Weather and Topography
Air quality is determined primarily by how much pollution is emitted and how much dispersion (or air movement and mixing) there is. Weather patterns, topographical features, and climate all affect how the air moves in the Puget Sound, and thus affect how clean our air is.
The Puget Sound is a unique part of the country. No other region in the United States at this latitude has weather as moderate, with mild temperatures and few serious storms. Our weather is largely a result of maritime influences and topography. The jet stream supplies us with air and weather from the Pacific Ocean, which gives us fresh air, a mild climate, and in the winter, lots of rain and storms - which helps keep pollution from building up.
We also sit in a valley formed by mountain ranges to the east and west. The Olympic Mountains can force air coming from west to divert and squeeze through the Strait of Juan de Fuca, which can cause stronger winds and precipitation (called a convergence zone) in the Puget Sound. The Olympics and Mt. Rainier can also block the prevailing winds and create stagnant zones on their leeward sides.
Another important driver of the air quality in our region is temperature inversions. During an inversion, colder air is trapped below warmer air. There is little wind and the air can't mix with cleaner air above, like it would normally. During wintertime inversions, air quality tends to decline very quickly and can reach unhealthy levels in a matter of a few hours.
Air inversions are common in our region and can lead to elevated pollution levels.