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Fog eats snow. Or does it?

post #1 of 28
Thread Starter 

The mountain is enshrouded in fog and I hear people saying that fog eats snow.  I take it that they think the level of the snow will go down because there is fog out there.  Really?

 

Does the snow level decrease at a faster rate when there's a cloud covering the mountain, independent of air temperature?  

Does some interaction between the foggy moisture and the blanket of snow on the ground cause the snow to melt and run off, or vaporize and rise up to join the fog?  

 

Does anyone know the mechanics of this phenomenon, or know for certain that it is just an old skier's tale?

post #2 of 28

Fog may be a sign that there is an inversion (these go hand in hand), so it is an indication that the snow is getting eaten due to the inversion.

 

So correlation but not necessarily causation.

 

i'm not a weather guy, but that's my guess.

post #3 of 28

I thought fog = storm cloud cloaking the mountain = more snow. I have many memories of this mystical phenomenon to prove it yahoo.gif

post #4 of 28

Fog is a byproduct of the snow being eaten.

 

I for one just got back here to NH from west and am positively stoked on the weather tonight as I want to start mtb season asap. RIP snow

post #5 of 28
Quote:
Originally Posted by JoeUT View Post

I thought fog = storm cloud cloaking the mountain = more snow. I have many memories of this mystical phenomenon to prove it yahoo.gif

 

That's the way it looks like it works around here.  We see a lot of fog and a lot of snow, at the same time.

post #6 of 28

Depends on temp, you can have freezing fog that accumulates on stuff during storms.  I would imagine that fog during above freezing temps would add moisture to the snow pack like rain does which eats snow.

post #7 of 28

I don't believe that fog eats snow.  I've never noticed it around here and don't understand a mechanism that would cause it.  What does eat snow is a "warm" dry wind.  I believe the term Chinook means "snow eater" in Inuit.  These winds, the Chinook, Mistral, Santa Anna...  Are caused by an air mass passing over a mountain range.  On the way up the air mass decreases in pressure with altitude and cools causing the relative humidity to rise past dewpoint dropping moisture on the windward side thus decreasing the absolute humidity of the air mass.  On the way down the leeward side of the mountain range this now drier air mass warms as the pressure increases with decreased elevation.  The relative humidity of the dry air mass plummets with the temperature increase and a vapor pressure is created on the snow surface that causes the snow to rapidly sublimate.  You may never see liquid water or run off and the actual temperature may even be below freezing, but the snow just goes away.   

post #8 of 28

My understanding of how this works (coming from an avy awareness class)...

 

The snowpack is constantly radiating heat upwards. Remember that the snow is heated not only by the sun, but also by the ground underneath it.

 

Cloud cover reflects the radiation back into the snow, keeping the snowpack warmer. It essentially acts as a layer of insulation.

 

Now, this was presented in an avy course specifically talking about night time, and how overnight cloud cover can affect snowpack- I have a hard time thinking that at the same temperature, fog would eat snow faster than direct bright sunlight, but it probably does so faster than a day with a high cirrus cloud cover.

 

I certainly notice the effects of clouds on snowpack around here. We usually have snow on our roof.  When it is above freezing after the sun sets on a clear day, we get a little bit of dripping melt. On cloudy warm nights, there is still a ton of melt happening.

post #9 of 28

The local FOX CT (or was it NBC CT) weather guys explained a few weeks ago that the fog formation was a result of the dew point of the surrounding air being just right (close to the actual air temperature) to cause condensation near the cold snow mass (thus locally colder temperatures near ground level), which is why the fog seems to be only near the snow and not elsewhere.

 

The following article describes it:

 

http://www.usairnet.com/lc/help/?q=dps

post #10 of 28
Thread Starter 

Anachronism,

 

I'm trying to understand how your post works with daylight fog on the mountain.  If a cloud is sitting on top of the snow causing your average white-out, and if at the same time the snow is warmer than 32 degrees for whatever reason, the cloud/fog will act as an insulating blanket assuring that the snow will stay warm and it will continue to melt out below its surface.  That makes sense to me, especially if the air above the cloud/fog is colder than the snowpack.

 

But often there is fog on the mountain when the air temperature is below freezing.  If the insulation factor is the reason people have come up with the saying that "fog eats snow" then fog won't eat snow on a 20 degree day.  It will protect the snow from the sun.  

 

Does this sound right to people?

post #11 of 28
Thread Starter 
Quote:
Originally Posted by tetonpwdrjunkie View Post

I don't believe that fog eats snow.  I've never noticed it around here and don't understand a mechanism that would cause it.  What does eat snow is a "warm" dry wind.  I believe the term Chinook means "snow eater" in Inuit.  These winds, the Chinook, Mistral, Santa Anna...  Are caused by an air mass passing over a mountain range.  On the way up the air mass decreases in pressure with altitude and cools causing the relative humidity to rise past dewpoint dropping moisture on the windward side thus decreasing the absolute humidity of the air mass.  On the way down the leeward side of the mountain range this now drier air mass warms as the pressure increases with decreased elevation.  The relative humidity of the dry air mass plummets with the temperature increase and a vapor pressure is created on the snow surface that causes the snow to rapidly sublimate.  You may never see liquid water or run off and the actual temperature may even be below freezing, but the snow just goes away.   

 

tpj, sounds like you know what you are talking about.  But I am still getting confused about dewpoint.  Can you explain when this process would cause white-out fog sitting quietly on a mountain, without any wind, to make the snow sublimate? 

post #12 of 28

 I think (please correct me if I am wrong)  the warmer airmass covers the snow, and then  is cooled to the point where  the fog forms.  Droplets then are released from the warmer fog at ground level releasing latent heat. This process melts the snow.  So fog "eats" snow.   

post #13 of 28
Quote:
Originally Posted by LiquidFeet View Post

 

tpj, sounds like you know what you are talking about.  But I am still getting confused about dewpoint.  Can you explain when this process would cause white-out fog sitting quietly on a mountain, without any wind, to make the snow sublimate? 

I can't, because it doesn't.  The snow gets eaten on the leeward side of the range because the relative humidity of the sinking air mass is far below dewpoint.  the moisture is pulled out of the snow across a strong vapor pressure gradient.  There are no clouds or fog under these conditions.

 

I don't believe that fog eats snow.  It can degrade the surface of the snow through warming and the surface might absorb some moisture from the fog, but I don't think the rate of this transfer is rapid enough to really melt much snow.  In my experience the two things that really eat snow are direct warm sunlight and "warm" dry winds.  Even rain doesn't eat snow quite as fast as these other factors.  At a certain point the snow just gets wet and the water percolates through the pack and runs off.

 

BTW Liquidfeet, the snow can't be warmer than 32 degrees.

Ozski, the air must be at or below dewpoint for fog to form.  It can't be "close enough".

post #14 of 28

Dusting off weather and chemistry from grades 9 and 10...

 

Fog is caused by warm air sitting on top of colder ground (or snow).  What happens is the cold ground (or snow) cools off the air to the point that the water vapour in the air condenses into water droplets.

 

A gram of water vapour has more energy than a gram of liquid water, which has more energy than a gram of ice.  You will notice that when you put an ice pack on your injured shoulder; as the ice melts it sucks thermal energy from you shoulder making it colder.  If you have ever beened burned by a jet of steam, you will know that steam gives off a lot of energy when it condenses.

 

When ice or snow melts and becomes a liquid it takes heat (absorbs thermal energy), when the liquid evaporates and turns into a gas it takes heat (absorbs energy).  When vapour condenses it gives off heat (releases thermal energy), when liquid freezes it gives off energy.  Ice turning into water at 0 C takes [Spock voice] approximately 333. 55 [/Spock voice] Joules; water turning to water vapour takes about 2257 joules per gram.   It's the same amount of heat going the other way.

 

What happens to the thermal energy of the water vapour when it turns to tiny water droplets in the fog?  for every gram of water vapour turning into fog, 2257 joules of thermal energy goes into the air, energy that could melt over 7 grams of snow.  It heats up the surrounding air, and the surrounding air gives up its energy to melt the snow. 

 

So, yes fog makes snow melt, but it doesn't eat it.  The water won't want to evaporate into the already saturated air; it will erode the snow pack from underneath.

 

Warm dry air might evaporate snow faster than fog can melt it though.

post #15 of 28

The snow is colder than the air, the snow melting.
 

I like the idea of the fog eating the snow, sounds more fun, as long as the fog isn't eating skiers! popcorn.gif

post #16 of 28
Quote:
Originally Posted by Ghost View Post

Dusting off weather and chemistry from grades 9 and 10...

 

Fog is caused by warm air sitting on top of colder ground (or snow).  What happens is the cold ground (or snow) cools off the air to the point that the water vapour in the air condenses into water droplets.

 

A gram of water vapour has more energy than a gram of liquid water, which has more energy than a gram of ice.  You will notice that when you put an ice pack on your injured shoulder; as the ice melts it sucks thermal energy from you shoulder making it colder.  If you have ever beened burned by a jet of steam, you will know that steam gives off a lot of energy when it condenses.

 

When ice or snow melts and becomes a liquid it takes heat (absorbs thermal energy), when the liquid evaporates and turns into a gas it takes heat (absorbs energy).  When vapor condenses it gives off heat (releases thermal energy), when liquid freezes it gives off energy.  Ice turning into water at 0 C takes [Spock voice] approximately 333. 55 [/Spock voice] Joules; water turning to water vapour takes about 2257 joules per gram.   It's the same amount of heat going the other way.

 

What happens to the thermal energy of the water vapour when it turns to tiny water droplets in the fog?  for every gram of water vapour turning into fog, 2257 joules of thermal energy goes into the air, energy that could melt over 7 grams of snow.  It heats up the surrounding air, and the surrounding air gives up its energy to melt the snow. 

 

So, yes fog makes snow melt, but it doesn't eat it.  The water won't want to evaporate into the already saturated air; it will erode the snow pack from underneath.

 

Warm dry air might evaporate snow faster than fog can melt it though.

 

Ghost is correct.  The thing is that the energy that the vaper gives off to condense into fog is a one time thing.  It happens at the moment that the fog condenses and is gone.  When the fog changes back into vapor, the enegry is reabsorbed.  I think that the effect of this energy exchange would impart less energy into the snow pack than direct solar radiation would.  In my opinion this implies that the net effect of fog is to insulate the snow.  In the atmosphere, the moisture and the temperature remain in equilibrium relative to each other and I don't think the ground absorbs a significant amount of this energy.  In fact if there was enough moisture colliding with cold air, the individual fog droplets would increase in size until they got heavy enough to fall out the atmoshere and it would then be raining.  Rain does more damage to snow than fog.  

 

I'm basing this on my college chemistry, I took a fair amount.  We did fractional distillation experiments where a mixture of different liquids is heated and the time/temperature graph is plotted as the different components of the mixture are vaporized, condensed, and collected.  The temperature flat-lines during the time that each fraction is being collected.  Even with surplus energy being pumped into the system, the temperature of the mixture can't increase past the individual vapor points until each liquid fraction has been fully converted to vapor.  Each collected component can be identified by looking up the vapor point in a reference manual.

 

In the case of fog this works in reverse.  The air (oxygen, nitrogen, and other things) can't warm up while the vapor is condensing.  If it does, the fog will burn off.  When we see fog, the temperature is at or below dew-point or we wouldn't see fog.

 

The other thing that is needed in the atmosphere is a condensation nuclei.  This is usually a piece of dust or something like that.  When there is a surplus of moisture relative to the temperature and not enough condensation nuclei to accommodate this moisture, the atmosphere becomes supersaturated with super-cooled water vapor and in the winter we get rime growing into the wind.  The supercooled water vapor flash freezes onto any cold object it encounters.  The rime ice grows into the wind.

 

Also warm dry air doesn't evaporate snow, it sublimates it.  Evaporation is the change of state from a liquid to a gas.  Sublimation is the direct change of state from a solid to a gas.  Dry Ice sublimates from solid CO2 to CO2 vapor at room temperature.  That's why its called "dry" ice.

post #17 of 28

The supercooled water vapor flash freezes onto any cold object it encounters.

 

If I may.............. supercooled water vapor comes into contact with any object and heats up from friction and then freezes to said object. You may mean the same thing and I misunderstand you.

 

So when you see the bottom of Cumulus Nimbus at the same height which it will always be that's also the D.P.

post #18 of 28

Wow, is it that time of year already? That is, time for ongoing threads with long-winded, technical debates over simple questions incidentally related to skiing? I thought we had at least another month or two. 

post #19 of 28

I ski in the fog a lot, and the fog absolutely condenses within the snowpack.

 

Powder snow is mostly air. 90% air or better. Snowflakes typically can have "wet" edges; they are often not just solid ice. Meaning liquid water is on the surface of the ice crystal.

 

There is water vapour in the air space of the snowpack and when the fog rolls in, the wet edges of the snowflake are a cheap place for the vapour to condense.  

 

As the fog rolls out, the equilibrium shifts, and the liquid edges of the snowflake vaporize. Hopefully enough to bring back the blower. 

smile.gif

 

 

There are lots of variables at play, and different climates will experience different weather phenomenon. 

post #20 of 28

I'm a meteorologist and just happened upon this discussion.  Water droplets in fog are large compared to typical ambient water vapor.  Large particles support lower vapor pressure.  This sets up an upward directed vapor pressure gradient resulting in net evaporation/sublimation.  Relative humidity is irrelevant.  Snow pack sublimates rapidly in fog.  Warm, dry winds are effective, but not optimal.  There is also a second order process at work.  Water releases heat when it condenses.  The thin layer of air against a snowpack is prone to cooling to it's dewpoint (i.e. condensing), thus releasing heat into the snowpack.  The primary driver, however, is the upward directed vapor pressure gradient due to the large size of fog droplets.

post #21 of 28
Here's a page from an old school aviation met text book. The picture seems familiar so I must have seen it about 30 years ago. Fog 101 in terms simple enough for aircrew to understand. Back to the OP, I don't think fog would eat snow, certainly not as bad as warm winds or direct sunlight but I don't see how persistent fog cover wouldn't degrade the snow cover over time.

http://www.aviationweather.ws/067_Fog.php


We get a lot of fog on the east coast of Canada, most of what I've seen this winter is ice fog in the harbour and on the coast. Now that it's March we'll see it more and more over land and it will be eating the snow because for us the season is done by 1 April (sadly)
post #22 of 28
You'd be surprised. If you want to melt snow, give me 40 and foggy over 50 and sunny and I'll win every time. It is counterintuitive for the layman, but it's physics at work. Read up on the role of vapor pressure gradient in phase change if you want to nerd up on it.
post #23 of 28
Nope, I will happily take the word or a meteorologist.
post #24 of 28
Since our normal season is cold with heavy fog and a way better snow pack and this season has been warm and sunny and hideous snowpack, I'm going with the idea that overcast and fog preserves our snow. But that's just based on personal experience. I'd go with the idea that there are other variables at work that explain why we have a better season when it's foggy, and yet the very BEST season when it's clear but really cold. I'd say clear skies are no indicator of the snow, but overcast skies reduce the impact of the sun when it's spring temps. And overcast skies and fog CAN mean it's creating rime, snow ghosts, goggle problems, and.. a thicker base (the rime grows on the hill, too, I have pictures) when it's cold.
post #25 of 28

Cabbage's got it- vapor pressure is a huge driver. 

Related the the 2nd-order effect he mentioned is that liquid droplets in the fog have a higher specific heat capacity than dry air- about 4 times as much.  Gram for gram, air heats up/cools down 4x faster than liquid water, given the same initial temperature difference with the heat source/sink.   And a gram of liquid water takes up far less space than a gram of air- so far more mass with far more heat capacity can serve as the heat source/sink in the same space.

 

Think about standing outside on a 60deg day, then do the same in a 60deg pool- which makes you colder faster?  The initial temp difference is the same between your body and the air/water.  But water will stay cooler longer as you radiate and conduct heat into it, and the temp difference is what allows the net energy transfer to continue.  The exact reverse happens to snow when its touching a lot of liquid water- the liquid simply has more energy to conduct and radiate into the snow than dry air does, and can maintain any temp difference longer, sustaining the energy transfer.

 

 

post #26 of 28
@TheCabbage, could you comment on sibhusky's response? I'm glad you brought up vapor pressure gradient, because the concept had faded to a mere ghost in the "weather trivia" portion of my mind, and I do believe that fog eats snow faster than sun at least when air temps are near or above freezing. But does the principle hold in cold temperatures, such as socked-in storms with temperatures far below freezing? Of course such storms deposit snow, but do they also degrade the existing snowpack?
post #27 of 28

@litterbug sibhusky is right that there are a lot of variables.  All else being equal, you will maximize sublimation/evaporation of snowpack in fog.

 

The sun obviously has an impact too, especially at lower latitudes.  Very cold temperatures also reduce the amount of water droplets in the air if it's foggy, so sublimation is not as pronounced.  For example, if you're at 20 degrees below zero and it's foggy, you obviously aren't going to wake up to a disappearing snowpack.  The nature of the snowpack itself can also be important.  A snowpack that has been through multiple freeze-thaw cycles and has become glaciated (e.g. icy/crusty on the top) will be more resistant to sublimation because the molecular structure is stronger (think of an ice cube compared to a pile of snowflakes and which one you would intuitively expect to break down more easily).  Rime is unlikely to contribute any significant amount to an existing snowpack, but it is still possible that in sibhusky's climate the better snow conditions occur when it's foggy.  After all, ski slopes have a lot of snow on them.  If conditions are in favor of good skiing with the single exception of some fog, then the snow base will withstand that just fine.  It's important to remember than the dominant process in fog is sublimation and not evaporation.  The direct phase change from solid to gas of the snow means that there is no melting and thus no liquid to make the snow wet/heavy/mushy.

 

Just because you lose snowpack most effectively in foggy conditions doesn't necessarily mean that foggy conditions are the worst skiing conditions.

post #28 of 28
Snow ghosts:
IMG_5929.JPG

When I get back from skiing today, I'll turn on the PC and find pictures of rime growing on the trail and my ski poles while I was at lunch. It also grows on the chairlift. When you see the amount it adds to the trees, which can happen several times a season - all shed, then regrown - you'll realize it's not a miniscule amount of "snow". It's often dropped onto the ground below rather than into the air.
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