height of a temp sensor from the ground question

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1

Is there a difference on the height of a temp gauge from the ground? How far off the ground should I have my sensor? I just moved it because it was reading way to high being on the ne side of the house. Its now on the north end of the house right below a bedroom window. It looks like my max temp today was 71.4 while happy valley got 1 degree warmer then me. Happy Valley usually is 2 degrees cooler then my station, and the other closet station got 3 degrees warmer then me. Should I put the sensor higher? Thx

http://www.wunderground.com/weatherstation/WXDailyHistory.asp?ID=KORPORTL110

2

The World Meteorological Organization (WMO) agreed standard for the height of the thermometers is between 1.25 m (4 ft 1 in) and 2 m (6 ft 7 in) above the ground.

3

thank you!

4

Are you using the original WMR200 radiation shield?

5

yes I am

6

So what you have to do is changing to a well known working one as Davis, Pete Bross, etc… That kind of “mushroom” is pretty useless.

7

do u have one that you would recompensed? link?

8

Do a search on Ambient Weather’s website for ‘SRS100’ and/or ‘SRS100LX’

9

http://www.peetbros.com/shop/item.aspx?itemid=54

10

Just a couple observations…

  1. The WMR100/200 is not a precision instrument. These stations are basement priced for good reason…

  2. Comparing your readings to some other station at another location is not the way to calibrate your sensor.

We all strive to do the best we can when mounting our sensors, compromises must be made in almost every situation…

11

I thought this was interesting: http://www.davisnet.com/product_documents/weather/app_notes/apnote_24.pdf

It suggests that even with a well-made non-aspirated radiation shield, you should expect temperature readings to be as much as 10 degrees F about actuals. I’m seeing that with my homemade shield and it was very frustrating until I read that PDF from Davis. Now I don’t feel so bad.

This fan-aspirated shield http://www.allweatherinc.com/pdf/8190.pdf is very unusual-looking. Could I make this myself out of PVC? I assume the temp sensor is on the right and the fan draws air from right to left, ultimately out the bottom left?

12

This page has a link to the manual, the probe is installed in the vertical tube on the right. Yes it should be a simple project, I think the only downside is the need for a 24 hour fan and you probably need some kind of fan monitor/alarm because if it stops when the sun is shining the sensor will heat pretty quickly.

13

It is a pity that they didn’t include a comparison to a traditional Stevenson screen. All the radiation shields in the study appear to be single-louver pie-plate designs. I think if you are going to go with a non-aspirated shield, you have to have double louvers, as in a Stevenson screen, to prevent reflected and radiated heat from ground sources.

14

I found a study that compares various radiation shields to a Stevenson screen, for what it is worth.

15

That’s a good one with some real world examples of simple screens.

16

I wonder what the reasoning is that the fan asp shield above needs to be so long? (42")

17

I noticed that too. A couple of reasons that occur to me are so that it doesn’t aspirate its own exhaust, and so that the intake can be well clear of whatever it’s mounted on/to.

18

One key thing I found, if you setup aspiration then make sure the temp sensor can’t get wet from dew / frost.

I had aspiration setup and on frosty mornings the aspirated sensor would drop several deg C when turned on (i.e error)

The other thing about aspiration is that it results will be different to non aspirated stations, especially on hot windless days. Eg Here in Australia the BOM don’t use aspirated shields.