A region of relatively low rainfall that occurs downwind of a mountain or mountain range.
To the left is a rain fall (precipitation) map showing the relative rainfall in inches on the Olympic Peninsula. The Yellow section covers all of Sequim, this section averages 15" or rain per year or less. You can see why Sequim has an Irrigation Festival and the western half of the Olympic Peninsula has a rainforest.
Sequim averages 15" per year due to the precipitation pattern caused by the storm blocking effect of the Olympic Mountains. Thus, this is how the term "Blue Hole" came about. From the ground it looks as if there is a blue hole in the clouds right above Sequim and the Dungeness Valley, because the Olympic Mountains at almost 7500 feet on average, are blocking the rain from falling on Sequim.
Sequim is known for its mild winters and moderately warm summers. This varied climate allows for participation in activities throughout all the seasons of the year. Sequim rainfall is about the same as Los Angeles or San Francisco California.