Evaporation rate question

[Beerbelly]

I brew outdoors year 'round, and I always seem to have trouble figuring evaporation rates depending on the weather. Today's Oregon weather was about 50˚ and rainy, and my brew came in 3/4 gallon over & 10 points lower than expected per BrewTarget. I returned it to the kettle, boiled another hour, and hit my target exactly. It appears colder, wetter weather really affects evap. rate; I don't have this trouble in the summer months.
Is there a general guideline or formula to address this?

 

[J A]

Humidity is your big factor. Temperature can have an impact on boil vigor because your burner may not be heating the pot as efficiently as it could. Since you have a good data point to work from, next time the weather is misty or rainy or muggy, assume less boil off and top up if your volume is lower than you need. I tend to estimate on the low side for boil-off in general because it's just easier to add it back, but I tend to hit it pretty close since I'm electric and "indoors".

 

[Craigerrr]

Yep ^^^

 

[Donoroto]

Ditto: humidity is a huge factor. A much smaller factor is temperature: easier to boil more vigorously at 80 F than 50 F ambient.

Keep track of the humidity and boil rate, eventually you’ll have enough data to make accurate predictions.

 

[BarbarianBrewer]

I brew outdoors as well and I notice the wind can affect my boil off rate as well. I've not measured it but I would estimate that I lose an additional quart/hour when sustained winds are ~15 mph vs a calm day.

 

[Yooper]

When I used to brew in the winter in Upper Michigan, even indoors the air was so dry I swear the wort got sucked into the air. I boiled off about 2 gallons per hour, even with just a slight rolling boil. Summer, less than 1 gallon per hour at the same boil. It plays a HUGE rule in boil off.

 

[Craigerrr]

Interesting, I boil indoors year round with a heat exchanger. I haven't done any real testing, but I suspect that the humidity isn't a factor in my situation. If it is it would be minute,funny how minute and minute are spelled the same ya?

 

[Limey]

When I used to brew in the winter in Upper Michigan, even indoors the air was so dry I swear the wort got sucked into the air. I boiled off about 2 gallons per hour, even with just a slight rolling boil. Summer, less than 1 gallon per hour at the same boil. It plays a HUGE rule in boil off.

LOL - I live in LM (Lower Michigan) so the winters are not as dry, especially since we are less than a mile from Lake Michigan which also mitigates the summer humidity. I brew outside, so I don't know about winter boil off, but my summer rate is reliably 1gallon/hour at a rolling boil.

 

[Zambi]

I actually do not know my boil off rate.
I just boil, then dilute