Rain Gauge and Wind

[wd40]

I was doing a bit of digging on rain gauge design and have come to a conclusion that calibrating or comparing a modified Fine Offset to a smaller diameter manual gauge can be misleading.

Here is a really good source of information. The History of Raingauges by Ian Strangeways

http://www.rmets.org/pdf/presentation/2 ... geways.pdf

The increased area of a Fine Offset rain gauge modified to reduce splash out, along with its profile may produce a higher measurement in higher wind conditions than a smaller area manual rain gauge due to wind blow over. That is what I have seen.

In a no wind shower, the manual. smaller opening area gauge and my modified fine offset gauge read the same. In a high wind rain storm, the Fine Offset reads higher. This higher reading I now believe, is due to the ability of a larger area gauge to capture blow-over.

The standard diameter gauge is 5"(12.7 cm) with straight sides best I can tell. The straight up sides seems to deal with the wind a bit better.

[apenwith]

Hi

That seems an odd result - you would expect that the cross section of rain entering any container would reduce as the angle of the rain changes until the ultimate of horizontal rain when in theory no rain would enter.
Looking at the picture I wonder if turbulence in the aluminium extension is tipping the buckets but then you would see phantom rain in high winds.

Regards
Alan

[wd40]

Alan, I may have not been clear. I am talking about high winds while it is raining. I know I have had high winds without it raining and have not seen any rain recorded. Still you have a good point.

This Dr. Strangeway, is the guru of rain measurement from what I have read. He has written a book on rain measurements. I think he ran test in a wind tunnel.

The shape of the gauge does effect blow-over. That seems to be true.
A couple of posters have noted using a manual gauge as a calibration tool for a modified bucket tip gauge but that may contribute its own error.

By the way it is interesting what you can find by doing a google search on designing your own rain gauge.

[beteljuice]

I was thinking there would be a more noticable difference with wind direction because of the 'apparent' csa due to the oblique angle on the rectangle, but my math is no longer upto calculating the apparent capture area difference between the 'squashed' rectangle (letter box) and the 'squashed' cylinder (elipse).

I suppose the point (as with most weather observations) is that a standard was created, and although modern technology / thinking can do better, the old standard must be adhered to for comparative purposes.

Consider modern truly electronic precipitation equipment that can even measure droplet / hail size - what can you compare those with ?

An old summary of rain gauge limitations can be seen here: http://journals.ametsoc.org/doi/abs/10. ... 2.0.CO%3B2
Although of course your gauge should be in the open but in a wind 'shadow'

[wd40]

The general conclusion I drew from this is, if your area corrected modified Fine Offset reads less than a good manual gauge, you still have a problem. If the modified gauge read more than your manual gauge, and your math is correct, then the modified gauge is correct.