Recent storm Ciaran

[old martParticipant @oldmart]

I live in Weston Super Mare in the West country, and just happened to notice the air pressure on Thursday 2nd November at 1:15 PM, it was 957mb, and in just 20 minutes it had risen 2mb. I have no idea how low the pressure got in recent days, but the 957 is the lowest I have ever noticed, and I live at 6m AMSL height. I would like to hear from anyone running a barograph.

[Clive Brown 1Participant @clivebrown1]

I read on the web that the Met Office recorded extreme lows of 953.3mb at Plymouth and 958.5mb at St. Athan, Wales, so your figure seems in line with those.

[DiogenesParticipant @diogenes]

Martyn Hicks has a weather station in Horfield, Bristol and recorded 957.4mb at 04:55 on the 2nd.

His website here;

https://www.martynhicks.uk/weather/data.php?page=records

 

[Nigel Graham 2Participant @nigelgraham2]

Mine, in Weymouth, dropped to about 940mB but with an aneroid barometer designed more as domestic ornament than meteorological instrument, and not calibrated since it left the factory (if at all), its accuracy is questionable!

Oh – it says in tiny letters, “Made in England”. I wonder how old it is?

[BazyleParticipant @bazyle]

Doesn’t anyone measure it in real units anymore? (ie inches of mercury)

[Nigel Graham 2Participant @nigelgraham2]

I can! The same instrument gives the equivalent scale: 7 inches Hg.

Could be worse. I have a small book written mainly for amateur sailors, explaining how the weather works. This uses hectoPascals.

Yes, the PA is the “preferred” unit according to the ISO but the same bureaucrats do accept the Bar (hence milliBar); and importantly for that book’s intended readers, all the weather forecasts including the Shipping Forecast use mB.

[old martParticipant @oldmart]

There are several ways of measuring global air pressure,the standard is 760mm of mercury, or 29.92 inches. This equates to 1013.25mb which is used over most of the world, the USA still use inches and most older barometers like the one on our wall are in inches, but I cannot remember what it was reading at the time. When using master barometers to calibrate aircraft altimeters, we used mm of mercury, but each time the temperature was taken and tables were consulted which took into account the expansion of the mercury and the glass barometer tubeand the changing levels in the system.

[Martin ConnellyParticipant @martinconnelly55370]

All the aircraft altimeters I had anything to do with had their settings done in mbar. This included F4 Phantoms and BAE Hawks so not a European or a USA thing. I also spent about a year in a section where I calibrated altimeters and machmeters and it was all done in mbar. The 1013.25 figure sticks in the brain after a while, since they didn’t have dials less than the unit value we had to set the altimeter local setting to it by turning the setting to 1013 then moving a bit further until the horizontal line of the 4 started to show at the bottom of the window so we were all working to the same standard setting.

Where I worked in industry the older parts were pressure/leak tested to a PSI setting and recent designs were all tested to a Bar setting. I think Bar is popular as it is close to one standard atmosphere but it is now derived from the SI unit of Pascals as it is equal to 100,000Pa, so a hectopascal (hPa) is one Bar. As a derived non-SI unit use of Bar is generally  discouraged.

Martin C

[Russell EberhardtParticipant @russelleberhardt48058]

The Weather underground station at Bristol Airport recorded a low of 935.63 hPa (mBar) between 4:20 and 5:50 am on November 2nd.

Russell

[old martParticipant @oldmart]

The F4 Phantoms used by the Royal Navy would have had altimeters with millibar scales, but the USN probably had theirs in inches.

[Nigel Graham 2Participant @nigelgraham2]

So my “home” barometer might actually be more accurate than I’d thought, by showing 940mB.

I am not a meteorologist but I would imagine the pressure in a weather-system is anything but uniform for any given radius, and vortices within the main circulation could well produce local depressions deeper than the main one.

It’s dropping again, back around 940mB, with less severe stormy weather forecast.

An effect I have seen in clouds is seemingly regular waves of the vapour, suggesting these are indeed marking extremely low-frequency sound-waves whose pressure oscillation influences the condensation. The clouds would lie along the low-pressure roots.

…..

As for those ridiculous little Pascals, designed to suit Official Mathematicians in ISOlandia more than practical engineers, MELG Boiler-admirers and motorists….  My work for quite some years involved sound, for which the Pa is now ridiculously large so has to be chopped into micro-Pascals (µPa – Cor, I can still remember that [ALT+0181] to type the Greek letter.)

The 0 deciBel level for airborne sound is 20µPa, the softest sound detectable to the fully-healthy human ear in its most sensitive frequency-range. In marine work, 0dB refers to a lowly 1µPa.

Making 0dB equate to 0[any unit] completely, i.e. utter silence, is mathematically impossible.

[old martParticipant @oldmart]

My 957mb was not from a calibrated source, just one of those Auriol weather stations with clock from Lidl. I have two of these, as I write this, one says 962 and the other says 967, and the old wall mounted aneroid barometer says 28.55″ which is 966.6mb.

The mention of the very low pressure at Bristol airport probably is a direct reading at the height of 190m which pilots set their altimeters to. This is not corrected to sea level because the altimeters need to read zero as the plane touches down.

[BazyleParticipant @bazyle]

Visited my sister this evening. They have a Fitzroy barometer. Might not be calibrated though.

[Bazyle]

On Bazyle Said:

Doesn’t anyone measure it in real units anymore? (ie inches of mercury)

yes! I do, I have an analogue barometer plus have made a number of electronic weather stations/devices that would normally show the newer iso readings which to me mean nothing, so all have been reconfigured/programmed to read in/hg.

The old scale is much easier to interpret, if it is down around 28 then get your sou’wester out, it;’s going to be stormy.

 

a reading of 29 depends on rising or falling, likely changable.

a reading of 30 get the sun cream out.

what could be simpler?

Pascals,hectapascals, millibars who cares, the numbers mean nothing!

 

[John HaineParticipant @johnhaine32865]

Groan!  Look guys, for a few decades now the world has used a nice coherent system of units in which force is measured in Newtons and length in metres.  So the natural unit of pressure is Newtons/square metre, which is a Pascal, shortened to Pa.  Think a Newton’s worth of sugar, about the weight of an apple, spread over a square metre, it ain’t very much, so we gather 100 or them to get an hPa, which happens to be the same as a millibar.  So:

1 hPa = 1 mBar

Numerically identical, and the abbreviation uses one fewer letters.  Get used to it.  I always used to call ’em mbar until I started using solid state pressure sensors which are spec’d in hPa.  As for using a unit based on Henry VIII’s thumb and a column of a liquid metal that no one in their right mind would have in the house, words fail me.

[mark costello 1Participant @markcostello1]

Worked well for a long time.

[duncan webster 1Participant @duncanwebster1]

I’m with John, in fact I’d dump bars and stick with kPa and MPa.

As for it’s worked for a long time, how easy is it to work out the force exerted by a vacuum cylinder which has 28″ mercury depression one side. Unless you’ve got a book of conversion tables handy you’ll have to remember the density of mercury. Take it from someone who has done engineering calculations all his career, SI is a lot easier.

[duncan webster 1]

On duncan webster 1 Said:

I’m with John, in fact I’d dump bars and stick with kPa and MPa.

As for it’s worked for a long time, how easy is it to work out the force exerted by a vacuum cylinder which has 28″ mercury depression one side. Unless you’ve got a book of conversion tables handy you’ll have to remember the density of mercury. Take it from someone who has done engineering calculations all his career, SI is a lot easier.

No one doubts that it is esier doing calculations, that is NOT the point.

The point is that the older in/hg is easier to interpret (in my humble opinion!) when dealing with barometric pressure,

On my weather display (before I converted it to in/hg) the reading meant nothing to me.

If the reading showed 985 where is that in relation to the scale limits?

With in/hg I can tell at a glance what the weather is doing (and knowing what the previous days reading was).

as for the analogue barometer, just tap the glass to see if rising or falling.

we are not dealing with calculations here just a simple observation of what the barometric preasue is and comparing that to the prevailing conditions.

 

[roy entwistleParticipant @royentwistle24699]

I have a piece of string hanging outside my window. If it’s wet its raining, or has been. If it’s dry, it’s not. If it’s waving two and fro, it’s windy  .

Roy

[old mart]

On old mart Said:

 

The mention of the very low pressure at Bristol airport probably is a direct reading at the height of 190m which pilots set their altimeters to. This is not corrected to sea level because the altimeters need to read zero as the plane touches down.

The pressure recording on this site was quoted as sea level pressure https://www.wunderground.com/history/daily/gb/bristol/EGGD/date/2023-11-2

Russell

[roy entwistle]

On roy entwistle Said:

I have a piece of string hanging outside my window. If it’s wet its raining, or has been. If it’s dry, it’s not. If it’s waving two and fro, it’s windy  .

Roy

Well average barometric pressure in the UK is ~1016 mB (it depends on where you live) so just remember that instead of whatever it is in in hg.

When I lived on the coast of Cumbria if you could see the isle of man it was going to rain, if you couldn’t it was raining. We’ve moved on a bit since then

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