View up the Willamette River toward downtown Portland on a summer morning. To the left/west the sky is clear and sunny; to the right/east the sky is cloudy and foggy. This weather is neatly divided by the river itself

River effect

Published 2015-05-28

A common Portland summer weather pattern is a dense layer of low clouds (but not usually fog as in California’s marine effect) forming late in the night, then boiling off by late morning.

Well, it’s common on the flatter east side of Portland, which is bounded by two large rivers on the north (Columbia) and west (Willamette). The west side is mostly hills, and the water there drains mostly into small tributaries of the Willamette. These hills take the shape of a table tilted to the southwest: the steep northeast side of the table (the legs) trends northwest to southeast. It forms the western edge of downtown and the eastside. Those hills fence in the clouds that form over the confluence of the rivers.

When we lived in Southwest Portland, I would experience this weather pattern in habitually underdressing for my morning commute. It was clear skies and sunshine overhead at our house after all! But then I’d be chilly and damp by the time I arrived downtown. This was most acute at our place near the top of Hummingbird Hill on its shallow southwest face. Some days I could stand at 11th and Hume, and see the edge of the clouds kissing the top of the hill around 8th and Hume.

Anyway I fully expected never to experience this pattern in our new place in flattish southeast Portland near to the Willamette. So this morning when I saw bright sun and clear skies over our house, I assumed it would be thus all the way to Richmond. I was wrong.