Welcome. We're going to look at the processes that add and move heat from the snow surface.
I'll comment on some of the persistent weak snowpack layers formed primarily by the energy exchange
and their role in slab avalanche formation.
Most of the radiation from the Sun that penetrates the atmosphere is shortwave radiation.
So the terms solar radiation, shortwave radiation, and visible light are sometimes used interchangeably.
When there are clouds, some of the shortwave radiation reflects off the clouds.
Some gets through the clouds as diffuse shortwave radiation.
When this diffuse radiation reaches objects at the surface, it casts no shadows or only indistinct shadows.
On days when there are no clouds, direct shortwave reaches the Earth's surface and casts distinct shadows.
When there are no clouds, more energy from shortwave radiation reaches the Earth's surface than on a cloudy day.
When shortwave radiation, either direct or diffuse, reaches the snow surface, about 90% reflects off the snow surface.
This is why you can get a sunburn on the underside of your chin while moving over the snow.
This percentage that is reflected is known as the albedo.
The albedo varies with the size of snow grains, moistures, and particles on the snow surface, but is typically around 90%.
Fresh dry snow can reflect about 95%, which is why on sunny days with fresh dry snow, you may find yourself squitting, even while wearing sunglasses.
So, if about 90% of the shortwave radiation reflects off the snow surface, that means about 10% is absorbed.
At the grain scale, part of the shortwave radiation is reflected in seemingly random directions towards other grains and part is absorbed.
The decrease in shortwave radiation with depth is called extinction.
As a ballpark number, only about 10% of the absorbed shortwave radiation penetrates deeper than 10 cm.
If you cut the roof of your snow cave thinner than about 25 cm during the day, you'll notice some light that is shortwave radiation getting through the thin areas of the roof.
Shortwave radiation that is absorbed warms the upper snowpack rapidly, so when the sun comes over the ridges, the first direct shortwave radiation will cause fast warming of the upper snowpack over many cm.
And Ed Laschappel reminds us that any rapid change in the thermal energy state of the snowpack can contribute to avalanching.
More on that in another video.
Now, when Laschappel talks about the thermal exchange, he is correct.
I have not been as correct when I talk about warming.
Let's take a side trip to talk about the effect of energy transfers on ice, water, and water vapor.
Here is a graph of what happens to ice or dry snow when heat is added to it.
Let's start with a kilogram of ice or snow at minus 100 degrees Celsius.
When you add heat energy, shown on this axis, the ice warms up.
When the ice gets to the melting point, it absorbs heat, a lot of heat, and stays at 0 degrees while it melts.
This heat that is absorbed in the melting process is called latent heat.
The latent heat of melting is the difference between heat energy stored in liquid water at 0 degrees and the heat energy stored in ice at 0 degrees.
And it's a big difference. At 0 degrees, each kilogram of liquid water contains more heat energy than is required to warm ice from minus 150 to 0 degrees.
If we continue to add heat to liquid water, it warms up along this slope.
To see what happens, if we continue to add heat energy, let's switch to a bigger graph.
Now, as we add heat energy to the liquid water, it warms up along this slope until it reaches its vaporization point at 100 degrees Celsius.
Then it plateaus again as liquid water absorbs all this heat energy which is required to vaporize a kilogram of liquid water.
Add more heat energy and the water vapor is warmed along this slope.
So far, I've been talking about adding heat to a kilogram of ice, water, or water vapor.
If heat is being released, the graph looks the same, but the terminology and the direction of the arrows change.
This latent heat transfer is now called condensation, and this latent heat transfer is now called freezing.
So these plateaus, when our kilogram of water or ice changes its state, but not its temperature, are called latent heat transfers.
These slopes, when our kilogram of water or ice changes its temperature, but not its state, are called sensible heat transfers,
because we can sense or feel these changes in temperature.
So in the rest of this video, when I informally refer to warming, the added heat energy could be increasing the temperature of the snow or melting it.
And when I informally refer to cooling, the release of heat energy could be decreasing its temperature or freezing it.
This is tricky stuff, so let's pause and answer a few questions before we get back to the energy exchange of the snow surface.
Here are two questions. Press the pause button at the bottom of your screen while you read and think about the questions.
When you're ready to see the answers, press play.
Question one. Clouds reflect that is block some of the sun's shortwave radiation from reaching the snow surface, so this is true.
More shortwave radiation reaches the snow surface when the sky clears.
Question two. This is also true. The rain at zero degrees contains latent heat that dry snow at zero degrees does not.
This latent heat is greater than the heat required to warm ice or snow from minus 150 degrees to zero degrees Celsius.
Let's get back to the sketch of the snowpack.
Everything radiates according to its temperature. The sun is hot and emits a lot of shortwave radiation.
The snow surface is cooler and emits diffuse longwave.
This longwave radiation, which is also called infrared radiation, is the same as the heat we feel from the heat labs in some fancy hotel bathrooms.
Now, the snow's emissivity is around 0.99, so it radiates like a black body.
The net direction of the diffuse longwave radiation from the snow surface is upwards.
The greenhouse gases in the atmosphere, including water vapor, carbon dioxide, and methane, absorb some longwave radiation and re-emit it in all directions.
On a cloudy day, water droplets in the clouds absorb and re-emit more of the longwave radiation than greenhouse gases.
Since part of this is downward, some longwave radiation cycles up and down below the clouds.
Thus, the greenhouse effect is strong on cloudy days, resulting in the air temperature where we measure it being close to the snow surface temperature.
On a clear sky day, much more longwave radiation is emitted by the snow surface than comes back down from the greenhouse gases.
This causes the snow surface to be cooler than the air where we measure it, sometimes 8 or more degrees Celsius cooler.
This outgoing longwave radiation is from the snow surface and not from below the snow surface.
At night and on shady or north facing slopes, this strong outgoing longwave radiation often creates a temperature gradient near the snow surface, which can cause fastening.
Now, let's consider a level or sunny slope on a sunny day.
Since the absorbed shortwave radiation warms the snow centimeters below the snow surface and outgoing longwave radiation will cool the snow surface, the warmest temperature typically occurs a few centimeters below the snow surface.
This means there is a temperature gradient in the top few centimeters.
In this thermal image of the pit wall, the snow 3 centimeters below the surface is 3 degrees warmer than the surface.
That is equivalent to a temperature gradient of 10 degrees over 10 centimeters or 100 degrees over a meter, which is 10 times the gradient necessary to cause fastening of the snow grains.
A strong gradient like this can cause near surface fastening on clear days. Note that the orange just above the snow surface in the thermal image is longwave radiation from the trees in the background.
Since the trees are well away from the surface we are looking at, they are not warming the surface near the pit wall much at all.
So, getting rid of the thermal image that was stealing the show, here are the key components of the radiation exchange that affect the snow surface.
Since I have mentioned near surface fastening, which can form a persistent weak layer, I should also mention surface whore.
When the sky is mostly clear and there is not much wind, the surface cooling can cause water vapor to deposit on the snow surface as surface whore crystals.
Surface whore formation is especially common on clear nights and on shady or north facing slopes where incoming shortwave is minimal.
It is another persistent weak snowpack layer that is primarily a result of the radiation exchange.
So, I have said the snow surface is cooler than the air when the sky is clear. But how much cooler? Let's look at a graph.
This graph shows the difference between the air temperature and the snow surface temperature for various sky conditions.
It is based on a small number of measurements I made while cross country skiing in the last two winters.
The snow surface temperature was measured with a handheld infrared thermometer.
Under clear sky, the snow surface was often 8 to 10 degrees cooler than the air, showing the strong effect of outgoing longwave radiation to clear sky.
Under overcast conditions, the snow surface was only 0 to 2 degrees cooler than the air.
With few scattered or broken cloud cover, the temperature difference varied widely, but generally fell between the values for clear sky and for overcast sky.
Time for some more questions.
Here are two more questions. Press pause while you read the questions and think about your answers. Then press play to continue.
Question 3. Yes, longwave radiation from trees can warm the snow surface near the trees.
We saw the longwave radiation from trees in the background of the small thermal image of the pit wall.
Question 4. Certainly near surface fastening can occur on north facing and shady slopes when the sky is clear and especially at night.
However, on clear days, on level terrain or sunny slopes, the strong incoming shortwave radiation and strong outgoing longwave radiation can create a temperature gradient and cause fastening in the top few centimeters.
So this is false.
Back to our sketch of the energy exchange.
Let's shift our focus away from the radiation exchange and consider wind.
Wind causes convection, which, depending on the air temperature, can warm or cool the snow surface.
This heat transfer increases with wind speed.
Warm winds can melt the snow at and near the surface.
Since less heat is transferred by convection as wind speed decreases, by the time we get to calm conditions, there is no convective heat transfer.
So that leaves only conduction of the still air next to the snow surface.
Heat transfer by conduction is so slow that we can't ignore it.
On some cloudy days, we have snowfall.
The snow that is falling can be warmer or cooler than the upper snowpack, hence it can add or subtract heat from the upper snowpack.
However, the temperature of the falling snow is usually not so different from the temperature of the upper snowpack before the storm, so this is a weak part of the energy exchange.
In contrast to snowfall, rain can add considerable heat to the upper snowpack.
That is because, as we saw in the first graph, rain contains latent heat that dry snow lacks.
Sure, rain at plus five adds more heat than rain at zero degrees, but this is a small increase compared to the latent heat in rain.
When rain-wetted snow freezes, it forms a rain crust, which can persist in the snowpack for the entire winter as a potential bed surface for slab avalanches.
Also, weak layers of faceted crystals can grow above or below a rain crust or other kinds of melt-freeze crusts, especially when those crusts are near a cool surface.
But woe now, I'm getting away from the energy exchange at the snow surface.
So far, we have been talking about the energy exchange on the level snow surface.
I'll just briefly mention how the energy exchange depends on the aspect or direction of snow slopes.
When the sky is clear, the sunny slopes will get much more shortwave radiation than the shady slopes.
Wind can warm or cool the snow surface on windward slopes more than on leeward slopes, that is, slopes that are downwind.
Rain usually has a similar effect on slopes with different aspects.
However, when there is rain with wind, the windward slopes can get more rain and hence receive more heat energy than the leeward slopes.
Since falling snow is usually a similar temperature to the upper snowpack, an even distribution of snow on a windy day usually has only a weak effect on the energy exchange.
Under clear skies, outgoing longwave radiation will cool slopes with different aspects similarly, unless trees or terrain blocks part of the sky view, which is the fraction of the sky that the slope faces.
For example, this slope, with a clear view of the sky, will tend to have a higher net loss of longwave radiation and cool more than this slope, which has its sky view partly obstructed by this cliffy slope.
Some longwave radiation from the cliffy slope will reach the sunny slope here, reducing its cooling by net longwave radiation.
Okay, the energy exchange at the snow surface is complex and especially complex when sloping terrain is considered.
Check out the swarm video to see the effect of radiation on the upper snowpack over simplified terrain as quantified by Laura Bakerman's swarm model.
However, this video is mostly qualitative. It refers to strong and weak effects.
The following components of the energy exchange can, under the right conditions, have a strong effect on the snow surface.
The incoming shortwave, the outgoing longwave, rain and wind.
Considering these effects helps us to anticipate the presence of some important snowpack layers and their possible distribution over terrain, even before we enter snow slopes.
Thanks to Mike Smith of Northwind, Avalanche and Weather Services for his review and feedback on an early version of this video.
And congratulations for watching the full length of this video.
