I noticed that in the PNW in USA and Canada, the rainy season is in the winter whereas the summers are relatively dry. However, move a bit inland to the interior of Washington/BC, and the rainy season becomes summer whereas the winters are fairly dry.
What causes the flip?
In: Earth Science
Rocky mountains changing prevailing wind patterns and temperature swings. For most of the interior more water gets dumped into the atmosphere when it’s hot and thus it rains more then. In the PNW the heat actually dries everything out as the wind blows all the moisture back out to the ocean. In the winter the more moisture can make it inland to create the steady drizzle that is common in the winter.
The Puget sound is a low spot between the high spots created by the Cascades and the Olympic peninsula. Is basically forms a bowl. Or at least a long wide valley.
The colder air gets the less water it can hold, the warmer it gets the more water it can hold.
Air is cooled going up mountains and heated coming down mountains because of the change in pressure.
In the winter the ocean and some extent the water in the sound is warmer than the air. This releases water into the air but it’s water the gator can be cold. As a consequence the water falls out of the air quickly and it rains here in the Puget sound area.
In the summer the air is hotter than the water. The air drinks up the water and is still warm enough to hold even more water. So it doesn’t rain anywhere near as much except on the coldest days.
Then the air blows up into the Cascades. Clouds form over the mountains and there is some rain. But the condensation of the water vapor into clouds actually heats the air. So blowing over the mountains does not completely dry the air.
But you’re still at a higher altitude so the air still can’t hold all the water because it’s continuing to cool.
As it cools more water rains out releasing more heat into the air as the water condenses which lets more of the water continue on farther east and so forth.
Once the cycle runs out and you start going down the eastern slopes of the Cascades you basically hit the high desert. Sometimes water gets there but not as much as you get in the mid cascade region which takes up most of Washington State.
So basically the terrain pumps warm moist air out of the Puget sound basin during the summer because the air is hot enough to hold the water long enough to escape the basin.
During the winter the air doesn’t quite get enough energy to hold on to the water for very long, so as the heat dissipates into space during winter the rain falls almost as soon as the clouds form and we end up getting winter rains in the basin.
Your premise just isn’t true.
* [Yakima, WA](https://en.wikipedia.org/wiki/Yakima,_Washington#Climate): Driest month July, wettest December.
* [Spokane, WA](https://en.wikipedia.org/wiki/Spokane,_Washington#Climate): Driest month August, wettest November.
* [Bend, OR](https://en.wikipedia.org/wiki/Bend,_Oregon#Climate): Driest month September, wettest December
You have to go further north, to places like [Kelowna, BC](https://en.wikipedia.org/wiki/Kelowna#Climate) (driest month February, wettest June, but Kelowna’s preciptation is much more distributed through the year) or [Kamloops, BC](https://en.wikipedia.org/wiki/Kamloops#Climate) (same) to find wet summers.
The reason for the seasonality is the migration of the [subtropical ridge](https://en.wikipedia.org/wiki/Horse_latitudes), which moves north in summer and south in winter, but doesn’t get far enough north to dry out British Columbia (which lies in the wet band north of it during the summer).
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