A primer on snow

Brief description

In this activity students explore what atmospheric conditions are needed for snow and develop an understanding for snowpack and how snowpack characteristics can change through time.

Background for Teachers

For this lesson your students will read about how snow is formed in the atmosphere and then look at atmospheric temperature charts to determine whether there will be snow, rain, freezing rain or sleet.  They will also read about snowpack and watch a video before diagramming a vertical snow profile. Finally, they will need to understand a bit about density, particularly snow density. All of the changes to snow (given below) will change the snow’s density. As you go out with your students this winter and collect snowpack data you will be calculating the density of the snowpack (or individual layers of the snowpack). The teacher’s version of the density worksheet gives all of the calculations you may need.

Snow forming in the atmosphere.

We need two things to get snowfall instead of freezing rain, sleet, or rain:

Moisture- It cannot be too cold for snow to form, but it can be too dry. Water from the ocean, lakes, rivers, plants and soils are taken up into our warm atmosphere and as the warm air rises and is moved by atmospheric currents it cools and becomes saturated. Cold air does not hold much water vapor (so very cold places without a source for warm air picking up moisture will not have a lot of snow… an example being the dry valleys in Antarctica).

Cold air- For snow to form and make it to the ground, the atmospheric temperature must be cold both up in the clouds where snowflakes form all the way to ground level. If the air between where the flakes form and the ground is above 0 degrees C, the snow will melt on its way down, changing to rain, sleet or freezing rain.

NOAA summarizes the effect of warm air on moisture to understand what we’ll get for precipitation this way:

Figure 1: from www.weathergamut.com, attributed to noaa.gov

In the eastern part of the United States, we can also get snowfall when warm air that brings moisture from the Gulf of Mexico rides up over (or is pushed up over by) cold air.

 Falling snowflakes are actually called snow crystals – regardless of their shape – by scientists. Snow crystals take on different shapes depending on – you guessed it – the temperature and moisture conditions in which they fell. (For more information see: http://www.weather.com/video/all-about-snowflakes-33779, or http://www.its.caltech.edu/~atomic/snowcrystals/)


A lot happens to snow once it hits the ground.

Snow is dynamic; it forms and falls but it does not stay in its fallen form for long. We have caught snowflakes on our mittens before, seen beautiful crystals, and played in fluffy snow. We have gone out the next day to find a somewhat less fluffy snowpack and two days later we have gone out to a dense pancake of snow. Even if the temperature has not changed, the snowpack is undergoing a change. Each individual snowflake is undergoing Destructive Metamorphism; a change from delicate crystal with a lacy shape to rounded ball. Through wind and constant settling the delicate snow crystals break and, much like sharp glass is buffeted by the ocean into smooth seaglass, the crystals become rounded. Once the flakes have become rounded they can fuse by freezing into larger crystals in a process called sintering. Snow crystals resulting from destructive metamorphism compact easily and form a snowpack, or part of a snowpack, that is hard and dense.

Figure 2. The destructive metamorphism of a stellar snow crystal. From: A Field Guide to Snow Crystals by Edward LaChapelle.

When it begins to warm and air temperatures rise above freezing, the ice crystals at the top of the snowpack melt (if the snowpack is thick). (But please note – during a big spring snowmelt when the snowpack begins to really disappear, snowpack melts from the bottom up, not the top down!) The water percolates downward through the snow pack and could refreeze. This is called the Melt-Freeze Metamorphism. Sometimes, during these warming periods the snow forms into a wet granular layer sometimes called "corn snow”. When freezing temperatures set back in the corn snow layer can harden into an icy sheet. Sometimes a melt-freeze event results in a hard, crusty layer that lighter weight people can walk on, but heavier people break right through.

Settling, destructive metamorphism, or melt-freeze metamorphism can all happen to the same snowpack. We can then get fresh snow, which will undergo its own changes, and affect changes to the snowpack below it. Therefore, not only do a lot of changes happen to snow once it hits the ground, but collectively a lot happens to the snowpack.

We end up with a layered snowpack, where layers have different densities, which can tell us something about the history of winter.

Figure 3: The backside (sunny side) of the snow pit has been cut away to allow light to pass through the snow layers (from: http://blogs.scientificamerican.com/expeditions/2012/04/12/snowstar-2012-hoars-and-drifters/)


Students understand:

  • That atmospheric conditions regulate whether we get snow or another form of precipitation
  • That snowpack is constantly changing, and layers form during the winter
  • Density of snowpack is an important characteristic


  • Students state the atmospheric conditions needed for precipitation to fall as snow
  • Students explain how different layers of snow can form in a snowpack
  • Students calculate the density of different snowpacks

Where does this lesson happen in the Project?

This is the second lesson in the project.

Getting Ready

Ask your students why we sometimes get snow in the winter and sometimes get freezing rain, or sleet, or rain. Maintain a list of all answers.

Ask your students about walking through snow. Ask if any of them have ever walked on top of snow or fallen through. Discuss what makes good snowballs, and what kinds of snow make poor snowballs. What snow is easy to shovel and what kinds of snow are hard to shovel? Maintain a list of all comments.

Ask your students how they would calculate density of snow.



Snow forming in the atmosphere

Websites: http://www.srh.noaa.gov/ama/?n=preciptypes (NOAA National Weather Service Weather Forecast Office Amarillo, TX)

http://nsidc.org/cryosphere/snow/science/formation.html (National Snow and Ice Data Center)

Worksheet: Atmospheric profiles


Readings from: The Snow Booklet: A guide to the science, climatology and measurement of snow in the United States  (pp. 8-13)

Video on website: http://www.fsavalanche.org/Default.aspx?ContentId=6&LinkId=12&ParentLinkId=9 (Forest Service National Avalanche Center)

Video: http://www.youtube.com/watch?v=9JQwO5iApbo (Forest Service 2013-01-29 Tuckerman Ravine: snow layers before the rain)

Worksheet: Diagramming the snowpack


Readings from: The Snow Booklet: A guide to the science, climatology and measurement of snow in the United States (pp. 14-19)

Worksheet: Calculating the density of snowpack

Student Prerequisites

Time needed

One class period

Doing the Activity

Snow forming in the atmosphere

Students read the information on the websites listed above and, using the atmospheric profile worksheet, decide and explain what type of precipitation will hit the ground with each profile.


Students read excerpt from The Snow Booklet: A guide to the science, climatology and measurement of snow in the United States and watch the National Avalanche Center video for a background understanding on how different layers of snow form.

They then watch the Tuckerman’s Ravine video and diagram the snowpack (as narrated in the video). Students share their diagrams and discuss differences in interpretation.


Students read excerpt from The Snow Booklet: A guide to the science, climatology and measurement of snow in the United States for background understanding on different snow densities and then calculate the density of different snowpack samples. When students collect snow for their own research, they will also be calculating density; this skill is important to master.


Prior to moving on to the next lesson students then fill out the Student Question Worksheet to begin thinking about possible research ideas.



Ensure that students can calculate the density of snowpack and can diagram the snowpack.


Students create a diagram of snowflakes from any particular snowstorm as they change throughout the winter. Explaining the processes that bring about each change. Include how each change affects the density of the snow.

Lesson Extensions and Supplements

Students can visually explore changes in snow cover with NOAA’s National Climate Data Center’s Snow Cover Maps (http://www.ncdc.noaa.gov/snow-and-ice/snow-cover.php).

Students can learn how to take pictures of snowflakes, understand the equipment needed.

Watch the weather and predict the amount of precipitation for any of the microclimates around school (in the woods, in the lee of the building, etc.).


Note: some students may want to research changes in the different layers of snow during field research. Throughout the field season (or from multiple places during one field sampling event) students can dig a snow pit and examine the different layers of snow, diagramming snow layers’ depths and densities as well as crystal structures. You will need a ruler and a hand lens, as well as printed snowflake chart, for this.




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