Does boil environment impact boil-off volume?

Did it take longer to reach an active boil? Did you have the propane turned up higher? Those are cold-related factors likely to affect boil off.

 

Lower relative humidity usually accompanies cold weather, so I think that could be a factor too.

 

I hated physics in college so I'm no expert, but I doubt external weather factors can influence boil off rates. That is driven strictly by the heat added over a given amount of time. Turn the heat up or boil longer and you'll have more evaporation.
I sweat just as much here in NM as I do in Mobile AL, the only difference is that here the sweat evaporates and isn't as visible, in Mobile it all sticks to my shirt.

 

I routinely plan for more boil-off when it is below 40°F, or windy, and especially when both. I have often noticed a 1/2 gallon less in the fermenter on such days, so I sparge a bit more (unless I don't care/am feeling lazy).

 

I am fresh out of food chemistry class where we did learn that the lower the relative humidity of the air the faster your reduction rate will be when making a sauce.

Very dry air will take up more moisture from boiling wort. This is because the higher the relative humidity the higher the vapor pressure on the surface of the boiling wort thus requiring more energy to get the water molecules moving enough to change from liquid to gas.

Here is a link that explains the concept pretty well.

I would like to add also that I'm no expert, I'm just a chef who took a 100 level food chem class so some chemist will probably come along and make me look dumb.

 

34F and its really cold out??!!??

I'll echo what Hopsandmalt said, but add that colder air tends to be drier as well, so you'll be likely to get a bit more boil off both from the drier air and from needing to pump more BTU's into it to keep it warm

 

I get more boil off in the winter. I account for that when figuring out pre-boil volume.

 

I brewed last week, it was somewhere between 35 - 40F.
I ended up with about a half gallon less than I wanted, and I even sparged a bit more than usual; I was expecting a bit more. Call it about .75 gallons more boil off than usual.
I'll just have to adjust even more for that next time.

 

The temp definitely affects your boil due to the atmospheric pressure; at the brewery there's a clear difference in the boil when it's 40F outside compared to 75F. The boil is way more vigorous at lower temps.

 

It's always a good idea to check your boil volume with a dip stick every 15 or 20 minutes so you can crank it up or back it down accordingly. Usually, I'd rather boil too much and add RO near the end.