Hi listeners!

I noted that the pressure changes around the passage of the Occlusion at about 2130Z on Fri 4th
had a pattern I had seen before as a Met Observer, but had not experienced (or not noticed!)
since I have had my Davis Vantage Pro.

Just before the surface front passed, the pressure rose from 992.0 at 2107z to 992.7 at 2122z
It then fell 2.4 hPa to 990.3 at 2134z (fastest being 2.0 hPa from 2126 to 2133) after which it
started to slowly rise.

So what causes such rapid changes of pressure? In this case up to 2 hPa in 7 minutes?

You can see a graph of weather parameters at the link below if you select 'Current last 24 hour
graphs' at bottom of page (at least up until 2100 tonight!)
--
Roy Avis
Bracknell Weather/Webcam updated every minute plus Historical Data at:
http://homepage.ntlworld.com/roy.avis/WDLive/index.html
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On 5 Jan, 11:27, "Roy Avis" wrote:

I noted that the pressure changes around the passage of the Occlusion at about 2130Z on Fri 4th
had a pattern I had seen before as a Met Observer, but had not experienced (or not noticed!)
since I have had my Davis Vantage Pro.

Just before the surface front passed, the pressure rose from 992.0 at 2107z to 992.7 at 2122z
It then fell 2.4 hPa to 990.3 at 2134z (fastest being 2.0 hPa from 2126 to 2133) after which it
started to slowly rise.

So what causes such rapid changes of pressure? In this case up to 2 hPa in 7 minutes?

Roy Avis
I see nobody has answered your question -so I'll give it a go.
Pressure rises of this type are usually due to the hydrostatic effect
at the cold frontal head- ie the pressure due to weight of a nose of
cold air at the leading edge of a the front. The effect is also seen
at the leading edge of thunderstorm outflows. The usual model for this
is a gravity or density current. Precipitation falling into the cold
frontal head from front to rear flow can enhance the effect.
See my recent post on the utility of such observations,
regards,
David

Thanks David - I thought no one cared!!!
I think I understand your explanation - though I am having to get my old forecasting textbooks
out!
I note your recent post of 08/1008z

Roy Avis
Bracknell Weather/Webcam updated every minute plus Historical Data link at:
http://homepage.ntlworld.com/roy.avis/WDLive/index.html
++++++++++++++++++++++++++++++++++++++++++++++++++ +++++++++++++++++++++++++++++++++++
PC and all mail in/out check by latest version of Norton AntiVirus/Internet Security


"Waghorn" wrote in message
...
On 5 Jan, 11:27, "Roy Avis" wrote:

I noted that the pressure changes around the passage of the Occlusion at about 2130Z on Fri
4th
had a pattern I had seen before as a Met Observer, but had not experienced (or not noticed!)
since I have had my Davis Vantage Pro.

Just before the surface front passed, the pressure rose from 992.0 at 2107z to 992.7 at 2122z
It then fell 2.4 hPa to 990.3 at 2134z (fastest being 2.0 hPa from 2126 to 2133) after which
it
started to slowly rise.

So what causes such rapid changes of pressure? In this case up to 2 hPa in 7 minutes?

Roy Avis
I see nobody has answered your question -so I'll give it a go.
Pressure rises of this type are usually due to the hydrostatic effect
at the cold frontal head- ie the pressure due to weight of a nose of
cold air at the leading edge of a the front. The effect is also seen
at the leading edge of thunderstorm outflows. The usual model for this
is a gravity or density current. Precipitation falling into the cold
frontal head from front to rear flow can enhance the effect.
See my recent post on the utility of such observations,
regards,
David