I have brewed 4 5gallon batches in a week of all grain. Two of the batches were boiled on a sunny day (high air pressor). The second 2 were boiled on a rainy day (low air pressor). The sunny day beer I hit my estimated OG reading and the rainy day beer were both -0.005 short of the target, do to less water boiling off. At lest this is my theory. How do I compensate? Or do you just not worry about this? V
For your next brews use the the rainy day boil off rate for estimating total beginning volume in your BK. Top off water can always be added in the last 15 minutes if boil off is greater.
There’s also a big difference between a warm/humid summer day and cold/dry winter day. In the summer my boil off ranges from 15-20%. In the winter it’s 20-25%. Kinda sucks, but I also don’t worry much about having 1/4 to 1/2 gallon more or less wort or missing by gravity by only a few points.
