Weather Watch

SLO’s temperature inversion layer is getting stronger; is greenhouse gas the reason?

A temperature inversion layer creates fog along Avila Beach.
A temperature inversion layer creates fog along Avila Beach. Special to The Tribune

Like the tides, the ebb and flow of low marine clouds along the California coastline is reassuringly timeless.

At times, the coastal clouds will surge into North County and give relief from the summer’s heat. At other times, “May gray,” “June gloom,” “no-sky July” and “Fogust” can persist along the coast for days on end. If you ever wondered why this occurs, the answer lies in an atmospheric condition called the temperature inversion layer.

Along the Central Coast during the late spring, summer and fall months, a layer of much warmer and less dense air often rides on top of colder air mass near the Earth’s surface, kind of like a layered ice-cream cake.

The height of the temperature inversion layer can vary from day to day, from a few hundred feet to more than 4,000. Along the Central Coast, it’s not uncommon to see morning temperatures in the 80s on the mountaintops, while at the same time temperatures in the coastal regions will be stuck in the 50s under a blanket of gray, moisture-laden ocean air.

Below the inversion layer, the air gradually cools as it rises and eventually reaches its dew-point temperature. When this occurs, the moisture in the air condenses on microscopic particles, and coastal stratus clouds are formed. Above the inversion layer, the air temperature can rapidly increase, by as much as 30 degrees or more. This area of increasing air temperatures with rising height is the region referred to as subsidence inversion. Within this region, the air sinks (subsidence) and warms by compression. As the air mass descends, not only does it heat up like a bicycle tire that’s being inflated, but it also becomes quite dry, less dense and clear as the dew-point temperature spread widens.

In late October, many locations over 1,000 feet or more in altitude in the coastal mountains never drop below 80 degrees during the overnight hours, forcing a few residents to install air conditioners.

Some of these folks have lived in these locations for decades without air conditioners. Tim’s weather station on top of the Irish Hills at 1,247 feet of altitude recorded an overnight low of 87.8 degrees Fahrenheit on Oct. 24! This station is located west of San Luis Obispo and not more than a few miles from the Pacific Ocean. To view this data, please visit Chris Arndt’s website at

After a careful review of data from the PG&E Diablo Canyon meteorological towers and other weather stations further inland, I discovered that the temperature inversion layer is becoming stronger over the long-term, and here’s why.

You see, if you put a balloon filled with air over a flame, it will pop in an instant. However, if that balloon is filled with water, the liquid absorbs the heat, and it will not burst. Water’s thermal conductivity (the rate at which it can remove or add heat) is much higher than air and has four times the specific heat capacity than air, according to Cal Poly thermal dynamics professor Jim LoCascio.

This is one of the reasons why the atmosphere is warming at a faster rate than the Pacific Ocean along the California coastline. Another cause is the ever-increasing amounts of greenhouse gases in the atmosphere, like carbon dioxide, which acts as a blanket reducing the amount of heat that can radiate in the space, according to PG&E Meteorologist Mike Voss.

There was an interesting story by Tony Barboza of the Los Angeles Times detailing how smog has worsened for the second straight year despite reduced emissions in Southern California. In Barboza’s story, the South Coast Air Quality Management District said, “One of the possible reasons is the ‘uptick of more days with hotter temperatures and inversion layers, weather patterns that trap pollution near the ground.’ 

We can all help by reducing our carbon footprint. By far, the most prominent single contributor to greenhouse gases in California is transportation. Our family purchased two electric vehicles several years ago, which dramatically reduced the amount of carbon dioxide we put into the atmosphere every year. The electricity we use to power our cars is very clean. In 2015, the energy that PG&E delivered to our house was about 60 percent carbon free. That number climbed to nearly 70 percent in 2016 and could be even cleaner this year.

John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at or follow him on Twitter: @PGE_John.

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