Wildfire turns San Francisco sky yellow over weekend

A yellow—ochre, to be specific—haze spread across the skies in San Francisco and the surrounding Bay Area late Saturday and Sunday, mingling with the seasonal fog and prompting Karl on Twitter to declare, “Apocalypse now.”

Of course, the skyward spectacle was indicative of neither the end of the world nor of Francis Ford Coppola’s aggressive art direction.

Rather, it was the product of the still ongoing County Fire, a huge conflagration burning over 32,000 acres just north of the Santa Rosa region.

After the blaze broke out around 2 p.m. on Saturday, smoke and ash from the inferno blew down to the bay to create a bizarre twilight atmosphere in the city.

Although Lake County has been battling wildfires all week, and a fire in nearby Concord even briefly prompted evacuations on Friday, the County Fire grew to many, many times the size of other regional fires in less than a day, throwing huge amounts of ash and particulate matter into our skies.

Note that if fires grow large enough they may affect views even thousands of miles away. In 2015, for example, NASA spokesperson Lynn Jenner credited fires in Siberia for turning skies red in the Pacific Northwest.

As Jenner explains, the phenomena is essentially the same as the one that gives us everyday sunsets, but here created by unexpected variables:

Particles from the fires allow sunlight’s longer wavelength colors like red and orange to get through while blocking the shorter wavelengths of yellow, blue and green. Those longer wavelengths give the sky a red or orange tinted appearance.

Similarly, during sunrise and sunset times when the sun is near the horizon, sunlight has to travel through more of Earth’s atmosphere to get to you. The additional atmosphere filters out the shorter wavelengths and allows the longer wavelengths to get through, providing reds and oranges during those times.

The Environmental Protection Agency’s [EPA] Air Quality Index for the Bay Area on Saturday was notably worse than usual, prompting the city’s Department of Emergency Management to issue a warning about the smoky atmosphere.

On Monday, air quality is back to its usual pristine state, although EPA projection suggest that conditions could deteriorate again to “moderate” poor conditions, meaning that “unusually sensitive people” with preexisting respiratory problems should minimize exposure to outside air.

This could also mean more strange hues in our local fogs, especially if the Lake County fires continue to grow.

• Karl the Fog [Twitter]
• Huge NorCal Fire [Curbed SF]
• Evacuations In Concord Fire [Curbed SF]
• Siberia Fires Cause Red Skies [NASA]
• SF DEM [Twitter]
• Air Quality, San Francisco [EPA]