In article ,
Alan writes:
If pointing at a clear sky you are looking through the atmosphere into
space. Space will be around absolute zero (-273 degrees C) - attenuated
by the atmosphere and limited to the range of the device. You may not
see the temperatures you expect to see

Actually, it's about 3K (-270C) due to heat left from the big bang
in the form of the cosmic microwave background. However, you won't
get a digital thermometer to register that.

If looking at low clouds the water vapour could be acting like a mirror
and reflecting the ground temperature.

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

If the OP isn't in a hurry for one, these things do turn up on
special offer from time to time.

--
Andrew Gabriel