Rate of atmospheric pressure change

CPD

CPD

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Somewhere I had a reference table indicating what mb changes over a period of time would translate into weather/wind wise. I also had a calculation showing windspeed depending upon distance apart of isobars. Its gone !. Does anyone have a link to such a table ?. Thanks.
 
they are two different things, the one that really has a theoretical foundation is gradient/wind speed

gradient is difference in pressure at a given moment over a given distance

once you know gradient (and location, hence latitude) you can determine geostrophic wind speed (those boxes on metoffice charts), then apply a correction for surface friction and a couple of other ones for curvature and air stability and you have a usually good estimation of sea surface wind

the hourly rate of change is *a lot* less significant: given that a depression usually has a speed from 5 to 35 knots, that boats have their speed, that isobars may be crossed at whatever angle, etc etc the hourly variation can hardly have any significance

if you are interested, I can find the values given by various textbooks, a lot of discrepancies as one may suppose.. /forums/images/graemlins/wink.gif
 
There is actually a very solid foundation for this approach, although it obviously must apply to a stationary point and not a moving vessel. However the the slow passage of a yacht is unlikely to make the method inaccurate.

The figures are:

1mb/hour: likely Force 6
2mb/hour: likely Force 7 - 8
3mb/hour: likely gale exceeding Force 8

You will note that these do not equate - but can be compared - to the Shipping Forecast terms of...

Falling (or rising) slowly: 0.1 - 1.5mb in 3 hours
Falling (or rising): 1.6 - 3.5mb in 3 hours
Falling (or rising) quickly: 3.6 - 6.0mb in 3 hours
Falling (or rising) very rapidly: More than 6.0mb in 3 hours

Thus anything faster than "falling slowly" should set the alarm bells ringing!

The wind force values are for at sea; for land winds the values should be reduced by one or two Force levels. Land winds are generally held to be two thirds the strength of offshore winds, excluding local effects.

This data is from the Weather Handbook by Alan Watts (2005).

Regards, Mudhook
 
Sorry, there are more technical pages on the Met Office site, and on many of the academic sites too.

And BIG sorry, metric measurements ARE the common language of science. /forums/images/graemlins/grin.gif
 
Picked this up somewhere - it's in my Little Black Book

Baro.................................Change in last 3 hrs

Steady...............................up to 0.1 mb
Falling / rising slowly..............0.1 - 1.5
Falling / rising.....................1.6 - 3.5 = F6/7+ ? - 12hrs ?
Falling / rising quickly............3.6 - 6.0 = F6/7/8 ? - 6hrs ?
Falling / rising very rapidly........6.0 mb + = F7/8/9+ ? - 3hrs ?

Note all the question marks - very rough guide.

Doesn't work with Mistral / Meltemi types of wind - I have gone from 2 - 8 in 5 minutes with a dead flat barometer, didn't move until the wind had dropped......

Might be what you were after ?

PS - I have been struggling for 5 mins to get this into a coherent list shape and it won't do it - sorry about the layout.

Thats a bit better but still not what it should be .......
 
[ QUOTE ]
its all in metres -not common practice.

[/ QUOTE ]

Huh? what century are you living in??? /forums/images/graemlins/confused.gif

I thought it was a good post and the Met site well worth bookmarking for symbol references.
 
I have the nasa meteoman which shows a graph of pressure tendency. I have searched for information relating this to imminent weather conditions. The best site I have found is WEATHER DOCTOR.
Here is a sample :-
GENERAL RULES FOR USING THE BAROMETER
TO FORECAST LOCAL WEATHER
Barometric pressure decrease indicates coming stormy weather, rain, wind, , heat.
Barometric pressure rise indicates fair, dry, colder weather.
Slow, regular and moderate fall in pressure indicates low pressure area is passing at a distance. Any marked change in weather unlikely.
Sudden decrease, even if small, indicates a nearby disturbance; normally bringing wind, and short showers.
Large pressure drop signals a coming storm in 5 to 6 hours.
Large, slow and sustained decrease indicates a long period of poor weather. Coming weather will be more pronounced if pressure started rising before dropping.
Sudden rise of pressure,when the pressure is about average or above average and the weather is fair, indicates approach of a low pressure cell, and the barometer will soon start to come down.
Rapid rise when the pressure is low, announces a short period of fair weather.
If pressure rise is large and prolonged, count on a many days of good weather ahead.
 
We have regularly been alarmed by drops of up to eight or even ten mb over three or four hours when sailing here in the Canaries - with no change in the weather conditions. These seem to be diurnal changes - the pressure drops during the hottest part of the day then rises again in the evening - or they may even be associatged with the venturi effect in the wind acceleration zones.

The pilot books for this area say a drop of 10mb indicats a Southerly gale of 35 knots or more, but we have yet to see one . . . so in our experience a falling barometer in this area doesn't necessarily mean anything at all. I think it is perhaps a more reliable indicator in UK waters though I would never read too much into it without also being aware of the general synopsis.
 
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