As we enter into fall, many folks refer to this as “Indian summer,” which from a meteorological standpoint refers to a period of above-normal temperatures after a spell of cool weather during autumn.
While no one knows for sure, the term may have been first recorded in the late 1700s by a New England farmer named J. H. St. John de Crèvecoeur, who wrote in a letter, “Then a severe frost succeeds which prepares it to receive the voluminous coat of snow which is soon to follow; though it is often preceded by a short interval of smoke and mildness, called the Indian Summer.”
Often during this season, an area of high pressure will build at the surface over the western United States and produce Santa Lucia (northeasterly) winds, also referred to as “offshore winds” because they flow from the land out to sea especially during the night and morning hours.
As if on cue this week, a strong ridge of high pressure will build over California on Sunday into Monday. This condition will produce dry and hot Santa Lucia winds.
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As these winds descend the coastal mountains, they become warmer, drier and less dense because of compressional heating. This condition causes the cold and denser Pacific air to accumulate near the ocean’s surface, while the hot, dry and less dense air from the land floats or along on top, forming a strong temperature inversion layer. This inversion layer can range from a few feet to hundreds of feet above the ocean.
When this condition develops, mirages are often seen along the coastline.
Mirages are caused when light rays are bent or refracted when they travel through mediums with different densities. For example, putting a pencil in a half-filled glass jar of water creates the illusion that the pencil bends at the water line.
Changes in air densities when the Santa Lucia winds blow across the ocean can create some fascinating optical illusions in the atmosphere.
I took the above photograph on a hot September day at Port San Luis looking southeast toward Guadalupe-Nipomo Dunes and Point Sal as the Santa Lucia winds blew. In the photo, the dunes and Point Sal seem to tower hundreds of feet into the sky.
This optical phenomenon is classified as a superior Fata Morgana mirage. A superior mirage floats above an actual object, and a Fata Morgana distorts vertically as well as elongates far away objects. For example, the sloping Guadalupe-Nipomo dunes in the photo appear to have lofty and elongated cliffs.
The Chumash people named an island after a mirage. Tom Reilly from San Luis Obispo wrote on Facebook that Anacapa Island off the Ventura County coastline got its name from a superior mirage.
He’s right; Anacapa comes from the Chumash word “eneepah” or “anyapakh,” meaning “mirage island.” I was stationed at Naval Base Ventura County at Point Mugu. If the atmospheric conditions were just right, Anacapa would tower in the distance as seen from the concrete runways at the naval base.
Mary Hanse read a book about her ancestor’s passage to America from Portsmouth, England to New York in 1831. One paragraph in the book read, “The apparition of a ship bearing down upon us nearly before the wind in full sail, and so close that to avoid contact seemed almost impossible.”
It seems that it was a mirage, in their opinion caused by the vapor of the cold weather and the warm Gulf Stream.
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Sunday and Monday’s offshore winds and hot temperatures will produce extreme fire danger. Because wildfires are a huge risk, PG&E works closely with Cal Fire and other state and local agencies to educate customers about fire safety. PG&E also provides funding to local Fire Safe Councils across its service area for fuel reduction projects and public education to help prevent dangerous wildfires.