Weather: Onshore winds keep SLO County coast clear and dry



A satellite photo shows how the marine layer has been pushed back off the California coast.

We may not see a persistent, deep marine layer until March, and here’s why.

Over the years, many Central Coast residents who live along the shoreline have commented on the reduced amount of coastal stratus and the fog and drizzle it can create.

Their observations are correct: Median cloud cover data from San Luis Obispo County Airport and Santa Maria Public Airport showed a decrease in marine layer over the past few decades.

Many factors affect the development of coastal stratus, such as storms, wind direction and speed. Over the past 37 days we have seen lots of Santa Lucia (offshore) winds which have warmed and dried the atmosphere and pushed the marine layer hundreds of miles offshore.

Currently, the atmosphere is locked in a dry pattern.

The Eastern Pacific High is firmly anchored off the California coast, with transient low and high pressure systems moving across the Great Basin. This condition will continue to produce gusty Santa Lucia winds and dry weather until mid-February, if not longer. Strong high pressure over California will continue to produce gusty Santa Lucia (northeast) winds.

Meteorologists often express atmospheric pressure in units called millibars. Standard atmospheric pressure is defined as 1013.25 millibars at sea level. Depending on the surrounding atmospheric conditions, levels above this value may be considered areas of high pressure. Typically, strong high pressure, estimated at 1,030 millibars and above, produces clear, dry weather.

In the United States, barometric pressure for aviation is measured in inches of mercury, or inHg. One inHg is the pressure exerted by a 1 inch tall column of mercury at the standard acceleration of gravity. So 1,013.25 standard millibars at sea level equals 29.92 inHg.

The highest pressure reading in the United States occurred on January 31, 1989 in Northway, Alaska, a town of 71 souls. That day, the air temperature fell to 62 degrees below zero and the pressure reached 1,078.6 mb or 31.58 inHg.

This reading is literally off scale for many analog barometers. If famous author Jack London were alive, he would probably be writing a story about the grounding of bush planes carrying mail and supplies because their altimeters could not be calibrated due to such pressure readings. You see, an accurate altimeter reading is crucial for planes to avoid mountains.

The highest pressure ever recorded in the lower 48 states occurred in December 1983 in Miles City, Montana, where it reached 1,064 mb, or 31.42 inHg, during an intense cold snap.

The official figure for the world’s highest recorded sea level pressure occurred at Agata, Siberia, which reached 1,084 mb, or 32.01 inHg, in December 1968. At the time of this reading, it was about 50 degrees below zero. On the other hand, and again depending on the surrounding atmospheric conditions, levels below the standard atmosphere can be considered low pressure areas.

Traditionally, according to wind data recorded at Diablo Canyon Power Plant’s weather tower, winds blow about 60% of the time out of the northwest quadrant along the Pecho Coast. The winds blow about 12% of the time out of the northeast quadrant and about 23% out of the southeast quadrant. The other 5% of the time, the winds are evenly distributed over the rest of the cardinal headings.

During dry years, northeast Santa Lucia winds are more common, while wet years will see more southeast prefrontal winds.

In January and the first week of February, the Santa Lucia winds (from the northeast) blew about 75% of the time; no wonder the skies were mostly clear, with cold mornings and warm afternoons.

When the winds blow from the northwest (onshore), they push moist sea air over the surface of the Pacific Ocean toward the California coast. This air near the ocean surface is cooled from below by the colder seawater and produces an inversion layer, which means that the air at the sea surface is colder than the air above.

Near the top of the inversion layer, cold air interacts with warm air and condensation produces clouds. Think of a cold glass of iced tea on a hot day. The water vapor in the air condenses on the outside of the cold drink.

The exact process occurs along our coastline, except water vapor condenses on microscopic dust or salt particles near the inversion layer, producing marine stratus. The colder the seawater temperature, the more condensation is likely to occur.

Think of it this way: if that glass of tea were the same temperature as the air around it, condensation on its surface would not occur.

Inversion layers can occur anywhere from a few feet above the ocean surface to thousands of feet above sea level. When stratus descends to the surface of the ocean, this is what we call fog. If the inversion layer is high enough, marine stratus can break into the coastal valleys (Santa Maria). As it increases in elevation, it can move through passes and gaps inland.

The NOAA Climate Prediction System model predicts a possibility of increased northwesterly and southerly (onshore) winds and a wet weather pattern developing in March and April.

If this pattern holds, it could be a repeat of 2020, when we had an almost parched January and a completely dry February, followed by heavy rains in March and showers in April. These onshore winds can also cause the redevelopment of a deep marine layer along the coastline.

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