The Puget Sound Convergence Zone

Suppose that when the bell for lunch rings in your classroom everyone immediately jumps up and runs for the door as fast as possible to be first in the lunch line. (Your teacher doesn't allow this--and for a very good reason!!) You would end up with some kids climbing over other kid's backs. In other words, not all kids can fit though the door at the same time (since it is too narrow). Some would have to go UP -- kids climbing over other kids.

Now imagine two groups of kids running along the outside of your school building toward the same corner. You would probably get a similar effect. This could be called CONVERGENCE. In the atmosphere, we sometimes have the same thing happening.

On the day of our snowstorm (Tuesday, December 18, 1990), we had cool moist air streaming in from the West just before and during the time it was snowing. The eastward flow of air from the Pacific first meets the Olympic Mountains. The air must do one of two things--go over the mountains or around them. The air follows the easiest path, which is around the left (North side) and the right (South side) of the Olympic Mountains.

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• Where are the two streams of air likely to meet?
  

Notice the convergence around the Olympic mountains. A Convergence Zone Animation (45K) is also available.

The animation showed two streams of air converging and forming clouds. We see that convergence can mean air rises. When air is lifted, it becomes cooler. In general, clouds form in rising air, increasing the possibility of precipitation (rain, snow, etc.). Enhanced clouds and precipitation can occur where the two streams of air mentioned above meet.

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. A Snow Depth Animation (80K) is also available.

• Explain why this happened in relation to the Puget Sound convergence zone.
  

The Puget Sound region has unique geographic characteristics. These characteristics provide differing weather conditions throughout the region.

To explore more about the unique geography of the Puget Sound region, go to Terrain of the Pacific Northwest.

Return to the "Where Did The Moisture That Turned Into Snow Come From?" page.