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Rain readings in exceptionally windy weather

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TwoTonTed
Posts: 12
Joined: Wed 26 Aug 2015 5:39 pm
Weather Station: Davis Vantage Vue
Operating System: Windows 7
Location: St Brides, Vale of Glamorgan

Rain readings in exceptionally windy weather

Post by TwoTonTed » Sun 17 Mar 2019 10:28 am

Sorry if this has been discussed endless times but I dont think I'm getting accurate readings when the wind is up above 25mph.

I have a Vantage Vue installed here near the coast in South Wales with no interference from any buildings, so a decent location. Over the past few days I'm getting gusts around 40 mph. I'm not sure that the rain gauge is capturing all the rain at that wind speed - like it just gets blown past! There's nothing I can do about it of course, just a matter for discussion if anyone feels like it!

Yesterday for example, a heavy rain front came through with the wind gusting 40 and I recorded around 5mm. Locally, according to Wundermap, I see that everyone else recorded around 7 or 8mm. The gauge is clear of debris and functioning ok I believe.

Thanks for any views!

freddie
Posts: 495
Joined: Wed 08 Jun 2011 11:19 am
Weather Station: Davis Vantage Pro 2
Operating System: GNU/Linux (Ubuntu 18.04)
Location: Dorrington, Shropshire
Contact:

Re: Rain readings in exceptionally windy weather

Post by freddie » Sun 17 Mar 2019 1:01 pm

You've come up against one of the shortcomings of PWS units such as the Vue. In meteorological measurements there are standard heights above ground for measuring the various weather elements:
  • Wind: 10 metres (33 feet): This is to reduce the effect of turbulence and friction from obstacles such as buildings, fences and trees.
  • Temperature: 1.25 metres (4 feet): This is to reduce the effect of radiative heat transfer from/to the underlying surface. Nearer to the ground the temperature difference is greater (higher maxima, lower minima).
  • Rainfall: 30 cm (1 foot): This is to reduce the effect of wind flow across the gauge funnel mouth. Ideally the gauge should be installed in a man-made depression in the ground, which should be equal in depth to the rain gauge height, but the edges of the depression to be a distance from the funnel edge to prevent splashing affecting readings.
With a unit such as the Vue, you will need to compromise on two of the three quantities measured. The only way around this is to use kit that can be separated into consistuent parts and each part then mounted at the appropriate height.
Freddie
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TwoTonTed
Posts: 12
Joined: Wed 26 Aug 2015 5:39 pm
Weather Station: Davis Vantage Vue
Operating System: Windows 7
Location: St Brides, Vale of Glamorgan

Re: Rain readings in exceptionally windy weather

Post by TwoTonTed » Sun 17 Mar 2019 5:20 pm

thanks Freddie

This explains matters I'm sure with it being an integrated unit. My station is on a pole about 2m above ground which explains things but interesting info you've provided nevertheless!

freddie
Posts: 495
Joined: Wed 08 Jun 2011 11:19 am
Weather Station: Davis Vantage Pro 2
Operating System: GNU/Linux (Ubuntu 18.04)
Location: Dorrington, Shropshire
Contact:

Re: Rain readings in exceptionally windy weather

Post by freddie » Mon 18 Mar 2019 5:35 pm

Also, remember that rainfall totals can and do vary over short distances. You probably didn't lose a massive amount at 2m, compared to what you would lose if your station was mounted at roof level. FWIW I think that a mounting height around the optimum temperature height is the best compromise, as you can apply a correction to the wind speed quite easily.

P.S: I live 20 km or so to the northeast of you (Rhondda valley) and recorded 30 mm on Saturday!
Freddie
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ExperiMentor
Posts: 103
Joined: Tue 24 Nov 2015 11:30 pm
Weather Station: Fine Offset & Davis Vantage Vue
Operating System: Windows 10; Raspbian Buster
Location: Switzerland

Re: Rain readings in exceptionally windy weather

Post by ExperiMentor » Mon 18 Mar 2019 8:13 pm

More often the problem is the other way round - high or gusty wind causes false (high) rain measurements. The wind batters your hardware and makes the see-saw mechanism of the rain gauge tip back and forward, which is recorded as rain even if it is perfectly dry. A secure mounting should help.

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