-1* C according to the weather network ...... you can probably take 2 or 3*C more off that. Supposed to be cooler tomorrow morning but I won't be home to take advantage.
Wow! That sounds so fun! Great experiment. Has anyone tried using a backup pump/heatsink/radiator/reservoir in MUCH colder outdoor temps using an antifreeze coolant? For example, where I am (and likely where bassnut is) it is not uncommon to reach temperatures of -25*C. I'm just ignorant of this practice and it sounds like a fun thing to try this winter when it gets cold.