SLO County had dense fog and heavy smoke for days. Why was September so cloudy?

You may have heard or read the terms “May gray,” “June gloom,” “no-sky July” and “Fogust” to describe the persistent marine layer that often develops along the coastal regions of California during late spring and summer.

Typically, by September, the coastal marine clouds give way to mostly blue skies until the first storms of the year arrive.

However, this September was is no way ordinary with endless days of dense fog and heavy smoke in San Luis Obispo County.

Zoe Hendricks of Los Osos coined a new phrase — “Souptember,” a reference to so-called “pea soup” fog — to describe the overcast.

During the Industrial Revolution, in London, pea soup fog was yellowish or greenish fog that was caused by the heavy smoke from the burning of coal to heat homes and power factories mixed within the moist air from the River Thames that runs through the city.

Like London during the Industrial Revolution, we saw pea soup fog on the Central Coast in September, and it significantly reduced the electrical output of my solar panels at my home.

In September 2019, my solar panels sent 365-kilowatt hours (kWh) to the grid, This September, it was a negative 147 kWh. In other words, due to the extensive cloud cover, instead of adding 365 kWh to the grid, I took 147 kWh from it to power my home.

So why was September so cloudy?

The first factor was the record size of California’s wildfires, which generated unprecedented amounts of smoke that traveled throughout the entire Northern Hemisphere. So far this year, a total of 8,155 fires have burned more than four million acres, or, 4% of the state’s 100 million acres of land, smashing the previous record of two million acres burnt in 2018.

The other factor was much warmer than average temperatures in the inland valleys this September.

Despite the shorter days of autumn, historically, the warmest months of the year along the beaches are October and September in the coastal valleys. In the inland valleys, the warmest months are July and August.

That’s despite Death Valley-like temperatures that developed in the coastal valleys on Sept. 6, when Solvang hit a high of 122 degrees Fahrenheit and Cal Poly 120 degrees.

This September, the coastal valleys — San Luis Obispo and Santa Maria — saw their high temperatures average about 3 degrees warmer than usual, while the inland valleys — Paso Robles and Santa Ynez — were nearly 8 degrees above average!

Typically, in the fall, the coastal regions become warmer than the inland areas.

All it takes is a small change in the surface air temperature, the temperature inversion layer, or the direction or speed in the wind to make the difference between a clear and or foggy day.

The warmer than average temperatures in the inland valleys last month produced an increase in the northwesterly winds along the coastline that brought cooler air with higher relative-humidity levels to the beaches. This, in turn, created conditions more favorable for the development of marine stratus clouds and fog.

The type of fog we see along our rugged coastline is advection fog. Advection simply means transport.

The northwesterly winds produce upwelling along the coast, bringing cold, subsurface water to the surface along the immediate shoreline. As you head further out to sea, the surface water becomes warmer.

The northwesterly winds transport the relatively warm air from further out to sea across the much colder water along our immediate coastline. The overlying air then becomes chilled and drops to its dewpoint temperature, producing coastal low clouds and fog.

The dew point is the temperature to which air must be cooled for it to become saturated. At that point, the air can no longer hold all of its water vapor, some of which condenses into water, as dew or fog. Dew point is simply the temperature when dew forms.

Fog can develop when air temperature and the dew point are within 3 degrees Fahrenheit of each other.

Dew point temperature can also help determine the height of cloud ceilings, such as the height of the base of cumulus clouds. When the dew point temperature and air temperature are the same, the relative humidity is 100%.

If the wind direction is out of the northeast, fog-free days will develop along the coastline as the air is warmed and dried as it descends along the sides of coastal mountain ranges due to compressional heating.

In conditions like these, the air temperature increases, but the dew point temperature decreases, and the dew point spread widens, leaving behind clear skies.

If you are curious about climate change, please visit PG&E’s website at www.pge.com. Other excellent sites are Dr. Ray Weymann’s Central Coast Climate Science Education webpage at www.centralcoastclimatescience.org and Climate Central at https://climatecentral.org.