Author Topic: Temperature measurement  (Read 408 times)

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Offline p3aul

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Temperature measurement
« on: September 25, 2021, 04:48:03 PM »
There are two types of shade I know there is open shade when you stand in the shadow cast by a tree and closed shade which is under the tree itself.

Which type of shade is the official temperature measured?

I remember seeing as a little boy, a box with ventilated sides where the air could flow through, with a thermometer in in to get the official temperature at a weather station. Is this still done today? I guess a sensor under a cup in a home weather station would qualify as closed shade?
Thanks,
Paul

Offline sky_watcher

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Re: Temperature measurement
« Reply #1 on: September 25, 2021, 07:48:18 PM »
I remember seeing as a little boy, a box with ventilated sides where the air could flow through, with a thermometer in in to get the official temperature at a weather station. Is this still done today?
The box is called a Stevenson Screen and they are still in use.
“The more a man knows, the more willing he is to learn. The less a man knows, the more positive he is that he knows everything...” ― Robert G. Ingersoll

Offline KathyLauren

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Re: Temperature measurement
« Reply #2 on: October 02, 2021, 10:34:23 AM »
The temperature you want to measure is the temperature of the air.  The tricky question is which air you want to measure.  Official temperature measurements are made in a Stevenson screen that is five feet above the ground. 

I have an auxilliary temperature sensor (the last surviving relic of my old weather station) in a home-made Stevenson screen, and it is interesting to see how its temperature compares to the sensor in my Davis VP2.  Warming up in the morning, the two sensors are very comparable.  After sunset, though, the sensor in the screen lags behind the VP2 by about an hour.  Clearly the structure of the Stevenson screen retains a bit of heat, especially if the wind is light.

To get an accurate measurement of temperature, you need to shield the sensor from direct solar radiation, and also from heat reflected and radiated from nearby objects, including the ground.  But you want it out in the open so that you are not measuring a forest or urban micro-climate.

Offline TrugWX

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Re: Temperature measurement
« Reply #3 on: October 02, 2021, 10:20:13 PM »
As others have mentioned, the box is called a Stevenson Screen and yes they are still a standard feature of modern automatic weather stations. This is essentially to maintain as close as possible the homogeneity of the observing environment over time.

I have my liquid-in-glass thermometers as well as my electronic temperature sensor inside an ex Bureau of Meteorology (AU) Stevenson Screen that I have had and maintained for over 40 years. If you are interested I have photos of the exterior and interior of the screen at http://www.truganinaweather.com/data/station-metadata.xml.

Offline DaleReid

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Re: Temperature measurement
« Reply #4 on: October 02, 2021, 11:04:53 PM »
It will be interesting to see what happens when (and if, although it is 'promised') will happen when the Screens get painted with this new ultra reflective white paint, with Barium Oxide in it, I think.

Reports are that it not only reflects nearly all incident radiation, especially sunlight, but also allows internal temperature to radiate outward.  If that is the case, the supposition is that things will be cooler than the surrounding air.  I'm not sure the final analysis is in yet and won't be until gallons of this stuff at hopefully affordable prices will be available for experimentation.  The hope was that this paint would allow roofs and trailer homes and all sorts of shelters (livestock shelters, too?) to be coated and make for more comfortable situations without as much air conditioning.  Humidity of course would be a continuing concern when dew points go up.

I hope this comes out soon.  I'd like to set up a couple control situations, not with a Stevenson Shield since I don't have any, but with a couple of radiation shields like RM Young or Campbell Scientific side by side, one painted with the ultra white and the other just a standard one.

But as far as the shields are concerned, as a kid in the 50s I can recall going to ranger stations and where the Weather Bureau had installations and seeing these cool louvered boxes on legs, and when the slight chance of seeing inside getting to see the max min thermometers, a recording humidity and temp (Bendix?) all in there and feeling such intense jealousy as to not live closer so I could come visit more often.  Of course my woodworking skills were deficient and I never could build one.

Hats off those who have or who were able to find one for sale before they became pretty hard and expensive to find.
 
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