When you looked at this morning did you notice the hazy whiteness in the sky? July 4th fireworks are too blame. And such fireworks have started wildfires, including a major burn near Wenatchee.
Let's start by look at the Seattle Panocam at 6:30 AM this and yesterday mornings (below). I bet you can see the difference. At my home, the sky looked white near the horizon. Why white? More later.
Today
Yesterday, July 4.
Fireworks smoke has caused significant air quality degradation over much of the region.
The western map at 830 AM (courtesy of the Puget Sound Clean Air Agency), show poor air quality (red and purple colors) over extensive areas downwind of population ceners.
And the US EPA's AirNow graphic also show poor air quality near Wenatchee (see below)
Why Wenatchee? Because someone ignited a major grass fire east of the town with fireworks (see map below). That individual has been arrested.
There is another grass fire, supposedly started by powerlines, southeast of Chelan. The visible satellite image at 6:30 AM shows the smoke from these fires, plus the Pioneer Fire up Lake Chelan.
Plots of air quality (actually small particle concentrations) at Seattle and Marysville (both shown below) illustrate the rapid increase in smoke last night. The Marysville numbers were crazy high (about 130).
Seattle
Marysville
Finally, why do these particles turn the sky white?
It turns out this is due to something call Mie Scattering. When light from the sun interacts with smoke particles is it scattered by the particles in many different directions (see below)
Image courtesy of Virtual Labs
Light from the sun includes all wavelengths of visible light (from red to blue/violet). Light appears white when all wavelengths are represented and if you were above the atmosphere the sun looks pretty white.
Large smoke particles from fireworks scatter all wavelengths of visible light similarly, thus producing a white-looking sky. This is what is meant by Mie Scattering. In contrast, small particles in a clean atmosphere (such as the typical gases) scatter shorter wavelengths (like blue) more (called Rayleigh Scattering), which gives the sky a blue cast.
Heavily polluted cities in China have a lot of big particles, so their skies almost always look white. Perhaps it is ironic that our fireworks come from China. They export their white skies to us during our day of celebration of independence.