Weather Watch

Onshore and offshore winds explained. It’s more complicated than you think in SLO County

I received this email last week.

“Hi John

Could you please explain the direction (onshore and offshore) of the winds you have in your weather report?

Thanks, Lynn”

Over the years, I’ve been asked this question, and no wonder! I have been often told that the terms “onshore” and “offshore” as applied to winds are confusing in two ways: the terms are vague and fuzzy in themselves (offshore sounds like it could be inland) and especially so since those terms don’t define the perspective from which the winds are considered.

A gale-force northwesterly wind (blowing out of the northwest toward the southeast) is an onshore wind along the north to northwesterly facing beaches near Los Osos and Morro Bay. But at the same time, it’s also an offshore wind event along the San Luis Bay coastline that is orientated toward the south-southeast.

However, if the winds are out of the southeast (blowing out of the southeast toward the northwest), they become an onshore wind at Avila Beach and an offshore wind event near Los Osos that surfers so dearly love.

The direction of the wind is reported by the direction from which it originates on a compass rose.

Onshore winds can bring moisture

However, onshore wind is when the air blows from the ocean to the shore, while offshore wind is the air that blows from the land to the sea, regardless of its cardinal heading, but its orientation to the coastline.

Northwesterly (onshore) winds can bring a cold moisture-laden Pacific air mass to Los Osos that can create fog, mist, drizzle or even light rain as this air is lifting and cooled on the north side of the Irish Hills, but warmed and dried due to compression as the air mass descends downward on the south side of the Irish Hills toward Avila Beach.

Due to the topography of the Central Coast, offshore winds are typically downslope winds that are technically called “katabatic wind,” from the Greek word katabatikos, which means “going downhill.” As the air mass descends the side of our mountain ranges, it warms at the rate of about 5.5 degrees Fahrenheit per 1,000 feet of descent. Meteorologists refer to this rate of warming as the “dry adiabatic lapse rate.”

On the other hand, a “wet adiabatic lapse rate” is less at approximately 3 degrees of cooling per 1000 feet of ascent. Remember, condensation is a warming process, that is why it gets warmer when it rains or snows.

If the air is warm at the top of the mountain range, it can be sizzling hot and bone dry by the time it reaches the valleys below, creating potentially horrific fire conditions.

Most common winds in SLO are out of the northwest

With that said, the northeasterly (blowing from the northeast to the southwest) winds are almost always (offshore) winds, while southwesterly (blowing from the southwest toward the northeast) winds are onshore.

Traditionally, according to wind data recorded at the Diablo Canyon Power Plant’s meteorological tower, the winds blow about 60 percent of the time out of the northwest quadrant along the Pecho Coast. The winds blow about 12 percent of the time out of the northeast quadrant and out of the southeast quadrant about 23 percent of the time.

The other 5 percent of the time, the winds are spread evenly across the rest of the cardinal headings. During dry years, Santa Lucia (northeasterly) are more common, while wet years will see more prefrontal (southeasterly) winds.

Diablo Canyon weather tour returns

Back by popular demand is the Weather Watchers tour of Diablo Canyon Power Plant and Lands. If you would like to participate in this tour on Wednesday, Oct. 23, please visit https://tourdcpp.pge.com/ to register. It will start at 9 a.m. at the PG&E Energy Education Center, 6588 Ontario Road in San Luis Obispo, and will finish by noon.

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 pgeweather@pge.com or follow him on Twitter: @PGE_John.
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