1. What is needed to make it snow?
   
   


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.
   
   


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?
   
   

We have seen that it takes moisture in the atmosphere at low enough temperatures to produce snow. The Puget Sound region is a sort of "crossroads" where moist air and cold air were able to meet. On 18 December 1990 we had the unusual situation where a mass of air over the Puget Sound area had enough moisture and was cold enough to produce snow when it was lifted by the action of the convergence zone. This was then followed by a push of much colder (but also drier) air which allowed the newly-fallen snow to linger on and create a major problem in many areas.

Extensions

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

• How much did it snow? Pick the location, time interval, and information source. The Web can be used as a resource for this information.
  
• What percentage of snow is liquid water? Is that figure always the same?
  
• 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.
  
• At what speed do snowflakes fall? Is the speed connected to the air temperature?
  
• 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.
  
• 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.