"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.