Seen at The Festival of the Tree

...if you would be happy all your life, plant a garden ~ Chinese proverb

Wednesday, 17 March 2010

ABC of Weather: Isobars

The above picture (courtesy of Meteorologica) shows the British Isles complete with mapped isobars. These are the many blue lines shown on the map and denote lines of equal air pressure. The term isobar is derived from iso, from the Greek meaning equal and bar, one of the unit names given for air pressure measurement.
We have High pressure (aka an anticyclone) over the UK at the moment: Tuesday's map* showed it was to the south-east with the pressure well over 1000 millibars (1027 in fact). High pressure means a period of calmer weather, which we've actually been enjoying over the past couple of weeks or so. You can tell the weather is calmer from the map if the space between the isobars is relatively wide.
However, whilst the weather has been quiet and sunny, the winds on the whole have been bitterly cold because they've been from the north or east. As air circulates in a clockwise direction around an anticyclone in the northern hemisphere, you can tell which way the wind is blowing simply by finding the centre of the High and following it round to its position over the British Isles. Luckily there's also been sunshine, so it's been fairly pleasant outside for gardening once the early morning frost has gone, though that cold wind has meant I've had to resort to my fingerless gloves from time to time.
Today, our weather is set to change to dull and wet as a Low (aka cyclone) is due to come in from the south-west tomorrow. It means the air pressure will probably fall to below 1000 millibars, the mapped isobars will grow closer together and their alignment over the British Isles will change as the wind picks up and veers round to the south-west.
Maps like the one shown are one of the basic tools used by weather forecasters. The data for the maps are collected from dozens of weather stations throughout the UK and the air pressure measurements are corrected to sea level values (as air pressure varies with height) before the computer produces a map like the one above. When they're shown on the TV forecast, I get much more out of that simple visual cue on how our weather will be over the next day or so, than any amount of description from the presenter. There's nothing like the tightly packed isobars of a remnant hurricane heading our way to get me rushing out into the garden to batten down the hatches!
* = I have no idea what the map will be showing when you read this as it appears the HTML code I'm using to show you the map updates the map every few hours or so rather than being a snapshot in time!
How's the weather with you today?
For many more Interesting posts in the way of I, do visit the ABC Wednesday blog.

13 comments:

  1. I do know how glad you are to see and feel the sun again after the winter that you have had! I hope it stays around for a while and warms up even more! Great post for the day!
    We've had the mildest winter on record here in Seattle, but we've still had lots of gray skies, rain and wind! And yet flowers and trees are blooming and they certainly do brighten up the gray! Hope your week is going well!

    Sylvia

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  2. Oh VP, you have tapped into Andrew's new love - his max/min thermometer a photo of which I have just posted. Weather is a hard language to follow! I just look out the window :)

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  3. Sunny and very warm today. Admit I don't fully understand the isobars but it's cool you showed them!

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  4. Interesting post for today's ABC.

    On the behalf of ABC Wed. team thanks for playing.

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  5. The weather is just dandy...if you don't mind snow and weather in the 40's. I just got back from a refreshing cross-country ski trip with the family. Ah, spring!

    Christine in Alaska

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  6. Way cool... I am always love Weather!

    Just want to wish you HAPPY ST. PATRICK'S DAY!

    Have you a great one!

    hugs

    shakira

    Oscar Wilde-The IRISH Gentleman

    ART OF GENEROSITY

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  7. A good weekend but much rain to come this weekend. Daffodils still not out round here.

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  8. Yes, those lines on the map show that we now have a break from the glorious weather we have had in wales for the past month or so, and it is now raining in wales!
    K

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  9. Your post made my head spin! LOL! OH loves anything to do with weather and just laps it up. He has a digital weather station plus a hand-held one, and consults them regularly. Me? I just rely on him to tell me what's going on. I do know that the closer together the isobars, the more likely it is we'll have strong winds though! Do I get points?

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  10. Yes its updated and the high has moved. Isn't technology wonderful.
    I tap on my barometer every morning as I go out, its over 1000 at the moment and fair, hurray, Great post.

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  11. Cool, cloudy, snow flurries inbound, unspringlike - bad isobars. :) Excellent post!

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  12. Albany, NY: started off a little cool this a.m., but turned into a lovely day.

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  13. Hi everyone - well, that wet weather has yet to arrive and we actually have sunshine this morning. The rain's now predicted for later on this afternoon. I don't mind - that means an extra half day out in the garden :)

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