Butane boils at 31.1 degrees F @ atmospheric pressure
Propane boils at -42.1 degrees F @ atmospheric pressure. (OOPS!) I
don't know what the boiling point is at say 20 PSIG gauge.

I know when charging a/c units, as I remove freon from the drum, the
remaining liquid in the drum gets cold. It is harder to evaporate the
remaining Freon. The same thing happens with propane. The colder it
is outside, the more gas you use and the higher the draw from the tank.
The remaining LPG in the tank acts as a refrigerant and the pressure
goes down

The regulator at the tank is at some hegher intermediate pressure. The
regulator at the house is normally set at 11 Inches water column. How
well the propane evaporates will depend on the outdoor air temperature,
the amount of liquid in the tank (as a heat sink), the surface area of
the liquid in the tank and the surface area of the tank (the size of
the tank) to absorb heat from the outside. The pressure at the burner
will also depend on pressure drop in the piping. They could also add
butane to the LPG in the summer but not in the winter. We used to buy
it as LPG, not as propane. I can see how that would affect things.

Boy, a little research first would have been good instead of shooting
my mouth off first! :-). I'm glad that I admitted it was a GUESS in my
first post!

Stretch