Suppose you check your bank account and discover that you have $1 as the net when you have received your net income and paid your bills. Not so bad. Suppose now you have forgotten how big your income was and you ask if from the present status of your bank account ($1), it is possible to figure out this number. If you can't, consider the following advice offered by the financial consultant of your bank:
This is relatively simple. Assume that your expenditures were $999, then your income was $1000 because you have $1 left. If you give it to me as consultant fee, then you will be even.
But you are left with a feeling that something in the argument is not convincing: Suppose you instead assume that your expenditures were only $99, then your income would be only $100, which is way to little for someone with your credentials. On the other hand, if you assume that you have spent $9999, then you would have had the impressive income of $10000, which is more than you ever dreamed of. </
After some thought you understand that you have been fooled by the bank financial expert and you want your $1 back: Clearly, it is impossible to tell what A is from the knowledge that A - B = 1, unless you know B.
The pyrometer does the same trick as the bank consultant: It measures temperature difference as bank net balance and reports total DLR and OLR as total income. In both cases it is guess work which is not worth of $1.
The guess work of the pyrgeometer is the outgoing radiation according to the formula sigma x T^4 which is Planck's law for radiation into surrounding space at 0 K. But the temperatur of the outside of the pyrgeometer is not 0 K, and so the outgoing radiation from the pyrgeometer as sigma x T^4 is only fiction.
This is the myth of backradition discussed in detail on this blog.
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