Hello there

I thought that this would be the best place to ask a question about
barometric pressure - it's a subject that I know nothing about so please
bear with me!

The question is this:

Taking the stormy weather that is apparently going to hit southern
England later on today as an example, does barometric pressure drop some
time in advance of the weather changing? And if so, how far in advance
can this happen? Or is it that the weather changes as the barometric
pressure drops?

This is probably a bit of a daft question but if anyone can answer this
for me I'd be grateful

All the best

Ruth

wrote in message
...
snip
The question is this:

Taking the stormy weather that is apparently going to hit southern
England later on today as an example, does barometric pressure drop
some
time in advance of the weather changing? And if so, how far in advance
can this happen?

.... there are no hard and fast rules for this (in terms of how far in
advance etc.), because it depends to a large extent upon the type of
system (low pressure) producing the change. However, I think we could
say to answer your first question that the pressure will fall 'some
time' in advance of the rain/high winds etc., without fear of criticism
(*) ... the problem is the second part of your question:
[(*): but falling pressure does NOT _automatically_ mean bad weather -
but that's another question for another day.]

For example, lets say the weather has been 'stable' in an anticyclonic
sense (fine, dry weather - usually plenty of sunshine) during the summer
/ early autumn. Day to day the pressure will vary slightly, but
generally not by much. Then a 'major' change in the overall (or
'broadscale') pattern is expected - the famous 'jetstream' changes
position, or the strength of same changes, or whatever, and the high
starts of drift away and decline. At your *particular* location, the
pressure will start to fall steadily, BUT, it may be 2 or 3 days before
all the ingredients are in place to produce rain, wind etc.

Now, think of an already 'disturbed' spell in autumn / winter, when
we've perhaps had several depressions crossing the country, and the next
one is a real hum-dinger! The pressure will/may recover (rise) behind
the last low (but only for a short time), but then fall (at an
increasing rate) ahead of the next one: however, the 'warning' from the
fall of pressure may be a matter of hours, not days - for a small (but
potentially vicious) low, perhaps 9 to 12hr, for a larger 'classic'
depression, up to 24hr.

For small disturbances like troughs, then it could be as short as 2 or 3
hours.

Hope that helps ... not a nice 'clean' answer, but it does demonstrate
how 'rules' based upon pressure change alone can collapse. You need to
factor in wind direction, change in speed, character of sky etc., as
well. Used to be called 'Single Observer Forecasting', and with
practice, you can get quite good at it.

ATB

Martin.

My barograph in Dorset started to fall from 1024 millibars at midnight
Tueday slowly down to 1010 at the moment - 14:20 BST Wednesday, where it has
flattened, curiously. Windy, temperature falling from a high of 23 Deg this
morning down to 18. Rain just starting as I type.

"Martin Rowley" wrote in message
...

wrote in message
...
snip
The question is this:

Taking the stormy weather that is apparently going to hit southern
England later on today as an example, does barometric pressure drop
some
time in advance of the weather changing? And if so, how far in advance
can this happen?

... there are no hard and fast rules for this (in terms of how far in
advance etc.), because it depends to a large extent upon the type of
system (low pressure) producing the change. However, I think we could
say to answer your first question that the pressure will fall 'some
time' in advance of the rain/high winds etc., without fear of criticism
(*) ... the problem is the second part of your question:
[(*): but falling pressure does NOT _automatically_ mean bad weather -
but that's another question for another day.]

For example, lets say the weather has been 'stable' in an anticyclonic
sense (fine, dry weather - usually plenty of sunshine) during the summer
/ early autumn. Day to day the pressure will vary slightly, but
generally not by much. Then a 'major' change in the overall (or
'broadscale') pattern is expected - the famous 'jetstream' changes
position, or the strength of same changes, or whatever, and the high
starts of drift away and decline. At your *particular* location, the
pressure will start to fall steadily, BUT, it may be 2 or 3 days before
all the ingredients are in place to produce rain, wind etc.

Now, think of an already 'disturbed' spell in autumn / winter, when
we've perhaps had several depressions crossing the country, and the next
one is a real hum-dinger! The pressure will/may recover (rise) behind
the last low (but only for a short time), but then fall (at an
increasing rate) ahead of the next one: however, the 'warning' from the
fall of pressure may be a matter of hours, not days - for a small (but
potentially vicious) low, perhaps 9 to 12hr, for a larger 'classic'
depression, up to 24hr.

For small disturbances like troughs, then it could be as short as 2 or 3
hours.

Hope that helps ... not a nice 'clean' answer, but it does demonstrate
how 'rules' based upon pressure change alone can collapse. You need to
factor in wind direction, change in speed, character of sky etc., as
well. Used to be called 'Single Observer Forecasting', and with
practice, you can get quite good at it.

Just in case Ruth does really know nothing about barometers I will
put my twopennies worth in.

Ruth,

On the barometer you will find a second pointer which you can
move by hand. After you have read the barometer, move the
pointer so that it is covering the black one. When you come to
read the barometer again, then the moveable pointer will
show you what the pressure was last time you read it. You
will then be able to judge how fast the pressure is rising or
falling. This will then tell you how soon the rain will arrive or
disperse.

Using the indicated pressure alone is not a lot of help in weather
forecasting, because when it points to rain it is usually raining!
For forecasting what you want to know is the direction of the
change (better or worse) and the speed of change (sooner
or later.)

Hope this helps,

Cheers, Alastair.






ATB

Martin.

"Alastair McDonald" k
wrote in message

Is there a copy of Admiral FitzRoy's work on the barometer anywhere
online?


--
Posted via Mailgate.ORG Server - http://www.Mailgate.ORG

Hi Martin

In article ,
"Martin Rowley" wrote:

wrote in message
...
snip
The question is this:

Taking the stormy weather that is apparently going to hit southern
England later on today as an example, does barometric pressure drop
some
time in advance of the weather changing? And if so, how far in advance
can this happen?

... there are no hard and fast rules for this (in terms of how far in
advance etc.), because it depends to a large extent upon the type of
system (low pressure) producing the change. However, I think we could
say to answer your first question that the pressure will fall 'some
time' in advance of the rain/high winds etc., without fear of criticism
(*) ... the problem is the second part of your question:
[(*): but falling pressure does NOT _automatically_ mean bad weather -
but that's another question for another day.]

He, he - yes,I suspected that there might be rather more to it

For example, lets say the weather has been 'stable' in an anticyclonic
sense (fine, dry weather - usually plenty of sunshine) during the summer
/ early autumn. Day to day the pressure will vary slightly, but
generally not by much. Then a 'major' change in the overall (or
'broadscale') pattern is expected - the famous 'jetstream' changes
position, or the strength of same changes, or whatever, and the high
starts of drift away and decline. At your *particular* location, the
pressure will start to fall steadily, BUT, it may be 2 or 3 days before
all the ingredients are in place to produce rain, wind etc.

Aha, OK - that makes sense....

Now, think of an already 'disturbed' spell in autumn / winter, when
we've perhaps had several depressions crossing the country, and the next
one is a real hum-dinger! The pressure will/may recover (rise) behind
the last low (but only for a short time), but then fall (at an
increasing rate) ahead of the next one: however, the 'warning' from the
fall of pressure may be a matter of hours, not days - for a small (but
potentially vicious) low, perhaps 9 to 12hr, for a larger 'classic'
depression, up to 24hr.

Yes, I understand....

For small disturbances like troughs, then it could be as short as 2 or 3
hours.


OK...

Hope that helps ... not a nice 'clean' answer, but it does demonstrate
how 'rules' based upon pressure change alone can collapse. You need to
factor in wind direction, change in speed, character of sky etc., as
well. Used to be called 'Single Observer Forecasting', and with
practice, you can get quite good at it.

That helps a lot - thanks very much indeed for the nice clear
explanation. I imagined that the answers could get quite complex but
that answers my questions nicely

All the best


Ruth

Hi Alastair


In article ,
"Alastair McDonald" k
wrote:


Just in case Ruth does really know nothing about barometers I will
put my twopennies worth in.

Excellent - the more info the merrier!

Ruth,

On the barometer you will find a second pointer which you can
move by hand. After you have read the barometer, move the
pointer so that it is covering the black one. When you come to
read the barometer again, then the moveable pointer will
show you what the pressure was last time you read it. You
will then be able to judge how fast the pressure is rising or
falling. This will then tell you how soon the rain will arrive or
disperse.

OK....

Using the indicated pressure alone is not a lot of help in weather
forecasting, because when it points to rain it is usually raining!
For forecasting what you want to know is the direction of the
change (better or worse) and the speed of change (sooner
or later.)

Hope this helps,

Yes thank you - very helpful indeed - that makes perfect sense!


Kind regards

Ruth

Martin Rowley wrote:
... the whole subject of how to interpret barometer readings, "sky
signs", etc., is one that is close to my heart! We have 'lost' touch
with a lot of accumulated wisdom of same, and sometime I'd like to have
a go at writing some of it up (but don't expect rapid results).

I have some simple 'rules' which couple the rise or fall of barometer
reading with season and wind-direction. They are not my original work -
having been cribbed from somewhere about 30-odd years ago, and as others
will point out, such 'rules' abounded in the pre-computer age.

It may be worthwhile having something like this in the FAQ.


--
Howard Neil

.... the whole subject of how to interpret barometer readings, "sky
signs", etc., is one that is close to my heart! We have 'lost' touch
with a lot of accumulated wisdom of same, and sometime I'd like to have
a go at writing some of it up (but don't expect rapid results).

I have some simple 'rules' which couple the rise or fall of barometer
reading with season and wind-direction. They are not my original work -
having been cribbed from somewhere about 30-odd years ago, and as others
will point out, such 'rules' abounded in the pre-computer age.

If you want me to send them along, let me know via e-mail (change the
anti-spam parts).

Martin.


--
FAQ & Glossary for uk.sci.weather at:-
http://homepage.ntlworld.com/booty.weather/uswfaqfr.htm

"Howard Neil" wrote in message
...
Martin Rowley wrote:

snip

I have some simple 'rules' which couple the rise or fall of
barometer
reading with season and wind-direction. They are not my original
work -
having been cribbed from somewhere about 30-odd years ago, and as
others
will point out, such 'rules' abounded in the pre-computer age.

It may be worthwhile having something like this in the FAQ.


.... indeed, it might: but (done properly) it will need a separate
structure etc. As I wrote, I'll get around to it one day ... but needs a
lot of research etc., to avoid just wholesale copying of existing texts.
It is something I intend to do though. There are lots of 'rules' based
on single readings relating to such as whether or not frost is expected
at night (based, for example, on afternoon temperature and relative
humidity or wet bulb), and the one I posted recently relating to
build-up of Cu during the day. Many of these could be useful.

Martin.


--
FAQ & Glossary for uk.sci.weather at:-
http://homepage.ntlworld.com/booty.weather/uswfaqfr.htm