The Physics of Snow

The Physics of Snow

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The Physics of Snow

Credit: Kenneth G. Libbrecht

Here is a nice picture of an actually snow flakethis is not a computer simulation but an actual snowflake

Id like to thank the center for the invitation to talk here tonight as well as Brie and heather for organizing and managing the logistics of the event. I would also like to thank my colleagues Dr. Pat Arnott and Dr. John All for supplying me with some of the slide for tonight's talk

Originally I had intended the talk to be all low key entertainment.

However I have to apologize in advance that I have done a bait and switch on you. I changed up the last part of my talk in light of last weeks data that shows that some of the glacier in the Andes are actually melting at their fast

rate in the last 12 years thanks to this El Nino.

Here is a nice picture of an actually snow flake.

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Sectored plate: note the stretched cavities3


More images many are what one would expect a snow flake to lookBut I want you to notice of the theme, the then is the number 64


Basic shapes

Snow flakes come in 4 basic shapes these six sides prism..The reason for 6 is that is the way water molecules can bond together.6

Cloud Formation

So what are the conditions neccseray for generating snowwell we need clouds. We need condensation.When the horizontal westerly winds encounter the Sierra, we get lift and the air ascend.As the air ascend it expands and cools (expands high to low pressure). If the air happens to cool below the temperature, at which the becomes saturated we get a cloud. If the point at which the air reaches saturation temp is above the freezing point of water we get a cloud of water dropletsI like this drawing who ever the artist was impresses meintentionally or otherwise this drawing is correct a deep levelnotice that the rain is associated a flat bottomed cloud.

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Rain Clouds

If the temperature at which the air reaches saturation is above freezing the cloud is composed of water dropletsThe base of the cloud is flat and located at the height where water condensesbelow that level water vapor, above that level water droplets8

Snow Clouds

No look at snow clouds an little more turbulentIf the temperature at which the air becomes saturated is below freezing, (typical for a snow cloud)There are actually two condensation heights or level one for water and one for ice.the height at which ice condenses is actually below the height at water condenses.(saturation Vapor pressure for water is greater than that for ice).Being the there are two condensation levels in the cloudthe cloud

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Snowflake Formation

You do not form a snowflake by simply freezing a droplet of water!...You have an ice cube tray in your freezer right not a snowflake tray. Recall that ice condenses lower down in the cloud than waterbefore water does and remember that we still have lift that the ice crystals are still traveling upward thorough the cloud.

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Snowflake Formation

https://www.youtube.com/watch?v=AsyTC8wafeM

Once an ice xstal forms it starts scavenging the water vapor around itself. Water vapor that encounters the ice xstal gets frozen to the xstal.This in turn lowers the vapor pressure which cause water droplets to start to evaporate off water vapor which is in turn scavenges up by the ice crystal.

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Snowflake Formation

So here is a video of a snowflake grown in a lab..this is a time laps video of a few 10 of minutes.I want you to take notice both the snow flake and the water droplets around the snowflake.12

Snowflake Formation

So that was a snowflake growing on a surfacein a cloud the initial ice xstal is being lifted through the cloud where is savages water vapor until it is heavy enough that it is no longer liftedat which point it falls and still continues to scavenge water vapor.13

Morphology

The shape a snow flake can take on depends an temperature and saturation profile in the cloud and trajectory the snowflake takes through the cloud

Question so where is man made snow on this slideit isn't

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Morphology

Snow-gun snow

So this stuff does look more like a frozen water droplet, which is exactly what it is.With man made snow You need the water to freeze before it hit the slope, you dont want to be spraying your ski run with water rightwith a snow gun water droplets are suspend for about 15 seconds.so you want them to freeze fastman made snow is not snowflakes it is a frozen water droplet.good snowball snow and real window breaking kind of snowballs15

Things to Ponder About Snow

When teaching students I love to give them paradoxical or Zen questions and then watch them try and squirm out a solution

or questions like did you ever notice that this weird thing happens whenever you blank.

So now its your turn.16

Can Two Snowflakes Be Alike?

NO two snowflakes are ever alike. Right you have heard that. So it is a great romantic notion but is it true?17

That really depends on how alike you want them to be

very small snow crystals can be identical


Large snowflakes can look similar


Here are two snowflakes that were grown side by side in nearly identical conditionsThey look simialr but are they identical?

Nope20

Very unlikely that there have ever been two identical snowflakes.

If earth is taken to be 3x109 years old

Then approximately 1034 snowflakes have formed during that time.

A snowflake contains 1018 molecules so there is 1018! ways to form a snowflake.

Very unlikely there have been two identical snowflakes.

Where does the white go when the snow melts?

So after the snow melts in the spring, why we are we not outside sweeping up white dust everywhere.

Ice/snow is transparent, right? Where does the white come from?

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Snow, salt and sugar are transparent but all appear white, why?

Here is a single plastic transparency23

Multiple internal reflections lead to an opaque, white appearance

But notice what happens once you start to stack the transparencies collectively it is less transparent....What is going on is basicalcally at every interface between air and snowflake or air sand sugar xstal, there is a 4% reflection.

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The Hush of SnowIs it really quieter when it snows?

The Hush of Snow

US Army Snow Acoustic Test

Why do ski tracks sometimes have a washboard appearance?

This is a tough one but basically this happens with any material that can be displaced laterallythink of skipping a stone on water.

Granular material it depends on you velocity and the how compacted the material is and once the patterns starts it grows.

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Why Study Snow?

Okay originally this talk was going to move on to skiing and snowboarding but in light of last weeks report that the glaciers in the Andes are melting out faster than they have in the last 12 years..I decided leave the skiing behind and to get a little dark on you.

Here I am making a snow measurement on Tocllaraju in the Peruvian Andes. Okay seriously why would anyone want to study snow?

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Why Study Snow? To better understand :

The Physics of Crystal FormationClimate Atmospheric Chemistry OceanographyBiology

So all of these disciplines benefit from studying snow

And my background isnt in any of these things, my background is in optical engineering and optical sciencespecifically space flight optics and instrumentation. 29

American Climber Science Program: The American Climber Science Program (ACSP) works to link scientist and climbers to help provide the knowledge necessary to preserve the alpine environment. The ACSP led expeditions to the Cordillera Blanca of Peru in the summers of 2011, 2013 and 2013.

I got involved with snow through the American climber science program..

These folks are trying to understand snow pack and water quality issues in the Peruvian Andeswhere the glaciers are melting out faster than anyplace else on earth.

I got involved for two reasons: one to help development optical instrument to analyze snow pack and glaciers

And two because I have done a lot of mountaineering in the Andesand could help provide safety for researches that have an interest in alpine environments but not a lot of experience mountaineering.

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American Climber Science Program Focus:

Develop field protocols for research teams that include scientists , students, locals and citizen-scientists.

Collect and store baseline data from which informed decisions can be made about resource management.

Develop partnerships between scientists, policymakers and the local population.

Educate the community through outreach.

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Peruvian Andes

The Lower level glaciers in the Andes are melting 2 to 3x faster than glaciers in other parts of the world.

It is estimated that the glaciers below 5500m are not going to be around in 20 years. And no one really knows why?

And as of last week we now know that thanks to this El Nino these glaciers are melting fast than they have in the last 12 years!

Scientifically speaking this is terrible loss, as the melting of the glaciers is basically wiping out a 20,000 year record of atmospheric and climate data...

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It terms of the humanitarian aspect this could be a disaster

As with California the local agriculture and population are really counting on the snow pack to supply them with water through the year.

water supply and water quality issues as well as hydropower shortages are already an issue. In the 2008 the water level in Laguna Paron fell below the dam creating a shortage in water in electricity in the city of Carazthis resulted in riots.

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Black Carbon Aerosol Concentration on Glaciers

The precise reason why these glaciers are melting out so much faster than other glaciers around the world is unknown

And will not be unsolved until there is more regional data

One thought is that it could be black carbon from human sources, i,e. pollution from burning fuel. Dark aerosols from human pollution are being laid down on these glaciers cause them to absorb more solar radiation and therefore melt fastI am involved with trying to make these kinds 34

Uganda: Mountains of the Moon

The same thing is happen for other lower latitude glaciers such as the ones in Africa.In 2005 Cam Burns, Charlie French and myself traversed The Ruwenzori mountain range: the mountains of the moon, the fabled source of the Nile. To discover that the Speke glacier is basically just a remnant ice-field35

Once gain the local and local economy is based on the the assumption that there is a reliable snow resiviour to get them through the year.36

Snow Pack, The Canary in the Coal Mine?