This drawing shows the basic internals of a simple passive pyrgeometer.
Heat flows from roughly earth ground temperature into the body, finds it’s way through the body to the underside of the thermoelectric generator, then through that and for a clear sky then radiates from the top side to space, unless there are heavy clouds or it is raining.
I repeat, heat flow is from the ground upwards. (under very rare meteorological conditions a minor reverse flow happens, the former is overwhelmingly dominant)
This is far from the whole convoluted story, which I will now try and explain. In reality it is very simple.
Some of you will have seen pyrgeometer data with values in the 200 to 400 Watts sqm range perhaps with mention of “downwelling Infra Red”. This has caused a great deal of confusion and contention.
These devices do not measure “downwelling Infra Red” and the figure is a computed number based on the actual outgoing IR and the temperature of the instrument body.
Put in another way, such a figure is a backwards reading with human added offset.
There is a basis for doing this but no excuse for obfuscation, or a failure to explain to lay readers there is a silent jump of context to theoretic.
I will explain more after the next graphics section of the article.
With the following graphics I have tried to take care of this confusion (labelled net_IR_out) so just concentrate on trying to understand what the variation of outbound is saying, how if varies with weather.
This is real data from a similar instrument in southern England during one day this winter when heavy cloud passed over laden the snow, then in the middle of the afternoon the sky cleared to hazy sun. Late on another bank of cloud started to arrive.
Sky photo of near blue sky does actually show thin high cloud, not fully clear.
Cloud, very little heat radiates from the ground. Clear sky, heat radiates from the ground.
The following overlay plots do not have matched zero, I’ve offset for clarity, we are dealing with shape here.
Overlay plot of IR outbound on 35GHz cloud radar graphic.
Overlay plot IR outbound on Lidar graphic. Notice lidar is less able to see the incoming higher cloud. The dense low cloud during the first half of the day shows as a red line with very little seen above that, a laser has less penetration than radar.
You will notice significant undulation in the trace around 15 hours. This will be small cloud not being shown by radar or lidar.
The thorny problem of computed offset
There are two kinds of these pyrgeometer according to the fundamental configuration
- single ended
- double ended, which is two single ended, on pointing up the other down at the ground
- full differential which like double ended sees both ways but without an intermediate
The commercial “full” instruments comprise four single instruments (or two full differential), where the additional two are measuring short wavelength light, sunshine.
This is necessary to try and measure both shorter wavelength radiation incoming and reflected, plus longer wavelength IR incoming / outgoing and reflected. Only from this data can a reasonable estimate be made on the total radiation energy flow into and out of the surface of the earth and only for that precise location.
For example the Kipp & Zonen CRN1 instrument is a net radiometer comprising four units, two pyranometers and two pyrgeometers.
Spectral response of CNR1, overlay to show as one. Strictly solar irradiance should always be shown using log-log plot axis. This would not only show that irradiance follows a log law but in this case would show the filters as somewhat less than ideal. It is possible to produce astoundingly good filters, is done by hand for aerospace and space application, at a very high cost, needing a 100 or more separate layers spluttered onto a substrate.
The difference in height of the two responses above doesn’t matter, calibration of the instrument adjusts that out.
Typical singled ended instruments additionally contain a body temperature sensor feeding a second output cable. An external computer digitises both the thermopile and temperature signals.
The temperature reading is then used as a second input to the formula computing the theoretic offset radiation value as the equivalent downward infra red radiation. There is no direct measurement, no such entity exists and this is the much talked about “downwelling” radiation or heat. (there is a jump of context)
This is literally the Stephan Boltzmann equation applied to an approximation of the temperature of the ground, or more precisely whatever is radiating. It is neither the body temperature of the pyrgoemeter nor the air temperature, but there is usually little difference. Also, no compensation is done for ground emissivity.
A full double ended instrument is more able to measure the up and down flow because it takes the very local ground and ground cover into the measurement.
This is the formula shown in the Kipp & Zonen manual for the CGR4 instrument.
Uemf is thermopile voltage, S is constant calibration factor for that particular instrument.
If it is raining the sensor output is zero the Uemf/S term in the equation equals zero which leaves the Stephan Boltzmann term 5.67E-8 . T4, where T is the body temperature of the instrument, or could be some other thermometer.
This data from 17th March 2013 is a reasonable demonstration of raining and zero output.
Top row second from left is the output of the formula for a CGR4 instrument showing about 325 Watts sqm downwelling IR during the rain storm but there can be no output from the thermopile. I have no data for the CGR4 body temperature but there is an air temperature reading. Since it is raining it is reasonable to whole lot is close to the same temperature, therefore it is fairly safe to back-calculate the air temperature via Stephan Boltzmann to Watts sqm for that air temperature and subtract this figure from the pyrgeometer reading.
This is the bottom right plot labelled net_IR_out where we find as near as makes no difference a value of zero during the rain storm.
And that is what the fuss is about.
Internally within a theoretic set of mathematics, figures can be split into what is going on, we hope, but it is not directly measured by these instruments. Actually measuring needs vastly more expensive equipment but that is an issue I am not covering here.
Those who think there is no “back radiation” are both right and wrong, two different situations. One way showing that internally within a system there must be a heat interchange is by considering time, any alternative would need knowledge of the future, the speed of light says no. (and this would be trivially easy to measure)
I hope it is obvious that water and water vapour dominate what is going on, clouds are dominant.
Independent confirmation, a table from the manual for Kipp & Zonen net radiometer (duplex device, four sensors)
Any effect from eg. CO2 is negligible in this data and would only be detectable under very clear sky with very low humidity, such as in some deserts or using very selective instrumentation.
None of this deals with phase change or gas movement as heat transport. The entire world thing is incredibly complex, as if we know much at all.
The Talkshop has several discussions to do with pyrgeometers, such as here, which has links to the Chilbolton data used.
Article by Tim Channon, co-moderator