Does snow falling from the base of a cloud lessen the icing risk?

[Scott Dennstaedt, PhD]

So winter is rapidly approaching and I know some folks here have already had their first snowfall of the season...but does snow falling out of the base of a cloud mean there's less of a chance you'll see supercooled liquid water in that cloud that is producing the snow?  Just curious what you've been taught about flying into snow-producing clouds (this questions isn't about flying through snow below the clouds).

   

[Yetti]

moisture, convection, hail, snow, moisture   Depending on their type, clouds can consist of dry air mixed with liquid water drops, ice particles, or both. Low, shallow clouds are mostly made of water droplets of various sizes.

[Hank]

I've flown through light snow, small flakes, going in and out between layers. Nothing stuck . . . Didn't like it.

But then, I'm a Southern boy, I don't like snow at all. And now that I'm back down south again, I'm gonna do like Lewis Grizzard and nail my feet to the ground. Err, tie myself to the airspace?

P.S.--ice is where you find it, but it is found more often in the tops of clouds, regardless what's coming out the bottom. I've dodged cloud tops at 10,000 msl over the Appalachians [between GSP and TYS] with OATs around 30ºF because of this. Plus the nasty headwinds had my groundspeed down to 68 KTAS, so any exposure would have been lengthy.

Edited October 30, 2018 by Hank

[thinwing]

Maybe...assumption is that temp is cooler than base alitude containing already frozen water droplets (snow)so unless there is some kind of inversion ,it is safe to climb higher....I only have 3/4 flights thru this situation...once at night and there was no ice acumulation....regardless my current strategy is to climb on top ,avoiding areas showing heavy radar returns...that seems to produce least drama ...

[Scott Dennstaedt, PhD]

38 minutes ago, Hank said:

P.S.--ice is where you find it, but it is found more often in the tops of clouds, regardless what's coming out the bottom. 

Hank,

That is certainly true of stratocumulus clouds, but not of nimbostratus that may have it's maximum liquid water content near the base of the cloud.  

[Hank]

3 minutes ago, scottd said:

Hank,

That is certainly true of stratocumulus clouds, but not of nimbostratus that may have it's maximum liquid water content near the base of the cloud.  

So, since my Latin is rusty and clouds are named sheerly after appearances, ice is often found at the top of tall, puffy clouds at any time, and near the bottom of short rain clouds in the winter?

[Scott Dennstaedt, PhD]

9 minutes ago, Hank said:

So, since my Latin is rusty and clouds are named sheerly after appearances, ice is often found at the top of tall, puffy clouds at any time, and near the bottom of short rain clouds in the winter?

Hank,

Supercooled liquid water can be found at any location of the cloud regardless of type.  But for those clouds that are typically limited in growth such as stratocumulus (these cover large areas and have a quilted look when viewed from the top) the highest liquid water content is usually near the tops.  Towering cumulus or cumulonimbus are those that have significant vertical development.  Depending on some other factors, it's not impossible to see supercooled liquid water at heights were the temps are -30°C (certainly to -25°C).  Nimbostratus can occur any time of the year, but are those clouds that we find in regions where there's deep moisture, but not necessarily air unstable enough to produce convection.  Typically these are highly layered over their depth.  Liquid water maxima can occur anywhere in the cloud, but rarely at the top and most of the time near the base.  

[carusoam]

So... where do the nice artistic snow crystals get manufactured?

I have heard them beating against my paint... but haven’t seen them in production...

Best regards,

-a-

[L. Trotter]

On 10/30/2018 at 12:27 PM, scottd said:

So winter is rapidly approaching and I know some folks here have already had their first snowfall of the season...but does snow falling out of the base of a cloud mean there's less of a chance you'll see supercooled liquid water in that cloud that is producing the snow?  Just curious what you've been taught about flying into snow-producing clouds (this questions isn't about flying through snow below the clouds).

   

Scott,

From your signature block It would appear that you are certainly my superior when it comes to weather. I am no weather professional.  However, I do have loads of experience flying in the northern tier-year round.

This I can say from personal observation....When there is snow in the clouds my concern for structural icing diminishes considerably. It would seem the atmospheric conditions which produce ice with supercooled liquid droplets and the conditions which are favorable to produce snow are competitive in nature.

However,  beware, depending on the moisture content of the snow, impact build up can occur and produce the end result of ice. This usually occurs when the temps are just hovering around the freezing point. When the temps are really cold the moisture content is low and the snow will do a great job cleaning your leading edge.

[jaylw314]

On ‎10‎/‎30‎/‎2018 at 10:49 AM, scottd said:

Hank,

Supercooled liquid water can be found at any location of the cloud regardless of type.  But for those clouds that are typically limited in growth such as stratocumulus (these cover large areas and have a quilted look when viewed from the top) the highest liquid water content is usually near the tops.  Towering cumulus or cumulonimbus are those that have significant vertical development.  Depending on some other factors, it's not impossible to see supercooled liquid water at heights were the temps are -30°C (certainly to -25°C).  Nimbostratus can occur any time of the year, but are those clouds that we find in regions where there's deep moisture, but not necessarily air unstable enough to produce convection.  Typically these are highly layered over their depth.  Liquid water maxima can occur anywhere in the cloud, but rarely at the top and most of the time near the base.  

I have to imagine that snow-producing clouds tend NOT to have rapidly freezing water vapor that is supercooled, since that would give time for droplets to accumulate into larger ice blobs rather than tiny crystals, and there are clearly freezing nuclei present--as such, the probability of icing might be lower.  Still, I'd be super nervous given not knowing how cold it is--if it's warm enough, impact with the ice crystals could melt them and then freeze on the airframe.

Am I even close?

[carqwik]

I have a non-FIKI plane...so I don't fly in cold (below +2C) wet clouds. 

My guess is that it is possible to have SLD in clouds that are producing snow.  Did I win anything?

[Scott Dennstaedt, PhD]

4 hours ago, L. Trotter said:

Scott,

From your signature block It would appear that you are certainly my superior when it comes to weather. I am no weather professional.  However, I do have loads of experience flying in the northern tier-year round.

This I can say from personal observation....When there is snow in the clouds my concern for structural icing diminishes considerably. It would seem the atmospheric conditions which produce ice with supercooled liquid droplets and the conditions which are favorable to produce snow are competitive in nature.

However,  beware, depending on the moisture content of the snow, impact build up can occur and produce the end result of ice. This usually occurs when the temps are just hovering around the freezing point. When the temps are really cold the moisture content is low and the snow will do a great job cleaning your leading edge.

Within aviation weather, my specific expertise is in icing.  There's a lot of bad information out there with respect snow and icing.  Precipitation, in general, can push the envelope either way depending on the situation.  In some cases, it can be snowing like crazy in Buffalo with lake effect topping out around 12K.  But at the tops, lots of supercooled drizzle-sized drops exist producing a nasty icing event.  Precipitation can often scour out liquid water in the clouds.  And when ice crystals start to form in a cloud they tend to grow at the expense of supercooled liquid water.   

In the webinar I did last week where I presented a scenario where a SIGMET was issued for severe ice after two Boeing 767s (UPS freighters) where landing in Louisville and reported severe ice.   I discuss this event in greater detail in this workshop.  Turns out that snow was reaching the surface in this situation.  

In a fairly recent study of accidents and incidents where it was determined to be ice related, 33% of these cases, it was snowing.  In other words, snow falling from the base of the cloud isn't an indication that it's depleted of supercooled liquid water.  In fact, it's fairly common for many clouds to be mixed phase, consisting of both ice crystals and liquid water.

[Scott Dennstaedt, PhD]

5 hours ago, jaylw314 said:

I have to imagine that snow-producing clouds tend NOT to have rapidly freezing water vapor that is supercooled, since that would give time for droplets to accumulate into larger ice blobs rather than tiny crystals, and there are clearly freezing nuclei present--as such, the probability of icing might be lower.  Still, I'd be super nervous given not knowing how cold it is--if it's warm enough, impact with the ice crystals could melt them and then freeze on the airframe.

Am I even close?

First, water vapor is harmless and won't freeze - it's a gas.  It's not until water vapor condenses on nuclei that they become a droplet.  Ice crystals tend to move over the aircraft and won't penetrate the boundary layer.  They are not an icing threat.  When ice crystals start to form, there's a better chance for that cloud to start to grow bigger ice crystals at the expense of liquid drops.  Also, those drops can rime onto the ice crystals as well.  So it tends to "dry" the cloud up in some locations...although it can still have a fair amount of liquid in the tops.  

[kortopates]

@scottd I assume cumilform or statocumulus clouds with larger vertical development or thickness and with potentially greater level of Liquid Water Content (LWC) concentrated near the tops would be more likely to provide an icing threat in clouds producing snow.  It seem the thinner stratus layer where the LWC may be more concentrated at the bottom of the cloud would be more likely to experience the drying you mention to support the ice crystal production? 

At least AC_91-74B makes the point that although snow itself, as long as it is dry, is harmless, but that we can't assume that the clouds producing it aren't an icing threat. But it too discusses ice crystal formation drying out the LWC. 

 

Never mind, I just watched your video and see the example event was in a stratocumulus layer that wasn't real thick.

Edited November 1, 2018 by kortopates

[Scott Dennstaedt, PhD]

10 minutes ago, kortopates said:

@scottd I assume cumilform or statocumulus clouds with larger vertical development or thickness and with potentially greater level of Liquid Water Content (LWC) concentrated near the tops would be more likely to provide an icing threat in clouds producing snow.  It seem the thinner stratus layer where the LWC may be more concentrated at the bottom of the cloud would be more likely to experience the drying you mention to support the ice crystal production? 

At least AC_91-74B makes the point that although snow itself, as long as it is dry, is harmless, but that we can't assume that the clouds producing it aren't an icing threat. But it too discusses ice crystal formation drying out the LWC. 

Stratocumulus clouds are highly capped, so they don't grow vertically beyond about 4 or 5K feet. Although, the highest liquid water content is usually near the tops.  Cumuliform clouds can condense out more liquid in the updrafts, that's why convective situations can produce a high liquid water content and SLD.  Often this liquid water can be carried upward to high altitudes and very cold temps. In fact, in order for lightning to form, it needs supercooled liquid water.  The coldest temp ever recorded by an instrumented research aircraft that found water in the liquid state was -37.5°C and that was in a thunderstorm.   

[L. Trotter]

42 minutes ago, scottd said:

Precipitation can often scour out liquid water in the clouds.  And when ice crystals start to form in a cloud they tend to grow at the expense of supercooled liquid water.   

This is what I was referring to in my less than eloquent written observation. Although my concerns for icing are diminished (when its's snowing in clouds) the concern for icing is not zero. I guess I can now say they are diminished 66% with a concern for ice of 33%.

I guess Snow impact on a leading edge is technically a form of "icing". I think for many in GA, this form of "icing" is often not considered as it is generally accepted that snow will just "pass over and not penetrate the boundary layer". My point is, wet snow can stick to leading edges and lead to the same issues as rime, clear or mixed icing. Maybe what I am really seeing is generic icing but that is not what it looks or seems like.

Thanks for bring up this timely topic. Dispelling myths to promote safety is always welcomed.

[carusoam]

Great discussion, Scott!

Thanks for sharing your knowledge.

Best regards,

-a-

[Robert C.]

I said it before, and will say it again  ..... @mike_elliott and @Seth we really should invite Scott to speak/lecture at the next Mooney Summit!

[Scott Dennstaedt, PhD]

11 hours ago, L. Trotter said:

This is what I was referring to in my less than eloquent written observation. Although my concerns for icing are diminished (when its's snowing in clouds) the concern for icing is not zero. I guess I can now say they are diminished 66% with a concern for ice of 33%.

I guess Snow impact on a leading edge is technically a form of "icing". I think for many in GA, this form of "icing" is often not considered as it is generally accepted that snow will just "pass over and not penetrate the boundary layer". My point is, wet snow can stick to leading edges and lead to the same issues as rime, clear or mixed icing. Maybe what I am really seeing is generic icing but that is not what it looks or seems like.

Thanks for bring up this timely topic. Dispelling myths to promote safety is always welcomed.

Yes, the tendency for any precipitation is to scour out the cloud of liquid water and condensation/ice nuclei.  In some cases, this can leave the cloud somewhat clean and the drop sizes can increase depending on the circumstances.  Maybe the same liquid water content remains in the cloud, but the drops are overall larger.  Airframe icing is a complex subject.  

Usually wet snow means the static air temperatures are above freezing and that greatly lessens the icing risk since the total air temperatures (especially on the immediate leading edge) are usually well above freezing.  Impact icing can occur in this case over areas where it can then starve the engine of oxygen.  Alternate air may become necessary.