You may recall that back in May I did a simple preliminary experiment to give me guidance on a hypothesis: That changes in paint on Stevenson Screens over time make a measurable difference on the temperatures recorded inside them. This stems from the fact that when the Weather Bureau commissioned the design in 1892, whitewash was specified. But whitewash is no longer commonly available, and the National Weather Service changed the specification in 1979 to be semi-gloss latex paint.
But, cured whitewash is composed of Calcium Carbonate, while latex paint uses Titanium Dioxide as a pigment. While they both appear “white” in visible light, they have vastly different properties in infrared.
My first simple experiment used thermistors in boreholes into 3 wood slats; 1 bare wood as a control, the other two painted with whitewash and latex, showed me that there was a measurable difference in the temperature of the wood by as much as 2-4 degrees at times. I needed to do that experiment before I embarked on the full scale test, since each of the Stevenson Screens you see here in the pictures cost me about $1000.00 Since I’m doing this out of pocket, with no funding or grants, I had to try a small scale test first.
The photos show 3 standard Stevenson Screens as used today in the United States. One is bare wood, unpainted, as a control, the middle one is latex, as sent from the supplier, and the third is painted with a historically accurate (for early 20th century) whitewash mixture that I obtained both materials and formula from the head chemist at the National Lime Company.
The device on the tripod is a stacked plate IR shield with a small fan to pull air through, commonly called an aspirated shield. It is the air temperature reference and placed at the same exposure height as the thermistiors in the screens. Also nearby but not shown is a pyranometer to measure solar insolation and wind speed/direction sensors that are being datalogged as well.
Each Stevenson Screen and the air temperature reference sensor are fitted with matched, calibrated thermistors, NIST traceable with certificates, that are connected to a calibrated data-logger, also with a certificate. The resolution is .01 degree Fahrenheit with an accuracy of +/- 0.1 degree over the range.
I expect that the air temperature differences inside the screens will be less than the 2-4 degrees I observed in the paint slat test. It’s possible that there will be no significant difference at all. I won”t know until I run about a months worth of datalogging.
The site, while not ideal due to the trees, is the best I could get permission to use. Fortunately the trees do not directly shade the screens except for a short portion of the day. It’s also out of the way, so vandalism will not be likely. Since it had to be an unwatered grass field, concerns over fire danger were raised from some I asked because of the electronics package, so I had limited choices. Perhaps later I’ll be able to find a better site but for now it will have to do.
The whitewash on the third Stevenson Screen is still curing, as the chemical reaction is not yet complete to convert Calcium Hydroxide to Calcium Carbonate. In about a week, I’ll make the data available via a web link in near real-time.
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