Freezing levels are probably the most misunderstood topics.
The best place to start is to tell you how these numbers are obtained. Every day across the United States the National Weather Service does upper air soundings by using weather ballons. These ballons contain technical weather instruments that measures air temperature, winds, and other data from above the surface into the upper atmosphere. In Washington State there are readings from Forks and Spokane. One of the major numbers taken is the freezing levels.