppler Observations in Regions of Radar-Detectable Snow
Journal of Applied Meteorology
Volume 39, Issue 7 (July 2000)
I. Zawadzki and W. Szyrmer, S. Laroche
Current Icing Potential: Algorithm Description and Comparison with Aircraft Observations
Journal of Applied Meteorology
Volume 44, Issue 7 (July 2005)
Ben C. Bernstein, Frank McDonough, Marcia K. Politovich, and Barbara G. Brown, Thomas P. Ratvasky and Dean R. Miller, Cory A. Wolff and Gary Cunning
Rain Production in Convective Clouds As Simulated in an Axisymmetric Model with Detailed Microphysics. Part I: Description of the Model
Journal of the Atmospheric Sciences
Volume 53, Issue 3 (February 1996) pp. 497–519
Tamir Reisin, Zev Levin, and Shalva Tzivion
Rain Production in Convective Clouds as Simulated in an Axisymmetric Model with Detailed Microphysics. Part II: Effects of Varying Drops and Ice Initiation
Journal of the Atmospheric Sciences
Volume 53, Issue 13 (July 1996) pp. 1815–1837
Tamir Reisin, Zev Levin, and Shalva Tzivion
No, that's the whole point. A lower probability of icing in the cloud that is producing the snow/sleet, because the presence of snow is an indicator of glaciation and the probable absence of supercooled water.
The weakness in your argument is the use of the word probable here - the fact is that to produce snow in the first place you must have supercooled droplets so that the water can evaporate from the droplets and form ice crystals on the freezing nuclei (the Bergeron-Findeisen process).
So, since both will exist in varying amounts depending on the conditions you cannot say with any certainty that the snow cloud will be a lesser icing hazard than the no-snow cloud.
So, since both will exist in varying amounts depending on the conditions you cannot say with any certainty that the snow cloud will be a lesser icing hazard than the no-snow cloud.
The Bergeron Process is one of many competing processes going on in the cloud. The cloud microphysics models tend to consider water in six states: water vapour, cloud water, cloud ice, hail/graupel, rain and snow. The processes that cause transition between these states are complex -- Lin et al consider between 20 and 30 of them, of which the Bergeron process is just one. If I'm counting correctly, there are 9 responsible for snow formation. In Lin's model example, the Bergeron process turns out to be significant, but not the most significant process in snow production -- simple accretion of cloud ice and cloud water by the snow is the most effective in making more of it.
Reisin et al have a more sophisticated model involving nucleation, though the phases considered are essentially the same. They run the model for a number of different types of cloud e.g. maritime and continental, with different densities (low and high respectively) of cloud condensation nuclei. What is noticeable about their results is that in every case where snow is produced, the cloud water concentration has fallen to a small fraction (say 10% of its maximum) by the time significant snow is present. It's worth noting that the simulations are for cumulus cloud with a relatively high base (about 4000 ft and 4 degC), so the model is not simulating the clouds we see as producing snow on the ground. Nevertheless, the microphysical processes are substantially the same. In every case considered, the cloud is substantially glaciated (water has turned to ice) before snow is produced.
Zawadski and Szyrmer suggest ways of predicting supercooled water content (SWC) from radar reflectivity. That paper is interesting for the assertion (based on another Zawadski paper) it makes in the introduction:
"In a study of the development of microphysics in an
Atlantic storm, Zawadzki et al. (1993a, henceforth to
be referred to as ZOL) showed that during the devel-
opment of a precipitating system, the SCW appears as
a transient phenomenon during the uplift of initially
clear air. Once precipitation develops within the super-
cooled cloud and snow grows at the expense of the
liquid, SCW vanishes rapidly. It is also possible for
SCW to coexist at equilibrium with snow if the vertical
air motion is strong enough so that the rate of the gen-
eration of water vapor excess overcomes the rate at
which snow grows by deposition."
Much of the rest of the paper is about working out the critical vertical velocity that permits SCW to coexist with snow. I'd interpret it as saying that some pretty substantial updrafts are required to make that possible. The more snow, the more difficult it is for SCW to exist.
Then finally, there's the paper that describes the NWS/FAA Current Icing Potential (CIP) model. CIP is a real-world prediction of icing potential, and is quite sophisticated. You can see the output here. The paper describes how data like numerical model output, satellite, METARs, radar and PIREPs are combined to reach the prediction of icing potential.
"In a similar situation in which only snow is reported
at the surface, ice crystals are clearly present beneath
and within the lowest cloud layer. These crystals scav-
enge SLW through riming and may completely glaciate
the cloud (Geresdi et al. 2005). In such cases, CIP de-
creases the maximum possible icing potential somewhat
by including a snow factor in the equation (see Table 2).
When the snow is associated with widespread radar
echoes of greater than 18 dBZ, there is likely to be an
abundance of large ice crystals aloft, implying more
riming, and the icing potential is further lowered. As
more of the grid box is filled with snow echoes, this
factor becomes stronger, further decreasing the poten-
tial for icing."
All that leads me to the same conclusion: seeing snow falling from a cloud significantly reduces the likelihood that the cloud will offer a significant icing hazard. Whether "significantly reduces" is good enough in the circumstances is a rather different debate!
Bulk Parameterization of the Snow Field in a Cloud Model
Journal of Applied Meteorology
Volume 22, Issue 6 (June 1983)
Yuh-Lang Lin, Richard D. Farley, and Harold D. Orville
Diagnostic of Supercooled Clouds from Single-Doppler Observations in Regions of Radar-Detectable Snow
Journal of Applied Meteorology
Volume 39, Issue 7 (July 2000)
I. Zawadzki and W. Szyrmer, S. Laroche
Current Icing Potential: Algorithm Description and Comparison with Aircraft Observations
Journal of Applied Meteorology
Volume 44, Issue 7 (July 2005)
Ben C. Bernstein, Frank McDonough, Marcia K. Politovich, and Barbara G. Brown, Thomas P. Ratvasky and Dean R. Miller, Cory A. Wolff and Gary Cunning
Rain Production in Convective Clouds As Simulated in an Axisymmetric Model with Detailed Microphysics. Part I: Description of the Model
Journal of the Atmospheric Sciences
Volume 53, Issue 3 (February 1996) pp. 497–519
Tamir Reisin, Zev Levin, and Shalva Tzivion
Rain Production in Convective Clouds as Simulated in an Axisymmetric Model with Detailed Microphysics. Part II: Effects of Varying Drops and Ice Initiation
Journal of the Atmospheric Sciences
Volume 53, Issue 13 (July 1996) pp. 1815–1837
Tamir Reisin, Zev Levin, and Shalva Tzivion
Joined: Apr 2000
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From: EGDC
Bookworm - some good info there - I think everyone has improved their knowledge of snow formation as a result of this thread.
I still think that my statement
Holds true since you cannot say that there will be no SCW/SCD, even if it is snowing and, since you have no way of measuring the SCW/SCD content, you are better off assuming that icing conditions exist.
I still think that my statement
So, since both will exist in varying amounts depending on the conditions you cannot say with any certainty that the snow cloud will be a lesser icing hazard than the no-snow cloud.
Fleet Manager
Joined: Aug 2006
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From: Ontario, Canada
Quote:
Does anyone have any thoughts on the flying challenges on a clear blue day in the summer, when you can see forever, and the temperatures soar to, say, 25C?
Carb ice.
Read the accident reports and weep.
Does anyone have any thoughts on the flying challenges on a clear blue day in the summer, when you can see forever, and the temperatures soar to, say, 25C?
Carb ice.
Read the accident reports and weep.
I'll watch for it today when I fly, it's a clear blue day out there...
Pilot DAR
Guest
Posts: n/a
Are there accident reports suggesting that carb ice was a risk factor at any temperature, on a clear blue day, when you can see forever?
It's also type dependent in my experience, as in airframe/engine combination.
Thus in a PA28 you'd be unlucky, in your trusty mount you'd have to be a little more careful in high humidity conditions.
It may also be location dependent, e.g. my local field
35007KT 310V030 9999 FEW015 14/11 Q1017 NOSIG, looking out of the window, you can see forever, but it is quite humid.
No big deal if you regularly check the carb heat.
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From: In the boot of my car!
Crab
Icing conditions may exist if there is visible moisture and the temperatures are correct for icing to occur.
But the keyword is visible moisture and whether there is snow there or just plain rain is irrelavant.
You should fear taking off into rain with no snow equally as much if the temperatures are in or near freezing.
The danger of implying otherwise is some poor S*D is going to tremble in his boots because of snow being present but happily take off into rain.
Snow looks cold and icy but its the temperature he should be looking at and the visible moisture not the snow.
Pace
Icing conditions may exist if there is visible moisture and the temperatures are correct for icing to occur.
But the keyword is visible moisture and whether there is snow there or just plain rain is irrelavant.
You should fear taking off into rain with no snow equally as much if the temperatures are in or near freezing.
The danger of implying otherwise is some poor S*D is going to tremble in his boots because of snow being present but happily take off into rain.
Snow looks cold and icy but its the temperature he should be looking at and the visible moisture not the snow.
Pace
Joined: Apr 2000
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From: EGDC
Pace - unless it is freezing rain which requires specific conditions to form and is well forecast, taking off into rain is not a problem. You need to be in cloud (if that is what you want to call visible moisture) below 0 degrees to be in icing conditions.
Snow is not visible moisture it is ice.
Carb icing can occur in humid environments at up to 30 degrees C OAT.
Snow is not visible moisture it is ice.
Carb icing can occur in humid environments at up to 30 degrees C OAT.
Joined: Aug 2000
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From: UK
I still think that my statement ... Holds true since you cannot say that there will be no SCW/SCD, even if it is snowing and, since you have no way of measuring the SCW/SCD content, you are better off assuming that icing conditions exist.
Joined: Sep 2001
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From: Toronto
Here in Canada, I can not remember any accident or incident where snow by itself brought down an aircraft as long as carb ice or alternate air were used if called for. The Transport Canada Aviation Safety Letter is replete with sermons on several common ways to crunch an aircraft, but I don't remember any article that declared snow by itself would bring down an aircraft; mind you, carb ice or alternate air may be required in some cases -- and if your helicopter's operating limitation prohibit operation in snow, then don't.
That said, whiteout and CFIT are hazards and if there's a significant snow shower over a runway, I'll not be using it until the snow shower has moved away. In fact, I may decide to land elsewhere.
And it won't hurt to carefully check out destination airfield conditions as there's lots of accidents where a wingtip has struck a snowbank on landing or during taxi
And please don't try taking off with any snow or frost on the flying surfaces.
That said, whiteout and CFIT are hazards and if there's a significant snow shower over a runway, I'll not be using it until the snow shower has moved away. In fact, I may decide to land elsewhere.
And it won't hurt to carefully check out destination airfield conditions as there's lots of accidents where a wingtip has struck a snowbank on landing or during taxi
And please don't try taking off with any snow or frost on the flying surfaces.
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From: In the boot of my car!
Pace - unless it is freezing rain which requires specific conditions to form and is well forecast, taking off into rain is not a problem. You need to be in cloud (if that is what you want to call visible moisture) below 0 degrees to be in icing conditions.
Snow is not visible moisture it is ice.
Snow is not visible moisture it is ice.
who is saying snow is visible moisture? certainly not me! taking off in rain at the wrong temperature most certainly is a problem in or out of cloud.Give me nice puffy snow flakes any day of the week
You will get rain out of clouds as well as in clouds. That may fall as rain as you climb through the clouds but then when the temp drops to zero that rain will start to form as ice on the airframe.
Pace
Joined: Apr 2000
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From: EGDC
Pace -
The last sentence of that quote implies you view both snow and rain as visible moisture - it may not be the way you intended it to be read but thats how it comes across to me.
Not quite sure what point you are trying to make here - all you have said is that icing conditions exist in cloud below zero, which we all knew anyway
How is taking off in rain a problem unless it is freezing rain? If the OAT is above zero it's not freezing rain because the airframe is above zero - I think this was one of your points many posts ago. If you are in cloud below zero you are in potential icing conditions whether it is raining or not.
Scooter Boy - I was trying to make the point that freezing rain is relatively easy to forecast because it generally requires quite specific conditions to exist (active warm front, sub zero layer beneath etc)
Icing conditions may exist if there is visible moisture and the temperatures are correct for icing to occur.
But the keyword is visible moisture and whether there is snow there or just plain rain is irrelavant.
But the keyword is visible moisture and whether there is snow there or just plain rain is irrelavant.
You will get rain out of clouds as well as in clouds. That may fall as rain as you climb through the clouds but then when the temp drops to zero that rain will start to form as ice on the airframe.
taking off in rain at the wrong temperature most certainly is a problem in or out of cloud.
Scooter Boy - I was trying to make the point that freezing rain is relatively easy to forecast because it generally requires quite specific conditions to exist (active warm front, sub zero layer beneath etc)
Fleet Manager
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From: Ontario, Canada
Wow, this is certainly going around and around.....
My point was missed; "clear blue day in the summer" = no moisture in the air = no risk of icing, temperature is irrelevent.
I would be much more fearful of flying in rain near freezing, than snow in any temperature. Yes, you can have "rain" below freezing, and "snow" a little above. "Snow" decribes a form of "ice", which is frozen, so it's not gong to freeze again (on your plane).
Would you rather fly your beautifully painted plane through paint dust (which is floating solids of paint - not sticky) or paint mist (which is floating liquid)? Your plane won't be bright pink after flying thorugh the pink paint dust, but it surely will after you fly throught the pink paint mist!
As I said somewhere around the beginning of this thread, the choice to launch into percipitation must be very carefully considered, and particularly in the context of training, experience, and comfort level of the pilot. If in doubt, don't go! The fact that one pilot doubts, does not mean another cannot fly safely!
If you want to learn, great! Read, write, ask, and try to wangle some right seat time with someone who has done this before. It's just amazing what you can learn from the wise and experienced person, who is very comfortable doing that which you have yet to try!
I regularly fly in snow, and in rain. If the percipitation can't decide which it is, I'm happiest on the ground!
Pilot DAR
My point was missed; "clear blue day in the summer" = no moisture in the air = no risk of icing, temperature is irrelevent.
I would be much more fearful of flying in rain near freezing, than snow in any temperature. Yes, you can have "rain" below freezing, and "snow" a little above. "Snow" decribes a form of "ice", which is frozen, so it's not gong to freeze again (on your plane).
Would you rather fly your beautifully painted plane through paint dust (which is floating solids of paint - not sticky) or paint mist (which is floating liquid)? Your plane won't be bright pink after flying thorugh the pink paint dust, but it surely will after you fly throught the pink paint mist!
As I said somewhere around the beginning of this thread, the choice to launch into percipitation must be very carefully considered, and particularly in the context of training, experience, and comfort level of the pilot. If in doubt, don't go! The fact that one pilot doubts, does not mean another cannot fly safely!
If you want to learn, great! Read, write, ask, and try to wangle some right seat time with someone who has done this before. It's just amazing what you can learn from the wise and experienced person, who is very comfortable doing that which you have yet to try!
I regularly fly in snow, and in rain. If the percipitation can't decide which it is, I'm happiest on the ground!
Pilot DAR
Joined: Jan 2001
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From: In the boot of my car!
If you want to learn, great! Read, write, ask, and try to wangle some right seat time with someone who has done this before. It's just amazing what you can learn from the wise and experienced person, who is very comfortable doing that which you have yet to try!
I regularly fly in snow, and in rain. If the percipitation can't decide which it is, I'm happiest on the ground!
I regularly fly in snow, and in rain. If the percipitation can't decide which it is, I'm happiest on the ground!
there are those that do and those that talk about it. I second your comments above as until a pilot has experience in dealing with certain situations and conditions no amount of talking can fill a void of lack of experience.
Like you I would rather take off into snow than rain near freezing point and like you I would recommend a PPL flying with an experienced pilot to get that practical immersion in the real conditions and in an aircraft/pilot combination able to deal with with those condiitions
Pace
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My point was missed; "clear blue day in the summer" = no moisture in the air = no risk of icing, temperature is irrelevent.
I could clearly see the upper parts of a mountain about 100nm away.
Air temp was 19 deg C and the RH was 78%. To be fair, despite the blue skies, there was some cloud at FL250. I know this, because I took a self brieifng from the airport.
Maybe it's different in Canada, but your point was not missed.
If the RH is high, carb heat can get you in the mid 20s, I've experienced it in the UK on a warm, sunny, clear 23 C day, in a Beagle Pup with a Continental O-200.
