During the spring it’s not uncommon to have sunny skies over Puget Sound, with a transparent zone extending northward up the Strait of Georgia. In distinction, away from the water, clouds reign.
The seen satellite tv for pc picture at 1:20 PM yesterday (Friday, May 15th) reveals the story. The lowest elevations are primarily clear. Generally extra cloudy even in japanese Washington…however look intently…there are some slender clear zones there too.
So what’s going on? Why are the clouds avoiding Puget Sound? And why are there no clouds instantly offshore?
Interestingly, no matter is occurring was captured by our high-resolution climate forecast fashions. The UW WRF mannequin run at very excessive decision (1.three km spacing between the grid factors) had a 21h cloud forecast that confirmed the impact.
Yesterday was an excellent day to see this impact: We are actually in a interval with a really robust solar and the massive scale winds have been gentle.
The robust, mid-May solar heated the bottom, inflicting the distinction within the temperatures within the vertical (the lapse price) to will increase, and the ambiance began to convect or combine within the vertical. Just like your scorching cereal pot if you activate the burner. And because the air rose, it cooled and finally turned saturated, producing clouds.
Certainly, there have been numerous clouds within the mountains, as proven by the fantastic Crystal Mountain 360 diploma cam at 1 PM Friday (see beneath).
In distinction, the Seattle Space Needle cam on the identical time reveals clear skies over the lowlands and clouds towards to foothills of the Cascades.
So what’s going on right here? It has to do with the native atmospheric circulations related to the land-water contrasts (see breeze) and with the mountains (upslope move on terrain).
During the day, the robust May solar warms the mountain slopes and land floor. In distinction, there’s far much less heating of the water floor (which has a big warmth capability and permits a few of photo voltaic radiation to heat beneath the floor). As a results of the photo voltaic heating over land, a sea breeze wind develops that’s directed from water to land round Puget Sound and upslope move begins on the mountain slopes (air tends to stand up mountain slopes….one thing all hikers learn about!). With air shifting away from Puget Sound at low ranges, air should sink over the Sound to takes its place, leading to a three-dimensional air circulation proven beneath (as illustrated by the arrows).
Sinking air is the enemy of clouds, stopping their formation or dissipating any clouds which might be round. In distinction, there’s numerous upward movement over and close to the terrain or away from the water, which helps produce clouds.
Thus, we get a sample of clear or practically clear skies over and close to the Sound (and the Straits of Georgia and San Juan de Fuca) however loads of clouds inland. And little clouds over the ocean the place the floor cannot heat.
Although visually it appears related, yesterday’s sample was not the results of a rainshadow, as a result of the overall move was weak. Rainshadows happen when air approaches a barrier and is sinks on the lee (away from the wind) aspect.
Similar results, however on a smaller scale, happen over japanese Washington: clouds over ridges and terrain options, however clear skies within the valleys. A superb instance of that is proven beneath for the area round Lake Chelan.