We’ve had a lot of days lately with morning clouds, but Tuesday morning those clouds dropped some drizzle on Portland and burned off differently than previous days. The drizzle was caused by an upper level storm that thickened the marine layer and lifted it enough to create (drizzle) droplets that were heavy enough to fall out of the clouds. But check out the way in which the clouds burned off. On the satellite image below at 9 am Tuesday, the valleys and coasts were cloaked in clouds. But there’s a hole in the clouds, a clear area, over the Coast Range.

Tuesday July 20, 2010 9 am PDT

That clear area was the key to how the clouds burned off  Tuesday.  Sunshine warms the air in the clear area and the clouds around the clear areas edges evaporate away because of that heating. This is different from previous morning when the entire area was clouded over. In that case, the clouds burn off somewhat uniformly from south to north, across the valley. Now check out the satellite image at 3 pm.

Tuesday July 20, 2010 3 pm PDT

You see the last patch of cloud is over Multnomah and Clark Counties. Because of the morning hole in the marine layer, the “clear” line progressed from west to east across the region. Washington and Yamhill Counties cleared earlier and consequently had higher temperatures. And people in downtown Portland, Vancouver and East County were the last to enjoy the sunshine on this July day.

It’s a subtle thing, but the way in which the marine layer burns off can have a big impact on how Portlanders percieve, and enjoy, their day.

Matt Zaffino