Questions and Extensions


  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.


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