Over Salem, Oregon there is a marine polar airmass most of the year including May
Air masses form as air gathers in a specific area, and all the air has stable conditions such as a similar temperature and humidity. Air masses are often large as they cover multiple kilometers.
Moreover, the air masses can be:
Salem, Oregon is in the west coast of the United States, in this area, the air masses are Marine due to the location near the ocean and Polar because the air is cold. Moreover, the location of this air mass is quite constant, this makes it likely the Marine Polar mass is over Oregon most of the days of the day.
Note: This question is incomplete because the year is not specified; however, I can answer it based on general knowledge
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