Weather Watch

Why it’s starting to feel like autumn — even though it’s still hot outside

Leaves turn and fall in Atascadero.
Leaves turn and fall in Atascadero. dmiddlecamp@thetribunenews.com

As we move into autumn, the days get shorter and shorter with less sunlight and cooler temperatures. And on a chronological basis during fall, an area of high pressure builds at the surface over the Great Basin — the land between the Sierra Nevada range to the west and the Rocky Mountains to the east — and begins to dominate our coastal weather.

This condition usually produces Santa Lucia (northeasterly/offshore) winds. In other words, the winds flow out to the Pacific, pushing the coastal low clouds out to sea. These Santa Lucia winds can also bring hot temperatures and very low relative-humidity levels. As the air mass descends the Santa Lucia Mountains, it warms at the rate of about 5.5 degrees Fahrenheit per 1,000 feet of descent. This type of warming is the dry adiabatic lapse rate.

This condition also creates bone-dry relative humidity levels that produce clear skies.

Without the insulating clouds, these vibrant skies allow much more of the atmosphere’s heat to radiate into space, leaving behind a colder air mass. In the inland valleys, the cold and crisp morning temperatures have already caused some leaves to change colors. Another sure sign of fall is the increase in the number of people walking around in coats during late-night and morning hours.

Just the other day, someone mentioned to me that it sure feels more like fall even though the temperatures remain warm. He said that the air felt different, dryer and more like what he would experience in the Sierra mountains than the heavier air of summer.

He’s right! The air is dryer because of the lower humidity levels.

Humidity is the amount of water vapor in the atmosphere. The “relative humidity” is the amount of water vapor the air could hold at a particular temperature. Warm air can hold more water vapor than cold. When air with a given amount of water vapor cools, its relative humidity goes up. When the air is warmed, its relative humidity drops. When the relative humidity reaches 100 percent, it has reached its dew point. This phenomenon causes clouds, fog, rain, snow or some other type of precipitation to develop.

October is a transition month where the coastal valley and beach temperatures exceed those in the inland valleys. Due to these offshore winds, coastal valley temperatures can often exceed those in the interior.

In 1987, a scorching heat wave developed in California, and San Luis Obispo was the warmest location in the nation, with 111-degree temperatures recorded at the airport for two days in a row. In October, the average temperature in San Luis Obispo is 62.4 degrees, while Paso Robles averages 62.1. In November, this temperature differential increases with San Luis Obispo averaging 57.6, while Paso Robles drops to 52.7.

October and November are also months that transition into our rainy season. On average, the Central Coast receives a little less than 1 inch of rainfall in October, while November jumps to around 2 inches of rainfall. Some of the long-range models are advertising the chance of light rain showers developing next Sunday. At this time, the long-range climate models are still indicating near to average rainfall this winter.

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I will be speaking at “Sharks After Dark” fundraising event at the Central Coast Aquarium in Avila Beach on climate change and its consequences for California on Thursday, Oct. 19, at 7 p.m. I would love to see you there.

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.

The festival, which coincides with the start of the Jewish New Year, Rosh Hashanah, was held Sunday, Sept., 17, 2017 in Mitchell Park near downtown San Luis Obispo. It was sponsored by the JCC-Federation of San Luis Obispo,

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