Sunday, April 26, 2009
An Ocean Full of Low Clouds
Take a look at the visible satellite picture for northeast Pacific Ocean (see image). Notice anything? Something that is quite different than a few months ago? Something that happens virtually every spring?
The ocean is covered for thousands of miles by low stratus/stratocumulus clouds.
We are talking about millions of square miles of low clouds. Every spring we see these low clouds blossom and dominate into summer. And it is the movement of these low clouds into our area during June that produces the familiar June gloom (there are other terms but this is a family-oriented web page). And many of the summer phenomena that I talk about in my book (the onshore push, alongshore surge, etc) are connected with the stratus/stratocumulus of the Pacific.
Why the low clouds? A hint can be found in the surface pressure analysis for this morning at 11 AM (see graphic). A large high pressure area has formed over the northeast Pacific. High pressure is associated with sinking air (called subsidence in the business), and sinking air produces warming aloft since air warms when it is compressed (the air is going from low pressure aloft to higher pressure at lower elevations). The sinking weakens towards the surface since air can't go through the surface! With stronger sinking-warming occurring aloft then near the surface the atmosphere becomes more and more stable--which means there is less tendency for vertical mixing. If you think about it this makes sense, warm air is less dense than colder air...and in nature less dense fluids natural rise above dense ones. This is a stable situation. A dense fluid on top of a less dense one is an unstable configuration.
The stable atmosphere allows moisture to accumulate at low levels, leading to low clouds. Another factor, that is particularly strong near the coasts, is that the high pressure produces northerly winds near the coast, which results in upwelling (see my book for an explanation of this), which brings up cool water. Cool water helps saturate the lower atmosphere....that is why the densest stratus is often along the west coast..particularly central CA.
Anyway, getting back to the story, high pressure builds northward out of the subtropics during the spring and the stratus follows . In the wintertime, as the high retreats and cold air moves in aloft behind Pacific systems, the atmosphere is much better mixed and the low clouds don't form.
So watch the satellite pictures and Pacific pressure patterns...you will note a close association of the low clouds and high pressure areas.
Seattle Math Adoption Decision is Still Up in the Air
PS: I will have an update on the current Seattle high school math situation this week. You won't believe what happened at the school board meeting last week...they split 3-3 and will try again on May 5th. I was amazed that three of them are still considering a terrible math series (Discovering Algebra, Discovery Geometry) that was found by the State Board of Education to be unsuitable. And dropped by San Diego as a failure.
Posted by Cliff Mass at 8:36 PM