Alter Type Windscreen

It can be fairly windy here especially in the spring, fall and during snow storms. My CoCoRaHS rain gauge does not catch all the precipitation it should when it is windy. This is a common problem and a WEB search or two turned up a number of different solutions. Most involve some sort of screen around the gauge to block the wind. The best ones are rather large multiple rings of snow fence around the gauge. I could do that but it is a bigger project than I want tackle right now and there is the problem of keeping the grass mode in and around the shield. The Alter Type windscreen looks like a good compromise and it also looks a bit like some lawn art mobile.

The commercial Alter type windscreens are 4 feet in diameter and have a movable section to allow access to the rain gauge. I did not want all that complexity so I chose to build mine 3 feet in diameter.

The shields are 20 gauge galvanized steel strips 14 inches long,  three inches wide at the top and 2 inches wide at the bottom. The edges of the strips are bent up to stiffen them and catch the wind.   Twenty gauge sheet metal is what heating furnace duct work is made of so I picked up some duct work parts at the local building supply store. I happen to have a metal shear/brake so making the shields was easy. One part that requires some care is drilling the mounting holes in the edges of the strips. Drilling sheet metal can be dangerous if you don’t take the proper precautions. The metal tends to catch  and ride up the drill bit. I solved this problem by clamping the entire stack of 24 unbent shields to the drill press table with a piece of heavy metal already drilled with the proper sized hole. By drilling down through the hole the sheet metal can’t catch or ride up the drill bit.

The strips hang on a metal ring supported by four legs. I chose to make the ring out of 1/4″ diameter steel rod because it seemed adequate and I happened to have some in the shop. To form the rod into a nice circle I wrapped it around a bicycle wheel rim about the right diameter. The ring came out 30″ in diameter not 36″ but I decided to call it good.

The legs consist of 1/2″ EMT (conduit) with holes drilled to fit the ring and the ends flattened where they are attached to the gauge support post. Additional pieces of 1/2″ EMT serve as spacers for the shield strips.

To mount the gauge and shield I planted a 5 foot piece of pressure treated 4X4 in the yard. This rain gauge will be in addition to the existing CoCoRaHS gauge at least until I find out how well it works or make any modifications that come from the testing. Initially it looks good but the supports may need some reinforcement for high winds.


The installed gauge with Alter type shield. This picture was taken before I adjusted the height of the shield. The top of the gauge is supposed to be 1 inch below the top of the shield.

There is a short video on YouTube of the windscreen in action during high winds.

For comparison here is a video of a professional double Alter type shield.

1 Year November 8 2014

For the past year I have operated the shielded rain gauge and an unshielded gauge. The two gauges are in similar locations. During the year the unshielded gauge collected 30.95 inches of precipitation. The shielded gauge collected 33.35 inches. For 7.75% more precipitation collected by the shielded gauge. Of the 109 days with precipitation 84 were rain and 25 were snow, freezing rain or sleet. There were five days out of the 109 where the shielded gauge had less precipitation than the unshielded gauge and 32 days where there was the same amount in each gauge. On the five days where the shielded gauge had less precipitation than the unshielded gauge the unshielded gauge collected 3.8% more. On days that it snowed the shielded gauge collected 56% more than the unshielded gauge. For rain only the shielded gauge collected 5.5% more than the unshielded gauge. I expected that the shield would be more effective for snow but the amount of improvement is surprising.

I believe it is reasonable to assume that the shielded gauge provides a more accurate measure of the precipitation at this site.




Alter Type windshield

Windshield types

This entry was posted in Amateur Science, DIY, environmental monotoring and tagged , , , . Bookmark the permalink.

5 Responses to Alter Type Windscreen

  1. Pingback: IoT Rain Gauge | Jim Hannon's Blog

  2. Peter Sharpe says:

    Dear Mr. Hannon – I really like this design and was wondering if I might have your permission to use the image in this blog for a presentation I’ll be giving at a workshop in Indonesia. This is a great example of a low cost way for folks to accurately measure precipitation.

    Pete S.

  3. jimhannon says:

    Yes you can use the image. I would appreciate credit for it.

    • Peter Sharpe says:

      Thank you very much – I will make sure your photo credit appears on the slide.
      Best Regards,

  4. cch001 says:

    Have you considered that you are now recording non-standard CoCoRaHS observations, since virtually the entire network do not have wind shields? One of the great things about CoCoRaHS is the standardization of the measurements.

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