When a gas leaks through an orifice, like a hole, it is called effusion.
Cold air is denser than warm air, and the molecules move slower.  The
lighter
molecules would leak out somewhat faster than the denser ones.

This would seem to contradict your premise....BUT
when is cold and the pressure inside the tire decreases, bead seal also is
poorer
and the leak may get worse.  I think this poorer sealing property is the key
to
what you are describing.

Air leaks
  - through the valves
  - through the bead
  - through the rubber by diffusion.

During cold weather, there will be more leakage through the bead, but
less leakage by diffusion.
--scott

True for rim seal leaks at least.  The cold seems to affect the bond
between the rubber and aluminum for one type, rims.

Every first cold snap we have multiple flat tires on the aluminum rims.
 This year we had 7 flats all at once.

I pumped them all up 5 psi 'over' spec for the vehicle sticker and they
have held this time.

If they didn't hold like last year, for the last several years we just
take them back for the flat tire warranty they have on them.  They get a
rim clean and seal and hold for another year usually.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
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