# What are the enemies of snow?



## billski (Jan 5, 2009)

Help me out with a list, going from worst to best.  Please rank for me precipitation events, from worst to best that wreak the worst damage to the snowpack.  I'm defining "damage" as reduction of snowpack, or anything that makes it less ski-able (like ice).

  I'll take a stab at it based on my pedestrian understanding.  I know there are other variables like sun and temperature and winds, but I'm trying to keep it simple for my simple brain.

Most Damage to snowpack
..
Fog
Freezing Fog (whatever that is)
Freezing Rain
Rain
Sleet
Hail
--- the line where we move from damage to base-building.  Is this right?
Wet Snow
Dry Snow
..
Least Damage to snowpack

did I get it right? What did I miss?  Thanks.


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## Greg (Jan 5, 2009)

I hear a lot how fog melts snow. It's my understanding that the fog _*is *_a result of melting snow, i.e. warm moist air over the cold snowpack. Bottom line is warmth is the biggest enemy of snow, whether from a southerly flow or the sun. Warm wind just amplifies the problem since it mixes the air thus causing you to lose the cold layer near the surface.


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## awf170 (Jan 5, 2009)

Greg is correct.  Here is my list.


Warmth + rain + wind
rain + warmth and wind + warmth
rain or warmth or wind (if there is powder on the surface)
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Freezing rain (despite making conditions awful I think it does help with base building) 
Powder (really doesn't do anything for the base unless you get huge quantities) 
Sleet (wicked good base material, but still might not cover large rocks, down trees, stumps, etc)
Dense snow (equal to sleet in base building ability but will also cover lurking objects in the woods)


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## dmc (Jan 5, 2009)

communism...


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## GrilledSteezeSandwich (Jan 5, 2009)

People from Jersey..since they skid their turns and scrap it down to the ice..


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## cbcbd (Jan 5, 2009)

I don't see how sleet, hail and freezing rain lower the snowpack... 
Sleet is wet, but still accumulates, and with a freezing after it should add to the snowpack, although not a fun add.
Same for hail and freezing rain - they don't lower the snowpack, they're just not the ideal additions to the snowpack for skiing.

Also don't see how wind should negatively affect the snowpack other than blowing powdery snow off a particular slope. If it's warm or raining, wind should only "help" by lowering the windchill and helping the precip to go away from the liquid kind.


Rain is the worst. Rain at 35 degrees is worse than 60 degree weather with no rain.


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## awf170 (Jan 5, 2009)

cbcbd said:


> If it's warm or raining, wind should only "help" by lowering the windchill and helping the precip to go away from the liquid kind.



When it is warm, but not windy a thin layer of cooler air forms above the snowpack.  So even if it is 60 degrees there could be a layer at the surface of 40 degree air.  If it is windy that layer won't be able to form and the snow will constantly be hit with the 60 degree air.


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## Greg (Jan 5, 2009)

cbcbd said:


> Also don't see how wind should negatively affect the snowpack other than blowing powdery snow off a particular slope. If it's warm or raining, wind should only "help" by lowering the windchill and helping the precip to go away from the liquid kind.



Wind chill has nothing to do with actual temperature. Windchill is an estimate of how it "feels" to exposed skin.

Again, wind will mix colder layers at the surface with warmer layers aloft, effectively raising the temperature closer to the snowpack where it does the most damage.



cbcbd said:


> Rain is the worst. Rain at 35 degrees is worse than 60 degree weather with no rain.



Disagreed. Cold rain can just drain through. 60 degree temps will eat snow.


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## KingM (Jan 5, 2009)

The Christmas one was a brutal example of this kind of stuff. It warmed to rain, then the rain went away and was followed by extreme warmth and wind. The end result was the almost complete annihilation of the snow pack over the course of what? 36 hours?


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## dmc (Jan 5, 2009)

cbcbd said:


> I don't see how sleet, hail and freezing rain lower the snowpack..



that would depend on the snowpack itself..  temp and water content...


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## cbcbd (Jan 5, 2009)

awf170 said:


> When it is warm, but not windy a thin layer of cooler air forms above the snowpack.  So even if it is 60 degrees there could be a layer at the surface of 40 degree air.  If it is windy that layer won't be able to form and the snow will constantly be hit with the 60 degree air.


Ok, I can see that.



Greg said:


> Wind chill has nothing to do with actual temperature. Windchill is an estimate of how it "feels" to exposed skin.
> 
> Again, wind will mix colder layers at the surface with warmer layers aloft, effectively raising the temperature closer to the snowpack where it does the most damage.
> 
> Disagreed. Cold rain can just drain through. 60 degree temps will eat snow.


And wouldn't then the precipitation also feel the same temperature as exposed skin? I know wind chill is not the actual temperature, but it affects everything hit by that wind since it removes heat more effectively from objects that have energy, right? Anyone take physics recently and can school us?

As for the rain... from what I have seen I still think rain will cause more damage. But then again, I would really have to get my physics books from HS out to really check this one since there are so many variables if you were to compare the two... I think I'm just going to step out of this one.


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## dmc (Jan 5, 2009)

they teach all this stuff in AVI class...  Or a lot of it...


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## cbcbd (Jan 5, 2009)

I took Avy 1 and it was mostly about the basics - triggers, the avy triangle, traps, planning, route finding, beacon use, etc.
At least from what I was told, Avy 2 went more into the snow science part of it.


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## dmc (Jan 5, 2009)

cbcbd said:


> I took Avy 1 and it was mostly about the basics - triggers, the avy triangle, traps, planning, route finding, beacon use, etc.
> At least from what I was told, Avy 2 went more into the snow science part of it.



AVIII get more into it for sure...


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## Greg (Jan 5, 2009)

cbcbd said:


> And wouldn't then the precipitation also feel the same temperature as exposed skin? I know wind chill is not the actual temperature, but it affects everything hit by that wind since it removes heat more effectively from objects that have energy, right? Anyone take physics recently and can school us?



I'm not a meteorologist, but wind is a result in changes in air pressure. It doesn't have any effect on changes in temperature per se. That's really an effect of wind direction as you probably know (northerly = cold, southerly = warm, at least in this hemisphere  ). So if you're on the backside of a departing high pressure system (spins clockwise) you'll get wind from the south which will usher in warm air.

Anyway, right - windchill is just an estimate of how it feels. A warm wind blowing on a cool surface probably does warm it faster and vice versa probably is true. I'm not a physics guy either, but I do know that wind doesn't inherently "cool". It depends on the temperature of the wind and the surface it's hitting as to whether it accelerates heat loss or gain.


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## polski (Jan 5, 2009)

My experience this season is that far and away the biggest enemy of snow is a Storm Speculation thread.





Sorry, couldn't resist.

On a more serious note, I suspect hail is rarely if ever a positive factor for skiing base because it forms in strong thunderstorms, which tend to happen outside of ski season. Even if there were a severe tstm on the shoulders of the season, like in the spring, hail at the front of the system almost certainly would be followed by heavy rain. I know there's occasionally "thunder snow" (some was forecast in SNE on New Year's Eve in fact, though I didn't hear any thunder while I was out that day) but you need lower-level warmth to feed the instability for the kind of strong tstm that could produce hail.


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## snoseek (Jan 5, 2009)

Does sun fall into tempature?


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## tcharron (Jan 5, 2009)

Greg said:


> I hear a lot how fog melts snow. It's my understanding that the fog _*is *_a result of melting snow, i.e. warm moist air over the cold snowpack. Bottom line is warmth is the biggest enemy of snow, whether from a southerly flow or the sun. Warm wind just amplifies the problem since it mixes the air thus causing you to lose the cold layer near the surface.



An additional issue is that fog allows the humid air to maintain tempurature, potentially causing more sublimation.  I'd asked a simular thing on some forums a while back, and it came down to, when the fog conditions happen, a nasty cycle begins, where more fog is made, where more snow turns to fog, etc..


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## Greg (Jan 5, 2009)

tcharron said:


> An additional issue is that fog allows the humid air to maintain tempurature, potentially causing more sublimation.  I'd asked a simular thing on some forums a while back, and it came down to, when the fog conditions happen, a nasty cycle begins, where more fog is made, where more snow turns to fog, etc..



Makes sense.


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## Marc (Jan 8, 2009)

cbcbd said:


> Ok, I can see that.
> 
> 
> And wouldn't then the precipitation also feel the same temperature as exposed skin? I know wind chill is not the actual temperature, but it affects everything hit by that wind since it removes heat more effectively from objects that have energy, right? Anyone take physics recently and can school us?
> ...



Somebody call me?

If wind becomes a net help or hinderence to the heat transfer to a snowpack depends on the temperature of the snow pack and the air.

If the air is warmer than the snowpack, wind will add heat to the snow pack faster by convective transfer.

If the air is colder than the snowpack, it will remove heat faster from the snow pack than stagnant air because of convective transfer.

All other conditions being equal, naturally.

Also, fog damages a snow pack so badly because much of the radiative energy emitted from the snowpack (mostly the infrared range of the spectrum) is reflected by fog (really just a water in air suspension).  That's why clear nights in the winter are often colder than cloudy nights.  All the radiation eminating from earth bound surfaces is lost to space with very little gain (stars... moon... tinker toys compare to the sun).  Clouds will reflect that back.

They other reason fog is killer is because of high humdity.  Humidty, as Greg sort of touched on, sets up a loop of melting and evaporation/sublmation that creates more fog as the conditions persists because of the radiative feed back, and because of the higher capacity for conduction that humid air has compared with try air.  The more humid the air, the better at transferring heat it is.


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## Marc (Jan 8, 2009)

tcharron said:


> An additional issue is that fog allows the humid air to maintain tempurature, potentially causing more sublimation.  I'd asked a simular thing on some forums a while back, and it came down to, when the fog conditions happen, a nasty cycle begins, where more fog is made, where more snow turns to fog, etc..



Ooops, sorry tcharron, you beat me too it and I didn't even see it.


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## tcharron (Jan 8, 2009)

Marc said:


> Ooops, sorry tcharron, you beat me too it and I didn't even see it.



I just didn't get as technical..  Mostly for fear of saying somethbing that is the ballpark idea, but perhaps not totally technically accurate.  :-D  But mostly, the act the the water going from the air and condensing releases energy, in the form of heat, causing more melt, potentially causing sublimation, causing more water to go into the air, potentially causing water to come out of the air, heating the air, etc..etc..

What I didn't understand is why the act of the snow sublimating didn't 'take' energy from the snowpack, making it colder and more stable.  I still don't quite understand why it wouldn't, but mostly it seems it's easier for it to get the energy from the air, which actually then cools the air, causing the entire process to go house.

And then there's the simply fact that the fog melting the snow is just like rain adding water to the snow, melting it additionally.


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## SkiBunny (Jan 8, 2009)

*Are you for Joisey?*



GrilledSteezeSandwich said:


> People from Jersey..since they skid their turns and scrap it down to the ice..



NOT! I am afraid that habit of skid and scrap cannot be awarded to only one State in the Union. That is an equal opportunity habit!;-)


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## SkiBunny (Jan 8, 2009)

*Always entertaining and educational*

Great info. here. These forums rock!


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## 4aprice (Jan 8, 2009)

Fog rips the snow off the mountain but the snow conditions can be just great.  The Sunday after Christmas I skied in the thickest fog I have ever skied in.  It was a crying shame to see the snow melting off the mountain but on the trails where the snow pack was plenty deep the conditions were just beautiful.  It turned into a 30+ run day.

Alex

Lake Hopatcong, NJ


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## cbcbd (Jan 8, 2009)

Just found this awesome post on TAY discussing loss of snowpack related to recent weather events in Snoqualmie Pass, WA - you can find the graph mentioned in the link below:

"I also think that the 30" decrease in snowdepth is almost entirely due to settlement (compaction) and not melting. It's a common misconception (or is it fear, for skiers?) that when rain falls on a snowpack, it must be melting the snow. The fact is that rain, especially when it falls within a few degrees C of the freezing point, is almost entirely incapable of melting snow.

What, you might say? No way...  But the simple reason (if you remember from science class) is the huge latent heat of fusion, which is the amount of heat (= energy) needed to melt a given quantity of snow or ice into liquid. For water, this energy for melting is roughly the same amount of energy as required to warm the same quantity of water by 80 degC, nearly to the boiling point.

So to just barely melt a kilogram of snow at the freezing point (0 degC), we'd have to put it in a pot and pour 1 kilogram (=1 liter) of 80 degC water onto it, and that water would be cooled all the way down to 0degC in the course of melting the snow. If we decided to try using 5 degC (= 41 degF) water, we would need 16 liters of it (80/5) in order to melt the same kilogram of snow. Using 1 degC (34 degF) water, we'd now need an astonishing 80 liters of it to melt the snow.

Now back to snowpack and rainfall: it's easier to think in terms of inches of snowdepth or precip instead of kilograms, so let's switch to that. How much rain fell at Snoqualmie Pass over 2 days, about 10" at close to 41 degF, right? That amount of rain would only be capable of melting 10/16 = 0.6" of snow-water equivalent (SWE). The 74" of snowpack prior to the rain probably had a density of roughly 25%, so it contained about 18" of SWE, and the rain could only have melted 0.6" of that, a fairly insignificant amount.

What about melting due to the actual warm air temperature? Again, air has a very hard time melting snow, even more so than rain because the heat capacity of air is about 4.2 times less than that of water. Therefore 4.2 times as much mass of air is needed to perform the same melting as a given mass of water. So melting a kilogram of snow using air at 5 degC (41 degF) would require 66 kilograms of it (330/5), which is a volume of about 55 cubic meters (almost 2000 cu ft), or the volume of a 12 x 20 ft room with 8 ft ceilings. And all that air would be cooled right to 0 degC in the process of melting 1 measly kilogram of snow. It's easy to see why strong winds are key to melting snow rapidly, because you've got to keep a continual fresh supply of 40 degF air coming through if you're going to melt any significant snow. With calm winds, the air right above the snow is quickly cooled to 0 degC, and it just sits there, incapable of melting any snow at all. The warmer air farther above is insulated from the snow by the denser now-cooled air, and without wind (or solar heating) the cold dense air can't move out of the way.

So two days of 40 degF temperatures probably melted even less of the snow than did 10" of rain at that temperature, but trying to calculate this is not easy. Let's just say it's lost a total of about 1" of SWE out of 18". With the current 44" depth and now 17" of SWE, that gives a density of about 40%, which makes perfect sense. That's the same as the typical springtime density of a well-consolidated maritime snowpack. Instead of having an unusually light-and-fluffy snowpack as we did before this storm, it's now settled to more typical Pacific Northwest density. But very little of it, only a few %, has actually been lost due to melting.

But how can we verify that any of these calculations are even close to correct? We need some way of knowing the actual SWE in the snowpack. Thankfully, even though NWAC doesn't use them, the NRCS SNOTEL network has a SWE sensor at every one of its sites (it's a rubber pillow filled with antifreeze, set flush to ground level, with a pressure sensor to measure the weight of the snowpack sitting on it). We just need to find a nearby SNOTEL site at a similar elevation, about 3000 ft: let's pick Cougar Mountain SNOTEL at 3200 ft (NOT the same as the Cougar Mountain near Issaquah), which has decent data (no big gaps or missing values like most other nearby sites) and this map shows how close it is to Snoqualmie Pass. I grabbed the last 7 days of hourly data and plotted it:



The green line shows that about 10" of precip have fallen the last 2 days, with temps (cyan line) around 40-43 degF. The blue line shows the rapidly decreasing snowdepth over the past 2 days, while the black line is the water content of the snow (SWE). Even though the depth has decreased from 50" to 37", the SWE has barely decreased at all, dropping only from about 17" down to 16". Meanwhile, the density (violet line) at this site has increased from 31% to 44% over the same time. Which illustrates numbers comparable to those calculated above, verifying that the basic point of this post is true: The snowpack at 3000 ft in the Central Cascades has settled during this Pineapple Express, but not melted in any significant way. Despite all the rain, only a few percent of the pre-existing snowpack has melted, well under 10% loss even at this fairly low elevation.

(Note that this SNOTEL site is west of the crest and isolated from any very cold easterly pass flow like Snoqualmie Pass gets, so it makes sense that the Snoqualmie Pass site would have had a much lighter-density snowpack prior to the rain, and thus settled more during the rain. Maybe John, i.e. Stimbuck, has the actually density numbers from his pit?)"

http://www.turns-all-year.com/skiing_snowboarding/trip_reports/index.php?topic=11823.0


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## Marc (Jan 8, 2009)

tcharron said:


> I just didn't get as technical..  Mostly for fear of saying somethbing that is the ballpark idea, but perhaps not totally technically accurate.  :-D  But mostly, the act the the water going from the air and condensing releases energy, in the form of heat, causing more melt, potentially causing sublimation, causing more water to go into the air, potentially causing water to come out of the air, heating the air, etc..etc..
> 
> What I didn't understand is why the act of the snow sublimating didn't 'take' energy from the snowpack, making it colder and more stable.  I still don't quite understand why it wouldn't, but mostly it seems it's easier for it to get the energy from the air, which actually then cools the air, causing the entire process to go house.
> 
> And then there's the simply fact that the fog melting the snow is just like rain adding water to the snow, melting it additionally.



It doesn't rob energy from the snowpack to sublimate because the snowpack is colder than the ice that's just about ready to sublimate.  Heat only flows from higher temp to lower temp unless you add work energy to the system (second law of thermodynamics).


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## tcharron (Jan 8, 2009)

Marc said:


> It doesn't rob energy from the snowpack to sublimate because the snowpack is colder than the ice that's just about ready to sublimate.  Heat only flows from higher temp to lower temp unless you add work energy to the system (second law of thermodynamics).



See, I'm a software engineer, and that sounds like a mechanical issue..  :-D

However, you're wrong.  :-D  The second law states that it will not happen *spontaneously*.  The the water going thru the state change isn't just randomly going thru this change.

If it's 104 degrees F outside, and you sweat, the evaperation of the sweat cools *U*, not the environment around you.  This is mostly becouse it is *easier* to get the heat from you, partially due to to the thermal conductivity of the stuff directly connected to the sweat, versus the thermal conductivity of a gas.  Specifically, anything in a fluid is more sense, and is 'more readily available', then the more parse stuff in the gas.


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## Marc (Jan 8, 2009)

tcharron said:


> See, I'm a software engineer, and that sounds like a mechanical issue..  :-D
> 
> However, you're wrong.  :-D  The second law states that it will not happen *spontaneously*.  The the water going thru the state change isn't just randomly going thru this change.



Yeah... I don't think I'm wrong here.  Heat won't flow spontaneously from low temperature to high temperature, unless work is performed on the system.  That's what I said the first time.  Tell me again exactly where I'm wrong?



tcharron said:


> If it's 104 degrees F outside, and you sweat, the evaperation of the sweat cools *U*, not the environment around you.  This is mostly becouse it is *easier* to get the heat from *you*, partially due to to the thermal conductivity of the stuff directly connected to the sweat, versus the thermal conductivity of a gas.  Specifically, anything in a fluid is more sense, and is 'more readily available', then the more parse stuff in the gas.



This is only because *I* generate heat internally, therefore there is a temperature gradiant from core (high) to sweat drop (low).  So the sweat drop can have heat transferred to it from the body because the body continues to generate heat and therefore continues to support a minimally higher temperature than the sweat drop once it has be secreted.

It will gain some energy from the air, provided the air is hotter than the sweat, as well as from you.  Yes it will get a higher proportion of energy from you, because the resistance to transfer is lower, or flesh, fat and body moisture is more conductive, however you want to phrase it.  That doesn't mean anywhere in that system there is heat flowing against a temperature gradient.

Back to the snow... the only way ice subliming to a warm atmosphere will pull heat from the snow pack is if the ground is warm as well.  In that instance, the temperature gradient through the snow pack allows for that transfer to take place, but that would only happen early season since ground doesn't warm sponteously from beneath (unless there's a volcanic source present).

So mid season with a stable snowpack, the energy to sublime still must come from the air, because the gradient through the snowpack starts high at the air interface and goes to low at the ground... and as we know because of the 2nd law, heat can't flow against a temperature gradient without a work energy input.


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## mondeo (Jan 8, 2009)

tcharron said:


> However, you're wrong. :-D The second law states that it will not happen *spontaneously*. The the water going thru the state change isn't just randomly going thru this change.


You're thinking of the wrong definition of spontaneous. In science/engineering, spontaneous is more along the lines of #5 from merriam-webster online, "developing or occurring without apparent external influence, force, cause, or treatment." Spontaneous combustion, for example, is not just random combustion but combustion that results from the internal heat from the object. Such as a stack of hay starting on fire because the chemical/biological release of stored energy during the course of decomposition.


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## mondeo (Jan 8, 2009)

Along the lines of cbcbd's post, with the right conditions I could see where rain could actually improve the base. Rain will add water and melt snow at the surface, but once it's at 32°, it does no more damage. Given cold enough rain and a deep and well conditioned enough base, I suppose it would be possible for it for the rain and whatever snow it melts to take long enough to leach/drain through the snow pack for colder weather to follow and freeze the snow pack before the additional moisture escapes.


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## jaja111 (Jan 8, 2009)

My $.02 is direct sunlight being the worst to snow. No science here, its just always seemed that way when outdoors. Seems as though the infrared radiation directly on the snow, no matter what the ambient air temp, kills the snow the fastest.


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## mondeo (Jan 8, 2009)

jaja111 said:


> My $.02 is direct sunlight being the worst to snow. No science here, its just always seemed that way when outdoors. Seems as though the infrared radiation directly on the snow, no matter what the ambient air temp, kills the snow the fastest.


If snow were a black body on a flat surface on an average day in Vermont, direct sunlight would be enough to melt 46kg/sq m per day, assuming no other heat transfer. That translates into about half a cm layer of water. In reality, fresh snow only absorbs 10-20 of sunlight' energy, maybe 50% with old snow.


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