I assume the cold weather would have nothing to do with it?
For a fixed quantity of gas in a fixed volume, as the temperature drops so does the pressure. So yes the cold weather will play a big part on how a the seat post works, especially with the small volume of air it uses.
Just to add the schoolboy Maths for those who care.
The relationship for an ideal gas is PV=nRT
P is pressure – the variable we are looking at
V is volume – a fixed conastant
n is the quantity – a fixed constant
R is the universal gas constant, just a number
T is the temperature – the variable the weather is changing
To keep the equation in balance you can see that when temp goes down, pressure goes down.
Oh, and not to mention that cold weather will make all the lubes that little bit more sticky and less effective.
Maybe take the post inside and let it warm up and see if it works then?