Weather Station Hardware > What Weather Station Should I Buy?
FARS vs Passive Temperature Shield
kcidwx:
For the first time I have both a FARS and passive radiation shield operating together. Now that November is over, I decided to take a look at the first full month of data. The FARS is a RM Young with a Nimbus temperature probe inside. The passive shield is a Davis with the Weather Monitor II temperature/humidity sensor inside. Before installation in October, both sensors were tested across the range at six different temperature points between -20°F to 100°F. Here's a photo of the sensor suite and a copy of the data collected for November.
Aardvark:
The difference isn't that much. I have the same thing as well set up and I am noticing a little bit between both. I was told that the differences are noticeable during hot weather and humid climates.
I am keeping data as well and probably within a year of amassing data, I will have a much better comparison that I have now
kcidwx:
That is true and I expect to see the biggest difference in next summers data. I was surprised at November's data. I was expecting the passive shield to be 1.5°F to 2°F higher.
Skywatch:
Here in Texas it gets pretty hot. I rarely ever see anything more than a 0.5 degree difference from neighboring stations. Some of them are Fan aspirated and most are passive but on the hottest days in summer or the coldest days in winter I rarely ever see more than a fraction of a degree off. Same for humidity.
As I mentioned in another thread The Weather Observer's Handbook the author did a similar test with the same radiation shields and for those who have the book provides a graph on page 106. Basically the same thing. No more than a fraction of a degree off. I'm not sure about the WMII shield but on my VP2 shield there is a ring of holes around the middle 3 plates. My theory is this allows for convection aspiration. In low wind conditions as the plates and surrounding air heat up the holes allow for excess heat to dissipate.
johnd:
Rigorous comparative studies of the 7714 Davis screen are of course reported on in both:
Davis's own application note #24 (admittedly some years old now, but no less valid for that)
http://www.davisnet.com/product_documents/weather/app_notes/AN_24-temp-radiation-shield-comparison.pdf
and in the WMO report at:
http://www.wmo.int/pages/prog/www/IMOP/publications/IOM-106_Ghardaia/IOM-106_Report.pdf
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