Weather Blog

Why has it been so foggy this summer?

Why has it been so foggy this summer?
Another foggy day at Ocean Shores. (YouNews contributor mikeyworld)

Some morning clouds and fog are not out of the ordinary for a Northwest summer, but this summer it seems the fog horns have been working overtime.

In Seattle proper, since July  there have been 19 days with fog at some point, of which 7 have had dense fog with visibility of 1/4 mile or less.  Over on the coast, it's been worse with Forks reporting 32 days of fog (out of 38) with 14 days experiencing dense fog. Forks averages 13 days from July 1-August 31 so they still have three weeks to add on to that.

Put another way, Seattle has averaged 4.6 hours a day with fog -- nearly triple the average of 1.6 hours. Forks has averaged 10.1 hours a day with fog (nearly half the day!) whereas they're supposed to get 7.3 hours.

In addition, we've received e-mails from a viewer in Port Angeles and along the western edge of Whidbey Island claiming they've lived here for ages and have never seen this much fog in the summer.

Turns out, they might be on to something….

According to Dr. Jim Johnstone with the UW's Joint Institute for the Study of Atmosphere and Ocean, the coast of the Pacific Northwest just set a July record for strongest average northerly wind component off the coast at 4.62 meters per second or about 10.3 mph. The previous record was 4.4 m/s. 

(The wind was courtesy of a ridge of high pressure that remained anchored in the northern Pacific Ocean through much of June and July, driving a general northerly wind our way. )

10.3 mph is not a lot of wind, but what that persistent wind does do is cause upwelling. The winds carry water off the ocean surface and push it away, much like putting a fan blowing across a dish of water would do.

This process then causes deeper water to rise to the surface to replace the blown-away water. Since deeper water has not been exposed to the sun, it is typically colder and the overall process can locally drop ocean surface temperatures.

Sure enough, take a look at this chart, which the dark blues and purples show sea surface temperatures were running around 3 degrees Celsius (about 5F) below normal for the period just off the Washington and Oregon coasts.

Note that the entire Pacific Coast has been colder than normal, and not surprisingly, just about all coastal locations up and down the West Coast have been foggier than usual this summer, Johnstone said. We noticed earlier that San Diego and coastal Los Angeles have been chilly since June. In fact, LAX has only been warmer than 75 degrees four times since June 1, and all four days were consecutive.

Now, this upwelling process is occurring all the time off the Pacific Coast -- it's the primary reason why our ocean waters are always relatively cold at around 50-55F through the year. But this summer the process has really been going strong.

So the colder-than-usual ocean waters could be a culprit, or at least a significant factor in the persistent fog. A colder water pool makes it easier for fog to form, as the water has more cooling power to bring the warm summer air above the water down to the temperature needed to condense into fog.  Factor in a fairly persistent pattern of westerly surface winds (the "marine flow") and you've got more fog out there to draw from, and a mechanism to blow it inland.

And, you've got a lot of very tired fog horns :)

Dr. Johnstone has done extensive research on coastal fog and you can find more information about his research and get updated fog statistics at www.jajstone.com