Barometer

[DavidofMersea]

Every Thursday evening the weather girl gives out what our barometer should read - and every week I adjust my barometer - To night it was 4 mb adrift, is there something wrong with my barometer, or is this normal to get out of line ?

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[l'escargot]

The pressure is usually given for a very specific location. If you are at a different height above sea level or further from/nearer to the high/low pressure then you will get a different reading.

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[LadyInBed]

Set it when there is a big high sat over UK then it will be about right, as the difference in reading from say Bracknell to your location will be minimal (unless you are based in Australia) /forums/images/icons/wink.gif

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[Cornishman]

BBC Spotlight South West will only tell us when it is good to be setting our barometer e.g. when there is a pressure gradient of about 2mb or less between Lands End and Taunton (the area they cover). Persuade your weather girly to do like wise for your area, I suggest.

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[VicS]

Good, reliable sources of barometric pressure are the BBC teletext p404 and <A target="_blank" HREF=http://www.xcweather.co.uk>http://www.xcweather.co.uk</A>. Avoid unofficial sources such as yachtclub websites. Set your barometer when the pressure is close to the mean of 1013 mb and steady enough for the time delay between the observation and the ajustment not to cause a significant error. Also remember if you are setting the barometer at home with the intention of then taking it to the boat it must be set low by an amount corresponding to your altitude above sea level. This is 29mb per 250metres.

How accurate your barometer remains across its range and with varying temperature will depend on its quality. A cheap one is unlikely to be accurate to within a couple of mb but one would like to think that an expensive one will be accurate to well within 1mb.

Are you perhaps talking about an electronic one? I have a sneaking suspicion that they are not as accurate as the level of precision of the indication might lead you to believe.

<hr width=100% size=1><font color=purple>Ne te confundant illegitimi.</font color=purple>

 

[DavidofMersea]

Thanks l'escargot, I think that you have the answer. The weather girl is 60 miles away and the different pressure between there and here depends on how near or far each of us is from the centre of the high/low, which differs each week. I cannot see the point in her giving us the pressure each week

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[peterb]

David

There are some occasions (often during one of those almost windless highs) when the pressure is almost constant across the entire country. Frequently the forecaster will then give the pressure so that people can check their barometers.

Remember that any wind will imply a pressure gradient, so that you can't then rely on a pressure measured at any distance away. To check on what pressure gradient you might get, use the geostrophic scale as given on the RYA/RMetS Metmaps. Roughly speaking, at your latitude, a pressure gradient of 1 mb per 40 miles gives a force 4 wind, 1 mb per 60 miles gives F3, and 1 mb per 90 miles gives F2. The pressure gradient is roughly at rightangles to the wind direction.

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[PaulJ]

It's an obvious point but if it is an aneroid barometer (which I assume it is), you do know that you should "tap" it to overcome any sticksion in the mechanism before taking any readings?

<hr width=100% size=1>I used to have an open mind but my brains kept falling out.....

 

[starboard]

Pressure is given as sea level pressure, an aneroid barometer works on the same principal as an aneroid altimeter. you can take that for every 30ft of altitude above MSL the pressure will change by 1 millibar. therefore your 4mb differance may be that you are 120ft above MSL. If you PM me I will give you the nearest pressure setting to your location from the aviation met site, this is updated every 30 mins.

Paul

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