crb wrote:

Andrew Gabriel wrote:
No, I get believable readings from clouds, typically -40 to -50,
which agrees with what aircraft report as the outside temperature.


Clouds at -40 to -50? - I don't think so. You have to go to around
30,000 feet or above to get down to those temperatures.

The error with low cloud is probably because their emissivity is less
than the fixed value of about 0.85-0.9 which is programmed into simple
IR thermometers. The instrument detects a certain quantity of IR
radiation, but reports it as coming from a cooler object than is
actually the case.

IR thermometers with an adjustable setting for the assumed emissivity
are much more expensive.


--
Ian White