Weather Watch

The science behind the recent high air pressure along the Central Coast

A view of Highway 1 near Morro Bay.
A view of Highway 1 near Morro Bay. Special to The Tribune

The storm door has been wide open for most of January, as a trough of low pressure parked itself along the West Coast, which allowed vigorous upper-level winds to bring a series of Pacific storms through the Central Coast.

In fact, this January is the wettest since 1995, when Diablo Canyon nuclear power plant recorded 16.72 inches. So far this month, the power plant has logged 10.31 inches. The most rain ever recorded in the month of January in San Luis Obispo occurred in 1969, when 24.63 inches was recorded at Cal Poly, the city’s home for climatology. In other words, that one month produced more rain than would normally fall during the entire season, which averages 22.40 inches.

Needless to say, January 2017 has pushed rainfall totals well above normal for the Central Coast. Most stations are near or above their entire rain season averages. A rain season is defined as the 12-month period beginning July 1 through June 30 of the subsequent year. Probably most striking is Rocky Butte near Hearst Castle, which has recorded more than 50 inches of rain. Typically, that station records 39 inches each rain season.

However, an extraordinarily strong high-pressure ridge over California has slammed the storm door shut, giving most of us a break from the much-needed rains of January. Even during wet years, a January dry spell is quite normal.

With that being said, the barometer readings have been unusually high over the past few days. So what is air pressure? It’s the strong, invisible force of gravity that accelerates trillions of air molecules toward the Earth’s surface and produces weight. The weight of the air is what we feel as pressure.

Meteorologists often express air pressure in units called millibars (mb). The standard atmospheric pressure is defined as being equal to 1,013.25 mb at sea level. Depending on the surrounding atmospheric conditions, levels above that value may be considered areas of high pressure.

Along the Central Coast, 1,030 mb and above is considered strong high pressure.

Over the past few days, air pressure readings reached 1,033.9 mb or 30.53 inches of mercury (inHg) at the San Luis Obispo County Regional Airport and 1,035.6 mb (30.58 inHg) at the Paso Robles Airport. These readings are some of the highest I’ve ever seen along the Central Coast. Air pressure indications like this occur during winter, when air temperatures are at their lowest and the air is at its greatest density.

If we saw pressure readings like this during the summer, temperatures would be higher than 120 degrees!

The highest pressure reading in the United States occurred Jan. 31, 1989, at Northway, Alaska, a town with the population of 71 souls. On that day, air temperature fell to 62 degrees below zero, and the pressure reached 1,078.6 mb (31.58 inHg). That reading is literally off the scale for many analog barometers.

If famed author Jack London was alive, he would probably write a story about the grounding of bush aircraft that carry mail and supplies, because their altimeters could not be calibrated because of such pressure readings. You see, an accurate altimeter reading is crucial for aircraft to avoid mountains.

The highest pressure ever recorded in the Lower 48 states occurred in December 1983 in Miles City, Montana, which reached 1,064 mb (31.42 inHg) during a severe cold wave. The accepted figure for the world’s highest recorded sea-level pressure in the world occurred in Agata, Siberia, which reached 1,084 mb (32.01 inHg) in December 1968. At the time of that reading, it was about 50 degrees below zero.

Many folks who live along the Central Coast have commented about the recent high pressure and how it’s affected them. I asked Scott Robertson, a physician in San Luis Obispo, about his experience with atmospheric pressure and human health.

“We all know someone who can predict the weather by the pain in their joints — and more often than not, they are right,” he said. “Many times, just listening to the patient is worth more than expensive diagnostic tests.”

According to the International Journal of Biometeorology, it appears there is a relationship between air pressure and our health. High pressure is beneficial.

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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.