erratic pressure readings

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erratic pressure readings

Postby Scott Whitehead » 2013 Jun 07, 15:53

Since 10am (when the sun more or less disappeared) the trend of my barometer on my AWS has been very unstable (a squiggly line) - it is usually relatively smooth, even with the approach of a low pressure centre. I understand the thundery activity affecting IoW and south - but can anyone add to what's going on at surface level further east.
10z 1021.6
11z 1021.5
12z 1021.7
13z 1021.7
14z 1021.5
15z 1021.2
16z 1020.9
Wanstead (east London)
Scott Whitehead
Wanstead, East London - 18m N 51° 33' 52" E 00° 02' 25"
Scott Whitehead
Posts: 21
Joined: 2013 May 03, 15:02
Location: Wanstead, East London - N 51° 33' 52" E 00° 02' 25"

Re: erratic pressure readings

Postby Stephen Burt » 2013 Jun 07, 22:13

Thundery conditions can and often do affect surface pressure over a much larger area than is directly affected by the storm's precipitation - sometimes hundreds of kilometres away. The effects are due to large variations in air density caused by convective activity and ensuing vertical motions within and around the storm, particularly downdraughts, resulting in gravity waves propagating outwards from the 'epicentre'. These gravity waves can be dampened, amplified or reflected by topography and atmospheric conditions and can propagate very rapidly - sometimes at or close to the speed of sound (some are effectively very low-frequency sound waves).

Today's pressure variations have been very minor; some storms produce fluctuations of 3-5 mbar within a few minutes. As a recent example, two years ago, on 28 June 2011, there were very marked ‘barograph jinks’ here during a short spell of hot, humid thundery weather. There was a fall of 2.0 mbar in 10 min (1015.7 mbar at 1036 utc to 1013.7 mbar at 1046) followed by a partial recovery of 1.5 mbar in 4 min (1015.2 mbar at 1050 utc). Apart from the highest gust of the day (14 kn) occurring at 1045, there was no other significant weather at the time. There were further fluctuations of the barometer 12-17h, ranging 2 mbar in all. I have many other similar or larger instances in my records!

Stratfield Mortimer, Berkshire - central southern England
51.4°N, 1.0°W, 60 m AMSL, station grade A - AAAA47R
Records commenced here 1987 - local records available back to 1862
The Weather Observer's Handbook:
Stephen Burt
Posts: 247
Joined: 2011 Dec 02, 19:36
Location: Stratfield Mortimer, Berkshire; a well-exposed rural site, 10 km SW of Reading

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