Is there a good reason why isobars on a surface pressure chart are spaced
at 4 mb? I would have thought 5 mb spacing would have made more sense.
TIA
--
MCC

"MCC" wrote in message ...

Is there a good reason why isobars on a surface pressure chart are spaced
t 4 mb? I would have thought 5 mb spacing would have made more sense.
IA
-
CC

4-mb gives slightly more detail and also can be easily divided by 2 to give
2-mb spacing if desired.

Ian Bingham,
Inchmarlo, Aberdeenshire.

MCC scrive:

Is there a good reason why isobars on a surface pressure chart are
spaced at 4 mb? I would have thought 5 mb spacing would have made more
sense. TIA

I have no idea of the correct answer, if there is one, but it would seem
possible that 4 was selected because it is a third of a dozen (or a fifth
of twenty) and the original measurements would have been 'imperial'
rather than the more 'metric' 5. Just a wild guess.


--
Gianna
Peterhead, Scotland

On May 16, 1:11*pm, MCC wrote:
Is there a good reason why isobars on a surface pressure chart are spaced
at 4 mb? I would have thought 5 mb spacing would have made more sense.
TIA
--
MCC

5 mb is too wide but is used by GFS maybe because it corresponds
very closely to the 4 dam interval in the 500-mb charts and thickness
charts.
In summer, particularly, a smaller interval than 4 mb would be
useful because of the weaker pressure fields. Also, in a
mesoanalysis, a smaller interval is needed, even 1 mb.
The interval of 4 mb has a long history, even going back to the
days when isobars were labelled in inches of mercury. It's about
right.

Tudor Hughes, Warlingham, Surrey.

My thanks to all who replied.
4 mb separation now makes sense to me.
I must admit I've never seen a chart with other then 4 mb separation, or if
I have, I've not noticed the difference.
--
MCC

"MCC" schreef in bericht
...
My thanks to all who replied.
4 mb separation now makes sense to me.
I must admit I've never seen a chart with other then 4 mb separation, or
if
I have, I've not noticed the difference.
--
MCC

October 1945 in the RAF Met. Trainingschool in Kilburn. Mr. Boyden told us
that isobars that should be drawn with an interval of 4 mbar. One of my
countrymen remarked that in The Netherlands the spacing was 5 mbar. 'O well,
you must have a very good Met' answered the (much esteemed) Mr. Boyden.

Sytze Stel

Op donderdag 16 mei 2013 22:06:55 UTC+2 schreef Sytze Stel het volgende:
... One of my countrymen remarked that in The Netherlands the spacing was 5 mbar.

.... and of course it still is http://www.knmi.nl/waarschuwingen_en...eerkaarten.php - Click on the chart to enlarge it.

Colin Youngs
Brussels

On Thursday, May 16, 2013 1:11:13 PM UTC+1, MCC wrote:
Is there a good reason why isobars on a surface pressure chart are spaced

at 4 mb? I would have thought 5 mb spacing would have made more sense.

TIA

--

MCC

It's an interesting question. When pressure was measured in inches, isobars were drawn at 0.2 inch intervals and I thought thee may be a correlation there but there isn't. Isobar intervals can be 2mb, 4mb or 8mb depending on the scale of the chart used so I suspect it's for that reason. ( hectoPascals (hPa)is the correct term these days!)

On 2013-05-17 12:26:36 +0000, Desperate Dan said:

On Thursday, May 16, 2013 1:11:13 PM UTC+1, MCC wrote:
Is there a good reason why isobars on a surface pressure chart are spaced

at 4 mb? I would have thought 5 mb spacing would have made more sense.

TIA

--

MCC

It's an interesting question. When pressure was measured in inches,
isobars were drawn at 0.2 inch intervals and I thought thee may be a
correlation there but there isn't. Isobar intervals can be 2mb, 4mb or
8mb depending on the scale of the chart used so I suspect it's for that
reason. ( hectoPascals (hPa)is the correct term these days!)

I remember gradient winds being measured on a geostrophic scale in the
'old days'.

Could it be that a 4mb interval was deemed the most convenient spacing
(on a UK and Eastern Atlantic chart) for accurate measurement with a
pair of dividers that had a physical limit to how far apart they could
be opened?

"yttiw" wrote in message
news:2013051713394928785-cuddles@britpostcom...
On 2013-05-17 12:26:36 +0000, Desperate Dan said:

On Thursday, May 16, 2013 1:11:13 PM UTC+1, MCC wrote:
Is there a good reason why isobars on a surface pressure chart are
spaced

at 4 mb? I would have thought 5 mb spacing would have made more sense.

TIA

--

MCC

It's an interesting question. When pressure was measured in inches,
isobars were drawn at 0.2 inch intervals and I thought thee may be a
correlation there but there isn't. Isobar intervals can be 2mb, 4mb or
8mb depending on the scale of the chart used so I suspect it's for that
reason. ( hectoPascals (hPa)is the correct term these days!)

I remember gradient winds being measured on a geostrophic scale in the
'old days'.

Could it be that a 4mb interval was deemed the most convenient spacing (on
a UK and Eastern Atlantic chart) for accurate measurement with a pair of
dividers that had a physical limit to how far apart they could be opened?


I thought it might be a hang over from a previous system and found that 4 mb
equals 3 Torrs. So if meteorologist at the time of the change were used to
thinking in cm Hg then perhaps they may have chosen 4 mb to make the mental
conversion back to the old system easier? Or, if they were working in inches
then 3 mm is appoximately equal to 0.1" and nearly equal to 1/8". Does
anyone know what the spacing was before mb/hPa was introduced?

Cheers, Alastair.