Weather Watch

It’s the first day of fall. Here’s what that means for SLO County’s weather patterns

Yellowing vines catch the late afternoon light along Orcutt Rd. in San Luis Obispo.
Yellowing vines catch the late afternoon light along Orcutt Rd. in San Luis Obispo. jjohnston@thetribunenews.com

Sunday marks the first full day of fall — or “the fall of leaves.”

The arrival of the autumnal equinox — the beginning of fall — occurred at 6:54 p.m. Saturday when the sun was directly over the equator.

On Sunday, we will lose the greatest amount of daylight of the year, about three minutes at our latitude.

As we move toward winter, the Earth will move closer to the sun, because our orbit is not perfectly round, but slightly oval shaped. Earth will reach perihelion, when the whole planet comes closest to the sun at 9:20 p.m., Jan. 2, 2019. Aphelion, when Earth is farthest from the sun, will happen this upcoming summer on the Fourth of July.

To be clear, autumn, like all the seasons, is not caused by the Earth’s distance from the sun, but by its axle tilt of about 23.4 degrees. However, as the Northern Hemisphere leans away from the sun when we’re closest to it, and toward it when we’re farthest from it, our axial tilt tends to moderate our temperature extremes, keeping our summers cooler and our winters warmer than they would otherwise be. The opposite is true in the Southern Hemisphere, were Earth’s tilt compounds both the heat of summertime and the cold of winter.

This Sunday, you would expect that we would have 12 hours of light and 12 of night. Equinox comes from the Latin phrase for “equal nights.” However, the sunrise and sunset times listed on tide tables are not exactly 12 hours apart, as one might suspect. That’s because Earth’s atmosphere refracts or “bends” light coming from the sun, so we see the sun a couple of minutes before it rises over the horizon and a couple of minutes after the sun sets.

Also, the sunrise is classified as the moment the upper edge of the sun’s disk appears on the horizon, and sunset is when the top edge disappears below the horizon, adding to the slightly longer days. In other words, we must wait until Wednesday, when the sunrise and sunset times will be equal.

By the way, one of the more interesting urban legends that I’ve often heard was the one about effortlessly balancing eggs on their ends during the equinox because of the equal length of the day and night. With enough patience and practice, you can balance eggs on their ends at any time of the year just as easily as during the first day of fall or spring.

In the fall, our weather begins to be dominated by an area of high pressure that builds at the surface over the Great Basin — the area between the Sierra Nevada range to the west and the Rocky Mountains to the east.

This condition usually produces Santa Lucia northeasterly (offshore) winds. These winds bring relatively dry air to our shoreline, pushing the marine layer out to sea.

My daughter Chloe mentioned to me that it sure feels more like fall even though the temperatures remain in the 70s. She said the air felt different, drier and more like what she would experience in the mountains, even though the temperatures were about the same. She’s right! The air feels crisp because of the lower humidity levels.

When the air mass moves in an offshore direction, it descends from the Santa Lucia Mountains toward the Pacific and then warms from compressional heating. As a result, the relative humidity drops. This lower humidity level makes it feel dry, more like fall than the air of summer.

During fall, the warmest temperatures along the Central Coast start to switch from the inland areas to the coastal regions.

Uncharacteristically, the start of this fall will feel more like June with plenty of low marine clouds with pockets of fog and mist with cooler temperatures in the coastal regions. An offshore wind pattern unfolds Tuesday and continues through the week for warmer temperatures and less coastal stratus. A few of the long-range models are indicating a chance of light rain starting Oct. 3 and continuing into following week.

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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