方齋夜話--殘夢軒

在南加州住了十幾年，今年一月份才真正體會到甚麼叫做聖塔安那風(Santa Anna winds，也叫焚風，因為它是濕度非常低的強暖空氣)。

幾天前晚上，只聽窗外狂風怒吼，老伴一直狐疑究竟是甚麼，因為從臥室裡聽起來，轟隆轟隆的聲音有點恐怖。我特地跑到屋外察看，才知道外面颳著強風。臥室外的車道有一面長圍牆，和鄰居的車道隔開，所以和臥室的外牆形成一個「隘口」，強風一颳，好像吹起極低音的大喇叭，聲勢驚人。
昨(一月二十二日夜到二十三日清晨)，聖塔安那風又來了，聲勢比上一回還驚人。
今天一看，外面的熱水鍋爐防護鐵皮門(像一般的門扇那麼大)已經被吹倒在地上，其實我原先就比較小心，還用一塊和它同大的野餐木桌面將它頂著，結果是雙雙被吹翻了。
後屋窗戶上沿的遮陽、擋雨鐵皮小頂棚也被吹下了好幾片，有點狼狽。
雖然沒造成任何損傷(最心愛的池中的魚兒們也都安然無恙，只是池水髒多了)，不過領教了焚風的威力。
南加州的聖塔安那風常常造成山林野火，因為它溫暖又乾燥，成了助長火勢的最大幫兇。
氣象學家Wendell Bechtold對聖塔安那風的形成和特性有如下的說明︰
The Santa Anna winds are caused by an atmospheric pressure pattern that
sometimes develops over the southwestern U.S. High pressure builds over the
intermountain region to the east of southern California, as low pressure
deepens off the coast. As the winds flow clockwise around the high towards
the area of low pressure to the west, they descend from higher altitudes
inland to near sea-level in Southern California. This descent causes
temperature warming in the air due to the pressure increase with the
descent. As the air warms during descent it becomes much drier, because no
moisture is added to the air mass at this time.
Depending on the pressure gradient between the high pressure center and the
low pressure off the coast, very strong winds with an easterly component can
develop...winds sometimes as strong as 100 mph. It is a very hot and dry
wind, as described above. The Santa Anna winds are a type of chinook winds
that occur in many parts of the world near mountain ranges.

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