Wednesday, 31 March 2010

ABC of Weather: Katabatic Winds


I need you to use your imaginations today and substitute the Antarctic Ice Sheet in the above diagram (courtesy of Hannes Grobe and available for use under Creative Commons Licence via Wikimedia) for the 1 in 10 slope in my garden and the ocean/sea ice to the left for the bottom of the slope complete with fence and hedge. If you do that, you now have the classic conditions for katabatic winds forming in a garden setting, more commonly known as a frost pocket or frost hollow.

As you can see from the diagram, katabatic winds are formed when cold air meets warmer air and the resultant difference in pressure between the two, aided by the downward pressure of gravity forces the cold air down the slope. As well as the Antarctic and my garden, these winds are commonly found in mountain areas and many other places where these kind of conditions occur.

Whilst you're probably not familiar with the name katabatic (from the Greek kata, meaning down) you might know some of the special, regional names given to this type of wind. There's the Bora, a northeasterly wind which blows across the northern Adriatic and the Mistral, a northerly which travels across the French Mediterranean coast in winter. We've experienced the latter on sailing and windsurfing holidays in the Mediterranean and NAH owns an Equipe, a racing windsurfing board made by the Mistral company. The speeds of these winds can be high: the Bora can reach over 100 miles an hour and they often exceed 120 miles an hour in the Antarctic.

It's usually difficult to see these kinds of winds, but I did find a rather beautiful example of it in action in the Antarctic to show you courtesy of Samuel Blanc, again under Creative Commons Licence and via Wikimedia. I also found a rather beautiful description of them (complete with diagram showing how they form in mountain areas) from a sailor's viewpoint.

Here in my garden any katabatic winds I get are very much gentler breezes and often go unnoticed as is the case with anywhere where frost pockets or hollows occur. The terrain isn't as dramatic or as high as that found in the Antarctic or mountainside, nor is the difference in temperature between the warmer and colder air. After all, frost does tend to form in relatively calm conditions.

However, the effects of my relatively minor katabatic wind does mean I have to be careful with my choice of plants for the bed at the bottom of my garden. They need to be much tougher than those I can grow as the top which also has the protection of the house and warming effects of the patio and garden walls. In an ideal world I should get rid of the hedge and fence completely as they're allowing the cold air to collect at the bottom of my garden.

Failing that, I should replace them with something that would allow the air to flow through the barrier. However, this is the boundary to my garden and to replace or remove them would leave us vulnerable to vandalism or burglary. So they'll have to stay and I'll choose my plants for there accordingly. Luckily my garden's south facing and in the southern half of Britain, so the days when my garden is truly a frost pocket are thankfully rare.

How's the weather with you today? Having come in like a lamb, the last day of March is living up to the rest of the saying and roaring out like a lion. It's windy with downpours of rain plus lashings of hail to add some variety. At least here in the south of England I'm not plagued by snow like the poor people in Northern Ireland and Scotland today.

ABC Wednesday is brought to you by the letter K and the ABC Wednesday blog.

12 comments:

  1. Love this meme because I learned a lot from every week letter. And Katabatic winds is my first time to hear that word. great info and thanks for sharing!

    ABC Wednesday~Keyhole

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  2. I am back in the Geography room at school with Miss Day and the blackboard-well roller board, I think-covered with annotated drawings of anabatic and katabatic winds. What a glorious reminder.
    I think my garden is suffering more from hilltop exposure today than yours-a lion with sharp,pointy teeth up here.

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  3. Hi!
    Great info! Thanks for sharing. I have one of those spots in my garden too. Have a great day!

    Sherrie
    Sherrie's Stuff

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  4. A phenomenon about which I knew absolutely NOTHING before. Well done!

    On behalf of the ABC Wednesday team, thank you!

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  5. I knew the term mistral but have never heard Katabatic. Greatly informative and interesting!

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  6. interesting post! Thanks for the weather lesson.

    Our weather in Western Washington is alot like yours it sounds like. This week we've had temps ranging warm to cold, wind, rain and hail too, then calm today with the full moon.

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  7. Interesting post! I have heard of katabatic winds before but had forgotten all about them! There are no dips and hollows in the fens, not to speak of, so it doesn't apply to us!

    I'd exchange the lack of katabatic winds for a few hills though, any day!

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  8. How is the spring weather here on the Southern High Plains?

    Windy.

    Really, really windy.

    Around these parts, we have a special word for the wind, too. I just can't use it in a family forum. ;-D

    (Actually, I love the wind.)

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  9. our day was beautiful, but a bit windy. Never heard of the Katabatic winds before.

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  10. I was never a good student, I guess i was dreaming when my teacher taught the katabatic wind. Shall ask my husband, he really likes this stuff.

    But I like gardening, we will get along really well.

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  11. Wondered what you would choose for K and would have never come up with that :) March exited like a lion here with wind and torrential hail but no damage done.

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  12. Hi everyone - glad you enjoyed this post :)

    TS - the weather was always 1 of my favourites in Geography at school. If my physics and maths had been better I'd like to have studied meteorology. All these latent tendencies are coming back to the fore now I'm blogging!

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