The San Gabriel Mountains were barely visible from Downtown Pasadena thanks to smog. (Jacob Margolis)

All of the major wildfires are finally out, the stench of smoke has blown away, but the air quality is still awful.

What's the deal?

As it turns out, this is ordinary L.A. weather for this time of year.

"We get a lot of fine particulate matter in the winter time in Southern California," said Sarah Rees, assistant deputy executive officer in planning at the South Coast Air Quality Management District.

While air often flows through different layers of the atmosphere, moving pollution away from us, right now there's an inversion layer, or a giant atmospheric dome, trapping that dirty, polluted air and stopping it from being cleared out.

There are a couple of reasons for this.

One is that there's a high pressure system, pushing down, keeping polluted air from flowing skyward.

The other is that right now temperatures are cooler closer to the ground than they are higher up in the atmosphere. Our trusty marine layer brought cold ocean air overland and reflected sunlight — and heat — away from our homes and streets. Since cooler, denser air falls and warmer air rises, the two layers of the atmosphere remain separated, with the pollution hanging out with the cool air close to the surface.

At around 4 p.m. today, surface temperatures were about 62 degrees, while readings taken 830 feet up were about 57, before climbing to 73 by 3,100 feet, according to the National Weather Service.

By Thursday night we can expect another Santa Ana wind event to push the pollution seaward with gusts up to 25-35 mph. Of course, with relative humidity around 6% to 15%, good air quality might be short lived as the fire risk increases.