"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.