Originally Posted by menschmaschine
This is very difficult to predict/calculate, so I would just plan on a percent or two more for outside. Unless it's really windy, then maybe 2 or 3 percent more than inside.
With all due respect, I don't think that the wind will increase evaporation rate by that much. I'm not going to do a mythbusters experiment to prove it, but it seems logical that while the wind can affect the boundary layer, improving mass transfer away from the boiling wort - it can also impact the amount of energy being transferred from the burner to the kettle as well as the amount of energy transferred from the kettle surface area to atmosphere.
This is why I don't think that wind will make a significant (or even predictable, without a supercomputer) impact.
A burner operating in zero wind is being more efficiently directed at the bottom of the kettle. When the wind kicks up, a greater amount of heat from the flame is being transferred to the air and away from the kettle.
Also, wind increases flow at the boundary layer around the entire surface area of the kettle as well, not just at the boiling liquid/air interface. This change at the entire kettle surface boundary layer can dramatically increase energy transfer from the kettle to the air, thus DECREASING boil off rate.
As far as humidity affecting the boil - it is really affecting evaporation. The impact of humidity on evaporation rate is irrelavent of the heat induced evaporation (vaporizing water by adding energy). If we fill the kettle up with water, and turn the burner off, there will be some mass transfer that occurs between the water and atmosphere - and that occurs at the boundary layer. If there is less moisture in the atmosphere, there will be greater mass transfer of water from the kettle, thus greater evaporation. If there is more moisture in the atmosphere (higher humidity) the evaporation rate will decrease. Now go fill your kettle with water and see how much water evaporates to the air only attributed to mass transfer during a typical brewing day 60 minute boil..... its not very much. Yes, increased humidity decreases evaporation. But the evaporation was so small to begin with (compared with the vaporization due to heat) that it is negligible.
Leave a cup of water out for a while and see how long it takes to evaporate. This illustrates my point that evaporation due to mass transfer is so small during a 60 minute boil that it's not worth considering.
In summary, I totally agree that wind and humidity affect boil-off rate, as do many other factors. However I still don't think they are significant.