Air does emit IR as anything else does. Its emissivity is low due to low
density, however. Iow, one needs a large amount of warm (not hot) air to make
the PIR see it. Too hot will not be seen, it needs to be at about 30-40 degrees,
and the best way to see it is against the cold night sky, but it *will* be seen
by the PIR. A similar situation can occur when a PIR faces outside in winter and
some hot air comes out of the house and passes in front of it.

Peter

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