September 30, 2020

Smoke and the Reddened Sun is Back

Last night, as the sun was setting on the Olympics, the approaching California smoke was apparent.

And this morning, the sun, obscured behind the spreading smoke veil, reddened the sky while fog threatened to engulf the city.  Beautiful, really.

The visible satellite image at 9:46 AM clearly showed the smoke over western Washington and the conduit of smoke stretching back to northern CA.

Fortunately for those of us who enjoy breathing, the bulk of the smoke stayed aloft, something often true of smoke imported from afar.   Air quality sensors at the ground show degradation along the Oregon coast and, to a lesser degree, in western Washington where some air quality sites have declines to "moderate" levels of small smoke particles.

For example, in Seattle (below), small particle concentrations have increased from typical background levels (around 5 micrograms per cubic meter for PM2.5 particles) to around 25.  Most people will not notice this level of smoke.

The biggest protection we have from the overlaying smoke is the inversion layer near the surface, where temperature is increasing with height.  Inversions suppress vertical motion of air.

The vertical sounding at the WA coast this morning, produced by weather instruments lofted by a weather balloon, showed the inversion clearly (see below).  Temperature values are noted on the x axis, height (in pressure) on the y-axis.  850 is about 5000 ft.  Temperature (the right solid line) increases with height to about 950 hPa...about 2000 ft above sea level.

Air does not like to mix through inversions, but as temperature at the surface increases, the inversion will weakening. Model forecasts suggest that enough of the inversion will remain to prevent large amounts of smoke from reaching the surface.  Just a gradual, modest degradation over the day.

The general weather pattern is not shifting much during the next day, so light smoke should remain for awhile.

I should note that the smoke can be traced to northern California where a Diablo wind event occurred over the weekend.  In such events, strong/dry northeasterly winds develop over the region that can start and spread fires, with Diablo winds driven by high pressure east of the Sierra Nevada.  Fortunately, the Diablo winds have declined, and northern CA smoke production has lessened (see satellite picture this AM below).

On Sunday, winds will switch to northerly over our Northwest and remove the smoke.


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