Probable dew forming on instruments, including pyrgeometer

Posted: February 17, 2013 by tchannon in atmosphere, Measurement, Surfacestation, Uncertainty, weather

Image

Probable dew forming on instruments at Chilbolton Observatory

I’ve been coming up to speed on watching a half pyrgeometer at the Chilbolton Observatory, Hampshire, England, close to where I live and so the conditions here are very similar. I’d previously observed what I took to be dawn dew/frost forming.

For investigation purpose I’ve scripted grabbing a set of images from their web site for the previous day, for occasional use. (Lua 5.1, ask if you want a copy, just a quick hack)

First set was for yesterday. On looking I noticed after dusk a notch in some of the data which looks right for dew forming. According to the BBC weather centre at 00:30, no frost in this area… I looked at the thermometer here, 1.2C not well exposed, checked outside, ground frost. I’ve been out late when there was already a heavy dew.

The images are aligned and vertical gauge lines added.

Commentary

Looks like the LIDAR suffers from dew too.

Humidity instrument is somewhat suspect on accuracy (reluctant to go exactly 100%), nothing unusual are poor instruments generally. The exact location relative to the other instruments will matter since the effect is progressive and localised. A difference in timing is to be expected.

A surprise is an effect on the wind data, although with such light wind maybe dew forms too. On the other hand strange minor airflow change does happen as atmospheric conditions change.

The pyrgeometer shows an excess incoming radiation figure, where for some parts of the world will often happen twice a day. Looks like a few percent increase when integrated. I don’t know what this would do to a net instrument.

Data

set of plots of thumbnail photo for 16th Jan 2013 here  (100k .zip)

Suggestion

Hourly sky photographs are taken, a very useful check on what is actually there. However, it’s a pity a low light camera is not used. This is not hard to arrange these days. Clouds etc. at night can be shown, anything is better than nothing. The cloud radar is only intermittently active.

I suspect this is outside of normal interest and yet night-time atmospheric conditions are under-represented in data and knowledge, as well as highly critical to planetary information. Half the data.

I thank Chilbolton and their supporters for providing plots via the web.
http://www.stfc.ac.uk/Chilbolton/weather/24987.aspx

Previous stuff http://tallbloke.wordpress.com/2013/02/11/discussions-on-pyrgeometers-ir-measurement/

Post by Tim Channon

Comments
  1. michael hart says:

    It’s not outside of my interest, Tim, and it is pertinent to actually collecting data.

    I have often wondered how surface-based temperature indices accommodate diurnal temperature ranges that regularly straddle zero Celsius (in locations that are not dry). Do we see an obvious signature for this phase change of water in the temperature records?

  2. tchannon says:

    Added a zip of all the plots and thumbnail sky photo

  3. tchannon says:

    I decided to omit information on the pyrgeometer which contain inbuilt heaters as too distracting.

    The general idea is transient usage during periods of dew, frost or snow formation.

    Discussion and argument says this is the least of two evils trading some degree of error from the heater for as much as 100Wsqm error from dew. Sensibly they say heater usage should be recorded as part of the data and quality flags set accordingly. Although not stated this is obvious a matter of software control. I haven’t seen a discussion of how conditions where heating is appropriate is detected.

    http://www.hukseflux.com/product/ir02-pyrgeometer

    Maybe obsolete
    http://meteocentre.com/StationUqam/instruments/Radiometre/kipp_manual_cnr1_1176.pdf

    Or a fan
    http://www.kippzonen.com/?product/7071/CVF+3+Ventilation+Unit.aspx

  4. AlecM says:

    Dew and ground frost prove that ‘back radiation’ do not exist.

  5. AlecM says, February 18, 2013 at 7:43 am: Dew and ground frost prove that ‘back radiation’ do not exist.

    AlecM, That’s an assertion of no value to anyone who doesn’t already agree with it. An explanation would be much appreciated.

    Thanks.

  6. Richard says:

    AlecM says:
    February 18, 2013 at 7:43 am
    Dew and ground frost prove that ‘back radiation’ do not exist.

    Please! You can describe an energy flow with one way arrows or bi-directional ones. Provided the total comes to the same thing all that is being described is a different viewpoint.

  7. tchannon says:

    AlecM, the situation might be reversed from how you see things, it hasn’t clicked.

    The normal situation is continuous loss from the ground day and night, including in bright sunshine. For the purposes here the sun does not exist even though it does, it has be notional magic of sun heat appearing as heat in the ground during daytime. (reason: comes in using wavelengths not in what we are talking about)

    If the ground cools enough, is wet enough during fading day through night dew and then frost form.

    However, if there is cloud it shuts off the way out, if it is day, most of the way in too. Whatever heat is there below the cloud bounces around flowing no-where, just internal flow summing to zero. We get those flat grey days of static temperature.

    The massive elephant is water. Anything else is very small. Leave that as just a complicating detail.

    Would it help if I drew up a picture of what is basically going on? I can do this as an article. I am probably making a problem for myself here, not so easy but if it helps needs doing.

  8. Max™ says:

    Please! You can describe an energy flow with one way arrows or bi-directional ones. Provided the total comes to the same thing all that is being described is a different viewpoint.” ~Richard

    If you’re treating the radiation from a colder surface as a source responsible for raising the temperature of the warmer surface, yes, it is completely different.

    When two surfaces are radiating towards each other, the colder surface doesn’t add energy to the warmer surface, it just reduces the losses.

    ______

    As I had to point out elsewhere recently, much to the surprise of the other poster I was discussing this with, you can’t use a pv panel and generate electricity with back-radiation unless you cool the panel down, or have some other sink for the waste heat radiation off of the panel itself.

  9. tallbloke says:

    Tim Channon says:
    “The massive elephant is water. Anything else is very small. Leave that as just a complicating detail.

    Would it help if I drew up a picture of what is basically going on?”

    Yes please. Make a new thread with it.

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