Why is Oregon Coast / Washington Coast Foggy When It's Hot Inland?

Published 10/12/22 at 5:15 PM
By Oregon Coast Beach Connection staff

(Newport, Oregon) – There's a long-standing, even worn-out witticism or saying among Oregonians: if it's sunny inland it means fog on the coast. While that's technically not entirely correct, certainly statistically, that does happen often enough to be a frustration to Oregon coast travelers in the summer. In the winter and other seasons, that's definitely not true. (Photo Oregon Coast Beach Connection: Hug Point near Cannon Beach under some fog)

Indeed, there is something to it. While the valley is sweltering in 100-degree heat, the Oregon coast and the Washington coast can get foggy quite often. There are some weather mechanisms that can cause that, and they were seen in action recently this past week or two, as a fog bank clung to the shorelines of Washington and Oregon while the inland areas were having record heat for October.

So why does it get foggy on Oregon coast / Washington coast when it's nice inland?

It's quite complex, but quite a bit of it comes down to the temperature and the pressure differences between the inland and the coast. Essentially, hot air to the east rises, and this causes low pressure there. Meanwhile, the ocean is much colder and that creates high pressure. Higher pressure air likes to move into lower pressure regions, so you get the moist, marine layer of the ocean getting pulled off the top layer of the water. All that moisture is sitting low – basically low-flying clouds. So it gets moved eastward towards the shoreline and beyond.

Other kinds of conditions need to be in play as well, but according to Brian P. Nieuwenhuis of the National Weather Service (NWS) office in Medford, “that's the jist of it.”

As Nieuwenhuis told Oregon Coast Beach Connection:

“Hot air rises. So, when inland areas get very warm, the air above them, as a whole, starts to rise. It doesn't do so quickly, given the large scale and other atmospheric processes, but it does begin to rise. When the air rises, it has to be replaced, so air from other places is drawn in, or 'pulled' in, from surrounding areas. This is what draws the moist, cool marine air into the coast. It is being pulled in, but because it's cold, moist air, it is blocked by the coastal mountains, and therefore mostly stuck at the coast.”

That's why if you're coming from a really hot Tacoma and heading to Long Beach, or coming from Eugene and arriving at Coos Bay, you'll find it socked in.

Yet meteorologists looking at the Oregon coast or Washington coast normally call this “marine pushes,” because it's the wind that gets stirred up and pushes the moist air towards the beaches.

“When the fog moves inland, the wind is pushing it in, as high pressure over the ocean to the northwest sends air towards the thermal trough (area of low pressure due to warm air rising) inland,” Nieuwenhuis said.

A marine layer off Cannon Beach

After all, fog is just low-level clouds. It's the same stuff you see flying high above you.

All this is affected by elements far overhead. If there's a lot of high pressure systems moving above the ocean waters of the Washington coast and Oregon coast, that pressure squashes the marine layer. The higher the pressure above, the lower that fog stays. Then it tends to stick close to the beaches and you get sunny conditions just barely inland.

Like what happened last week. The NWS kept predicting sunny conditions for the Washington and Oregon coast, but you kept having fog right up against the beaches of areas like Cannon Beach, Bandon and Ocean Park. Drive a couple blocks away from the sands and you had the warm, sunny weather the NWS was talking about.

“How far it goes inland, and if it can make it over the mountains, is due to other conditions in the atmosphere such as surface fronts, upper level lows and highs, etc.,” Nieuwenhuis said. “If winds shift to the east, then the fog will dissipate or be pushed back out to sea.” (Also see Manzanita Is Indeed 'Banana Belt' of N. Oregon Coast - Science Behind It )

Those temperature differences between the inland areas and coastal shores aren't as pronounced in early fall, such as September and early October. This creates conditions that make for the best weather of the entire year on the Oregon and Washington coast, called the Second Summer here. This year, of course, that's all a bit out of whack with 80-degree days still in the middle of October.

Nieuwenhuis said most fog along these shorelines is due to the marine layer.

Another fun fact about fog: “To technically be considered fog, it also has to obscure visibility to less than 5/8 of a mile (or less than 1 km),” Nieuwenhuis said.

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