Katabatic wind

 

A recurring theme of mine over the years has been that of a katabatic wind: the gentle stream of air that flows down a mountainside overnight. Indeed, I wrote about it fifty years ago, again this year with 23 cm/s, and with many postings in between.

Barney Gilmore, who has a grand view across the North Arm of the Lake, today sent me a wonderful picture illustrating a katabatic wind. It was taken in the light of dawn and shows smoke (from slash burning) being transported down the mountain slope and out over the Lake by a katabatic wind.

I am really impressed with such compelling illustrations of a physical process as this.

Barney Gilmore’s picture is used with permission.

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4 Responses to Katabatic wind

  1. Grace says:

    50 years ago you wrote about it! That’s impressive. Definitely can be considered as a life long passion. Thank you and all the best to you and your family for this holiday season. Grace

  2. Kelly Hoogendoorn says:

    Hello, it’s been awhile… So happy my old email was still there. Beautiful photos as always, an I learned something new!

  3. Melissae says:

    Alistair, Does this process contribute to fog spilling over the mountains near San Francisco? https://vimeo.com/69445362

    • Alistair says:

      Melissae, yes and no. Yes, in the sense that the fog is in colder air that spills over the mountains to the west of SF and drains down the easter slopes. No, in the sense that the process that produced the cold air is different.

      The katabatic wind starts with a sloping mountain surface that is cooling as a result of a net loss of infrared radiation (usually on a clear night). This cooler mountainside cools a thin layer of air adjacent to it, and this air, now being denser than the air over the valley, drains down the mountain side. This is a typical behaviour in the mountains.

      The air that flows toward SF was cooled by the upwelling of cold ocean water offshore and this has nothing to do with mountains per se. A fog forms in the air as it cools. If the cold offshore air is not very deep, the mountains to the west of SF block it from flowing into the Bay. But, with a deeper layer of cold air it can flow over the mountains and down the eastern slope. This process is characteristic of some western coastlines and is different than that encountered among mountains and valleys.

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