Snowstorms in Western Washington

Teacher Background

The goal of this module is to investigate the factors involved in the occurrence of snowfall. Some of the content will relate to the unique geography of Western Washington. Students will examine data (weather observations) obtained during a snow event which occurred in the Puget Sound area several years ago (December 18, 1990). They will collect "snow stories" about this or other memorable snowstorms and conclude the module by forecasting the likelihood of snow.

A significant snowstorm occurred on December 18, 1990 which affected neighborhoods in North Seattle as well as communities east of Lake Washington such as Bellevue, Redmond, and Issaquah. Snowflakes fell most of the day but did not "stick" until early afternoon when a blast of cold Arctic air arrived from the North. The roads throughout the city became glazed as the temperature plummeted, causing city-wide paralysis. You may recall horror stories of people being stranded on busses for many hours, school children spending the night at school, and commuters being forced to check into downtown Seattle hotels due to the total transportation gridlock.

This module is composed of an opening computer-based activity and branches to numerous classroom-based activities. The first four pages of the module are: Snowstorm Hits Seattle, Snow Stories, Tell It All, and What Makes It Snow. They are intended to engage students in issues related to snow, both fun and troublesome, to provide a personal context. Students branch to Cold or Moisture to explore the physical causes of snow more closely. Some of the activities within each of these branches use data from the December 18, 1990 snowstorm for concept development.

Students are given the opportunity to participate in the process of analyzing the weather in a manner similar to that used by a professional meteorologist (Weather Observations For December 18, 1990). Also, a simple experiment showing what happens along the 3-dimensional boundary between cold and warm air masses is suggested (Cold And Warm Air Masses On The Move).

The module concludes with a page named Questions and Extensions. This page, along with other "input" pages throughout the module are used to gauge how well the concepts were understood and to give extending activities. You can keep records of students' inputs by printing the pages, copying them, and having students answer the questions in the spaces provided.

Introducing the Module

Engage students in the first part of the module for about twenty minutes. They should explore the audio and links to gather background and interest. Use your experience from regular classroom activities to determine depth, pace, etc. Students should understand a few basic concepts before entering the unit: snow is a form of "precipitation;" experiments always have a factor of uncertainty; and science is a method of collecting and analyzing information to compare with an hypothesis. Consider these options for introducing the module:

Students interview other people in Snow Stories to gather information about others' experiences related to snowstorms. Have students interview at least three people outside the classroom who have relatively clear memories about a snowstorm (adults are preferred). Students may interview in person or by telephone. Print and reproduce the form for the students or have them copy it themselves.

Weather Map Plotting Exercise

A milestone in the development of modern meteorology occurred when the invention of the telegraph made it possible to assemble weather observations from a number of geographically dispersed sites at a central location. The reports could then be plotted on a map in real time and thereby make possible an instantaneous look at the condition of the weather at the time of observation.

In the following exercise students will examine weather observations actually made during the day of the snowstorm and plot them on a series of weather maps. The exercise can be carried out in different ways:

  1. Individual assignment -- Blank base maps are given to each student and the weather observations are "plotted." The resulting maps can be displayed for all to see by posting them "museum" fashion, making a bulletin board of the clearest ones, or on overhead transparencies. Consider this an opportunity for student self and peer assessment.
  2. Cooperative Groups -- Students are divided into groups in which student reads off the information to be plotted and another plots the data on the blank base map. Students may switch jobs occasionally so that everyone participates. Those not working can be "checking" the finished map as it is being produced and helping locate the various stations (cities and towns where the observations were made).

Teacher Preparation of Map Plotting Exercise

Begin by reading the student directions for the exercise (Weather Observations for December 18, 1990). They contain instructions for plotting a series of maps for that day. Depending on your browser, your view of the base map may run off the screen. One remedy for this is to print a reduced version by selecting 90% under the print set-up "Reduce or Enlarge" option. A postscript version is also available for those who can use that format. Make enough copies of the base map for each individual or group to have three blank maps.

Students make three maps, one for 4:00 am, one for 1:00 pm, and one for 10:00 pm. Students plot the station data according to their invented method or by the method scientists use. Click here for the symbols used in station plots.

Note--students can use a program that plots symbols and therefore makes the exercise interactive. The program can be downloaded from: WxGraphics

Answers to questions within the module

In Snowstorm Hits Seattle!! --

Q--Write about a snow storm you remember. Include both the "fun" part and parts that were not fun.
A--Students should have some experience with snowstorms and be able to relate various aspects. On the "positive" side may be school closure, building snow persons, sledding, etc. On the "negative" side may be such things as getting cold, missing something fun (such as a party or a visit to a friend's house), being stranded, or losing something in the snow.

Q--What do you think causes snow?*
A--This is the first of many times this question will be asked. Students may have informal ideas about snow formation, which should be more refined as they progress through the module. The kind of answer we expect students to eventually give will (hopefully) be like: "The atmosphere must be cold (below freezing), have moisture, and have particles (CCN). Snowflakes will form on the particles within a cloud and grow in size as they fall though other ice crystals and supercooled water."

Q--Is it snowing somewhere in the world right now? Check this reference to see: . Where is it snowing? How do you know?
A--The online resource is the best place to look, and will list "snow" in the column titled "WEA" (weather). Newspapers may also be used as a resource.

Q--One family created a Web page devoted to their experience with a blizzard. Check out the Blizzard in Delaware: . What strategies did this family use during the blizzard?
A--Answers will depend on the version of this resource students see. It will be updated "regularly" with newer snowstorms, but look for answers like "shoveling and changing travel plans."

Q--Deicing solution: . What is there about snow and ice that makes flying dangerous? A--Snow and ice build up on the wings of airplanes and decrease their lift. This can happen while on the ground and while flying (the air is much colder at flying altitudes). The ice also adds weight to the airplane.

Q--Find out the location of snow or ice in North America. Look at satellite pictures from DMSP Snow and Ice: Where is there snow or ice in North America?
--Caution: This site has many links that may take students off the question at hand. Be sure to check out this link first to familiarize yourself with its contents and potential for distraction.
A--Answers will depend on the version of this resource students see. It will be updated "regularly" with each satellite downlink photo. Look for white patches.

Q--What's the weather like in Seattle right now? Check this picture and make a guess:
A--Answers will depend on the version of this resource students see. It will be updated approximately every minute. Students have a chance to practice being a meteorologist while being handicapped with only one bit of information (what is looks like outside).

In Tell It All!! --

  1. List some problems people had because of the snowstorm.
    A--Answers will vary on interview information. Possible responses include transportation, closure of important resources (bank, store, etc.), and losing items in the snow.
  2. What was the most important problem to the kids?
    A--Answers will vary on interview information. Responses need to be connected to question #1.
  3. In what ways is having snow fun?
    A--Answers will vary on interview information. Possible responses include missing school, sledding, constructing, and playing.
  4. In what ways is having snow NOT fun?
    A--Answers will vary on interview information. Possible responses include those related to inconvenience and fear.
  5. What is needed to make it snow?
    A--Moisture and freezing temperatures are necessary for snow to happen. This is virtually the same as the question (marked with an asterisk) on the first page. Students' expertise will probably not be developed enough yet for many to arrive at this answer.

In What Makes it Snow --

Q--Why does ice form on the glass (in the freezer) and not as crystals that fall out of the air (like snow)?
A--The ice needed a "surface" on which to form. The glass provided that surface. The atmosphere has particles known as "cloud condensation nuclei" (CCN) upon which vapor deposits to produce ice crystals or water droplets. "Snow" does not form in the freezer because there is essentially no vertical depth within the freezer. The ice particles have virtually no room to fall during their formation.

In Cold And Warm Air Masses On The Move --

  1. Where is the air moving into the freezer
    A--Near the top.
  2. Is the air moving into the freezer warm or cold? How do you know?
    A--Warm air is moving into the freezer. Air from the room is entering the freezer and room air is warmer than freezer air.
  3. Where is the air moving out of the freezer?
    A--Near the bottom of the freezer.
  4. Is the air moving out of the freezer warm or cold? How do you know?
    A--Cold air is moving out of the freezer. Air from the freezer is entering the room and freezer air is colder than room air.
  5. Summarize what you know about the movement of warm and cold air.
    A--Cold air tends to push its way under adjacent warmer air. The warm air moves over the top of the sinking cold air.

In What Makes The Air Turn Cold? --

Q--There are several different ways the air in an area can change temperature. List at least two ways the outside air can become colder. Be sure to list specific causes. Discuss your list with your classmates. Input your consensus below:
A--There are numerous ways air can become colder. (1) Air can be replaced by cooler air that has moved in from another location. This is called advection. (2) Air can cool by being in contact with the ground. The ground lost its heat to space, called radiational cooling, which is typical of clear nights. (3) "Lifted" air expands because of the lower pressures it encounters. The expansion of lifted air causes it to cool. (4) Cooling occurs when evaporation is present.

In The Puget Sound Convergence Zone --

Q--The eastward flow of air from the Pacific first meets the Olympic Mountains. It follows the easiest path, which is around the left (North side) and the right (South side) of the Olympics. Where are the two streams of air are likely to meet?
A--Near and around Seattle, the central part of the Puget Sound region.

Q--Here is a map of the "snowbelt" for the December 1990 storm. Notice where the snow was deepest. Also, notice where snow did not fall. Explain why this happened in relation to the Puget Sound convergence zone.
A--The "snowbelt" happened because of the two streams of air coming around each side the Olympic Mountains. The streams "collide" in the central Puget Sound region and miss other areas. The region where the two air masses meet will experience stronger cloud development and enhanced precipitation. This region is often referred to as the Puget Sound Convergence Zone (PSCZ). The Convergence Zone does not always happen in the same place, and it may move during a storm. Compare the wind patterns with the snowbelt. A "shadow" effect from the Olympics resulted in no snow for some locations. The convergence of the air masses was greatest in other locations, resulting in heavy snow accumulations. The most snow fell where the convergence was strongest.

In Where Did The Moisture That Turned Into Snow Come From?--

Q--Locate the source of the water that becomes snow in the Puget Sound region. Look on the map of the Pacific Northwest. Where is the largest body of water located?
A--The Pacific Ocean. It is directly West of the region.

Q--Assume water in the air has evaporated from this large body of water. Evaporated water is in the form of water vapor. From which direction must the wind blow to carry this moisture to the Puget Sound area?
A--The wind must blow from the West. Note the emphasis on the direction. Meteorologists describe winds from their "source."

In Particles --

Q--If you have a pencil that uses lead that is .5 mm thick, how many of these particles (CCN) could be lined up side by side across the width of the lead?
A--500 particles would be needed to be as thick as a .5 mm pencil lead.

In What Do The Weather Maps Tell Us? --

  1. At 4:00 am were temperatures at all 7 stations above or below freezing?
    A--All the stations were above freezing.
  2. At 10:00 pm were temperatures at all 7 stations above or below freezing?
    A--All the stations were below freezing.
  3. Which wind direction(s) accompanied the above freezing temperatures?
    A--The winds were from Southwest to Northwest.
  4. Which wind direction(s) accompanied the below freezing temperatures?
    A--The winds were from North to Northeast.
  5. As the day progressed, what happened to the temperatures at the different stations on the map?
    A--The temperatures dropped as the day progressed.
  6. How did the air in Seattle "get cold?"
    A--The air over Seattle was replaced by colder air.

In Where Did The Moisture Come From?--

Examine the weather maps you made (from the page Weather Observations For December 18, 1990). Use the maps to answer the questions about moisture in our area and where it came from.

  1. What was the wind direction BEFORE it began to snow?
    A--The wind was blowing from the Southwest or West.
  2. What was the wind direction WHILE it was snowing?
    A--The wind was blowing between the Southwest and Northwest.
  3. What was the wind direction AFTER the snow had stopped?
    A--The wind was blowing from the North.
  4. Would the air from the land be moist or dry?
    A--The air will be relatively dry.
  5. Would the air from the ocean be moist or dry?
    A--The air will be relatively moist.
  6. From where do you think the moisture for the snow came?
    A--The moisture ultimately came from the Pacific Ocean.

In Questions and Extensions --

  1. What is needed to make it snow?
    A--Moisture and freezing temperatures are necessary for snow to happen. Students should have a better answer to the question by now than earlier in the module.
  2. Moist air comes toward the Puget Sound region from the West (which is where the Pacific Ocean is located). The surface of the ocean off the coast is usually around 45° F in the wintertime. What does this tell you about the possibility of it snowing down at sea level? Remember the air near the ground wants to take on the temperature of the underlying surface.
    A--The possibility of snow at 45° F is remote. Snowflake can still form in the upper atmosphere but will melt on the way down and turn into drops. This process is common, and can even happen in the Summer when very tall clouds release precipitation. Rain often results from melted snowflakes, we just never notice the flakes because they melt long before reaching the surface.
  3. Dry air may come down from the interior of British Columbia into the Puget Sound region. This air often is well below freezing but it usually has very little moisture because it had dry ground under it rather than the ocean. What does this tell you about the possibility of getting rain or snow out of it?
    A--The possibility of snow from dry, cold air is remote. The cold air must encounter wetter air for precipitation to result.


The following are optional. They extend the content of this curriculum module and require time and resources beyond the primary scope of this module.

Q--How much did it snow? Pick the location, time interval, and information source. The Web can be used as a resource for this information.
A--The answer will vary according to time of year, location, etc. Look for students to connect such variables as proximity of large bodies of water, prevailing winds, latitude, and topographic features. Have some students look into high latitudes to challenge the misconception of continually heavy snowfall in polar regions.

Q--What percentage of snow is liquid water? Is that figure always the same?
A--The answer will vary greatly. The intent of this question is for students to see that not all snow is equal.

Q--What atmospheric signs indicate the likelihood of snow? Develop a "rating" method for the signs that reflects your confidence in each one as a predictor.
A--Accept whatever students create and test it over time. This question will allow students to see how difficult it is to predict snow and to produce a potentially useful tool.

Q--At what speed do snowflakes fall? Is the speed connected to the air temperature?
A--Students' process is important for this question. If students depend on their own reflexes their variation may be more than the snowflakes themselves. The rate of fall of snow will vary according to flake size and mass. Typical speeds will be from about 0.5 m/s to 3 m/s.

Q--How does the annual snowfall of polar regions relate to mid-latitudes? Try to control for variables like how far the land you examining is from the nearest ocean.
A--Answers will vary, but polar regions will generally have less snowfall than mid-latitudes.

Q--Gather data on how air temperature varies with altitude. Try to obtain the data directly using tall buildings, model airplanes and rockets, etc. Remember to control for the effects of radiation from surfaces near your thermometer, wind speed, motors, etc.
A--Students are developing the "lapse rate" of the atmosphere. It is about 6.5°C per 1000 m (about 3.6°F per 1000 feet). These values vary according to such factors as humidity.

Discussions of Important Concepts

Phases of Water

One of the difficulties in teaching how snow forms is related to our use of the word "water." We usually mean "liquid water" when we say "water." In general, water can exist as a solid, liquid, or gas. Water's phase is mostly dependent on its temperature and pressure.

"Ice" is water in its solid phase. The water molecules organize into a crystalline structure and form a variety of shapes, which are mostly temperature dependent. The following table describes the crystal types which can form in the air and grow into snow:

Environmental Temperature
°C °F Crystal Form
0 to 4 32 to 25 thin plates
4 to 10 25 to 14 columns
10 to 12 14 to 10 plates
12 to 16 10 to 3 dendrites (traditional "snowflakes")
16 to 22 3 to 8 plates
22 to 50 8 to 58 hollow columns

Please note: the temperatures listed above relate to the air. Upper air temperatures are usually much colder than surface air temperatures. Temperatures typically decrease 6.5°C per 1000 m (about 3.6°F per 1000 feet). That is why temperatures will be suitable for snowflake formation at cloud altitudes (from 1000 m to 6000 m, 3300 feet to 20,000 feet) when the surface air temperature is above 0°C (32°F).

We are most familiar with making solid water by freezing liquid water in a cold environment (a freezer, of course). Snow is typically made through a different process--water vapor "deposits" on particles at altitudes where temperatures are well below those found near the ground.

"Vapor," the gaseous form of water, is invisible and odorless. Water vapor cannot be sensed directly but is always in the air. Lots of water vapor in the air (high humidity) can result in "bad hair days" or that "muggy" feeling.

Water can exist as a liquid, solid, or a gas for many typical temperatures. The maximum amount of water that can exist as a gas above a surface of liquid or solid water is called the saturation vapor pressure. The maximum amount of water vapor that can be in the air increases as temperature increases. The saturation vapor pressure above ice is different (lower) than over liquid water.

The Formation of Clouds

A key idea to understanding the way snow or rain forms is that clouds form in air that is cooled, usually because it is rising. This is linked to the idea of saturation (discussed above). As air rises in the atmosphere, it enters regions of lower pressure, and it will expand and cool. If it cools enough, it may reach saturation. This means that it will contain the maximum amount of water vapor that it can, at that temperature. Further cooling will result in condensation--water vapor will turn to liquid or it will turn to ice if the air is cold enough.


Students will explore various aspects of weather, like precipitation and air movement, from this curriculum module. Some details of weather, like cloud formation, are mentioned but left for future modules to develop more fully. This module is successful if students end up with many more well- formed questions than they had before.

Other weather resources can be found on the World Wide Web:

University of Washington Department of Atmospheric Sciences

UIUC Online Guide to Meteorology

University of Michigan Weather Underground

The Weather Channel

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