Cliff Mass explains Diablo Lake area weather

We were very excited to have Cliff Mass as one of our instructors for the 2010 Northwest Naturalists Retreat, and then thrilled when he posted a piece about the weather in the Learning Center’s neighborhood on his popular blog:

A green-blue mountain lake with towering mountains, snowfields and glaciers, as well as a fascinating meteorology–the is what I found during a pleasant stay this weekend at the North Cascade Institute this weekend, where I was one of the instructors for their Naturalist Weekend Retreat. The location of this beautiful facility is on Diablo Lake (see maps below), behind Seattle City Light’s Diablo Dam. A very pleasant place to take environmentally oriented classes or to use as a base for exploring the North Cascades.

The lake has a green-bluish tinge due to the very fine particles produced by the surrounding glaciers (glacial flour). Why greenish blue? Why is the sky blue? A similar reason–what is known as Rayleigh scattering of visible light. Very fine particles scatter short wavelengths (like blue or green) far more than longer wavelengths (like red or yellow). Thus the shorter wavelengths are scattered back to your eye producing the bluish or greenish tint.
Some of the most exceptional meteorological features of this location are the diurnal (daily) winds. Nearly every day in summer the winds pick up on the lake around noon, with the flow accelerating up to 12-25 mph, often producing whitecaps. The wind is from the west, flowing directly up the Skagit  River valley (see map above). During my stay I noted a strong correlation between this westerly wind and the pressure difference across the Cascades; when eastern Washington pressure fell relative to the west, the winds accelerated. Thus, the winds appeared to be gap winds, which are roughly proportional to the pressure difference across the gap. The interesting thing for me, is although the gap is very clear to the west (the Skagit River Valley), to the immediate east there is considerable blocking terrain until one gets to Mazama. But clearly the air rushing up the Skagit is going somewhere as it pushes to the east. Since the pressure difference increases during the day (eastern Washington heats up, air there becomes less dense, and thus the pressure falls), the wind strengthened late morning into the afternoon.



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