A clean boundary between clouds and sunny skiesAugust 3rd, 2012 by cdunagan
UPDATE, Aug. 4
Gary Peterson got back to me with detailed observations, which I’ve added in a block quote below.
Gary Peterson of Seattle was headed north Saturday in his sailboat Windswept, a Perry Custom 47, when he spotted this abrupt break in the clouds off Seattle’s Alki Point.
One of the two ferries in the distance was half in sun and half in shade.
I was able to get some meteorological ideas about what caused this phenomenon, but first these observations from Gary:
“I noticed up ahead the sky looked like it was starting to clear a little, which was a welcome sight after a couple hours of motoring due to the lack of winds. As I approached the Alki Lighthouse, the separation of clouds from the clearing sky was so distinctive that it was borderline eerie. I couldn’t help but have the theme tune from the old TV program ‘Twilight Zone’ run through my head.
“The seas were dead calm with not a whisper of wind in the air… As the skies were turning blue and the sun was burning off, the remaining clouds to my starboard, it seemed the front was headed for the hills (Olympics), creating the seemingly black and white to color experience I took in the photo.
“It was a beautiful, yet eerie, experience with the warming feeling of the sun as I entered the brighter side of my voyage slowly pealing off my sweatshirt to feel the warmth of the sun. The waters were still glassy, but the skies were blue, the sun was out and the seagulls were flying around and singing their nautical tunes. Life was good.”
To see if we could come up with a scientific explanation, I contacted Nick Bond, a researcher with the University of Washington’s Joint Institute for the Study of the Atmosphere and Ocean.
“It is rare to see such a clean break in the clouds,” Nick told me, adding that such a boundary was probably the result of two air masses coming together from different directions.
Consulting weather records for July 28 at 11:22 a.m. (when the photo was taken), Nick said it appears that moist marine air, accompanied by clouds, was pushing in from the south, coming through the lowlands between the Olympic and Cascade mountains. Meanwhile, another air mass coming over the Olympic Mountains lost its clouds as it dropped downslope and pushed in from the north.
“When we have two convergent zones coming together, you can often see the effect of winds on water,” he said. “But in this picture there is no obvious change in that. You see the shadow of the clouds on the water, but not much more than ripples. Perhaps the winds were really subtle.”
According to the weather records, conditions were changing slowly Saturday, which is confirmed by the photo. When the clouds bumped up against the clear air, they probably went vertical, thus the reason for the distinct edge.
By 3 p.m., nearly four hours after the photo was taken, records show that the air from the north had cleared out the clouds over Central Puget Sound, while farther south the clouds persisted.
“This is a good example of how the terrain conspires to produce these very dramatic changes in the winds, cloud properties, precipitation distributions and sometimes temperature,” Nick said.
Areas socked in by clouds tend to remain cool, he noted, while areas under clear skies may warm up rapidly.
“These kind of conditions make it hard to forecast in the Puget Sound region, because it all depends on where the boundary sets up,” he added.
This photo was originally published on the blog “Three Sheets Northwest.”