Weather Watch

Autumnal equinox is Sunday

Special to The TribuneSeptember 22, 2013 

John Lindsey

The arrival of the autumnal equinox — beginning of fall — will occur at 1:45 p.m. Sunday when the sun is directly over the equator. Today, we will lose the greatest amount of daylight of the year, about three minutes at our latitude.

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 actually rises over the horizon and a couple of minutes after the sun sets. In addition, the sunrise is classified as the moment the upper edge of the sun’s disk appears on the horizon, and sunset is when the upper edge disappears below the horizon, adding to the slightly longer days. In other words, we have to wait until Thursday, 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 easily 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 heavier air of summer.

During fall, the warmest temperatures in San Luis Obispo County start to switch from the inland areas to the coastal regions. This is a good thing. You see, I coach my son’s Under-12 South Bay Soccer Association team called the Black Gladiators from Los Osos. Last week, the Gladiators ventured to the North County to play the Gray Hippos from Templeton.

Our team from the coast was not acclimated to warm North County summer temperatures. Nevertheless, it was a fun game enjoyed by all. The Central Coast soccer season bridges the late summer and early winter seasons, transitioning from hot summer afternoons to frosty winter mornings.

Come December, when our players participate in the Atascadero Freeze Soccer Tournament, I’m sure they will miss the heat.

A little rain falls on Central Coast

Residents of San Luis Obispo County woke up to a damp morning Saturday because of a weak cold front that dissipated by the afternoon.

The system brought heavy mist and drizzle to beaches and coastal valleys of the Central Coast, while the North County stayed mostly dry. The system also produced thundershowers across the Sacramento Valley.

Rainfall totals for the 24 hours ending at 5 p.m. were from 0.01 to 0.07 inches.

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Did you know that PG&E delivers some of the nation’s cleanest power? More than 50 percent of the electricity the company provides to customers comes from sources that are renewable and/or emit no greenhouse gases.

John Lindsey’s column is special to The Tribune. He is a media relations representative for PG&E and a longtime local meteorologist. He is president of the Point San Luis Lighthouse Keepers. If you have a question, send him an email at pgeweather@pge.com.

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