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Archive for April 19th, 2018

Santa Ana, Santa Barbara winds

Santa Barbara helt yderst til venstre, mens Santa Ana (bydel i LA) ligger på den anden side af LA downtown i retning af San Diego

      

A sundowner is a northerly offshore wind in Santa Barbara, California. It occurs when a region of high pressure is directly north of the area, the coast of which trends east–west. This contrasts with the more typical onshore flow. The winds blow with greatest force when the pressure gradient is perpendicular to the axis of the Santa Ynez Mountains, which rise directly behind Santa Barbara. These winds often precede Santa Ana events by a day or two, as it is normal for high-pressure areas to migrate east, causing the pressure gradients to shift to the northeast.

Sundowners are particularly dangerous during wildfire season because the air heats and dries as it descends from the mountains to the sea. Gale force hot, dry winds can make firefighting impossible. A sundowner quickly burned a swath from the mountains through populated areas and across Highway 101 into Hope Ranch during the 1990 Painted Cave Fire. The most intense periods of the Jesusita Fire’s destruction have also been blamed on sundowner winds. The Sherpa Fire grew to 4,000 acres (1,600 ha) overnight due to the sundowner winds, destroying the water system for El Capitán State Beach at the beginning of the 2016 fire season.

Et lignende opslag findes for Santa Ana Winds. Desuden forekommer samme fænomen langs hele kysten i det sydlige Californien; det er samme fænomen, som man i Schweiz kalder Föhn vinden, og som er mest udpræget, når ørkenluften (fra Arizona eller Sahara, respektive) presses op over en bjergryg. Også Norge kender til den virkning: Vestsiden af Oslofjorden kan somme tider få sådan en omgang lufttørring, når varm luft fra Nord presses henover Telemarken; men om det også medfører skovbrand ved jeg ikke (det er sjældent at der kommer varm luft fra nord!)

The Santa Ana winds are strong, extremely dry down-slope winds that originate inland and affect coastal Southern California and northern Baja California. They originate from cool, dry high-pressure air masses in the Great Basin.

Santa Ana winds are known for the hot dry weather that they bring in the fall (often the hottest of the year), but they can arise at other times of the year. They often bring the lowest relative humidities of the year to coastal Southern California. These low humidities, combined with the warm, compressionally-heated airmass, plus high wind speeds, create critical fire weather conditions. Known as “devil winds”, Santa Anas are infamous for fanning regional wildfires.

Written by Donald

Thursday, April 19, 2018 at 18:02 GMT+0000

Posted in Skov og have, Vejr

Tagged with , ,