Slow1 said:
My point was to emphasize the point that even with a hot day the wood may not be drying nearly as much as one thinks - even the cooler (70* days) may be out performing in terms of drying days.
So many folks get hung up on the temperature. As others point out - the temperature is a factor in the calculation (the warmer the air, the more water a given volume can hold thus the relative humidity goes down), but just because a body of air is warmer doesn't mean it hasn't already found water somewhere.
I got curious to find out (based on reading other comments after mine yesterday) and found that yesterday's stats for my town were rather dismal for drying:
Temp min/max - 69/89 *f
R Humidity min/max/ave - 62/100/80
With an average RH of 80 and the minimum of 62% (assume that hit when temp was 89*f) that explains the poor drying conditions for me.
So far today doesn't look too good either... At the moment (10:45a) it is 91*f with RH of 70% - it feels very uncomfortable out there. I split up some black birch this morning (8a) and couldn't keep my glasses on due to the sweat flowing off my face. (My biological evaporative cooler was not functioning very well eh?) After 30 minutes I was done with that and ready for the shower and several large glasses of water!
Slow, in outside conditions you are probably right about the temp thing not being all that important. You just aren't going to get the wood hot enough for the temp to be a major driving force in wood drying, even in full sun on a hot day. For academic purposes, however, we should at least acknowledge the fact that the diffusion rate across the grain increases as the wood temp increases.
Rising temps in some areas bring about a correspondingly large increase in the
absolute moisture content (how much water by weight is in a given volume of air at STP), leaving RH basically unchanged as temperatures rise. The Corn Belt is notorious for this. All that evapotranspiration of water from the corn fields at high temperatures is largely responsible for a lot of severe thunderstorm activity out that way.
http://www.weather.com/outlook/weat...potranspiration-corn-belt-humidity_2011-07-13
Still, in most areas of the country, rising daytime temps have the effect of lowering the RH somewhat. Below is a table of today's forecast in my area. RH begins at a not too shabby 80% at 78º at 6 AM, will drop to 49% as the temps climbs to 90º by noon, will get even lower at the same 90º temp later in the day (indicating that drier air is expected to move into the area), and will rise to 76% at 70º by midnight. Since the RH at a midnight temp of 70º is actually expected to be lower than the 6 AM RH at 78º, this is a further indication that drier air (i.e. lower absolute MC) is expected to move in later in the day.
When you are considering the drying rate over the long haul, you really only need to consider the average RH in your area. A lot of drying still goes on at 80% RH, especially if there is good air movement. In fact, if you had an invariant RH of 80% day and night, both your 80% MC oak and your 160% MC cottonwood will eventually get down to 16% MC dry-basis. That is less than 14% water by weight...
plenty dry for any stove out there.
Too dry if you split it really small.
So, although it's interesting to examine these things and to discuss them on this board, in real life there is nothing we can do about it all unless we dry in a kiln. The wood will be dry when it gets there, and it
will get there in time... even at a constant 90% RH. After all, the EMC at 90% RH is 20.5% MC dry-basis, or about 16% wet-basis... the MC at the low end of the EPA testing protocol fuel requirement.
The drying experiment I did last winter was meant to try to establish a drying curve that would allow one to predict the time that the fuel would be ready based on weight data only. I discovered that, after the initial fast-drying period of free water leaving primarily from the end grain, the drying curve closely resembled an exponential decay function. Funny thing, all anybody seemed to get out of that is that I was trying to convince folks to skip outdoor seasoning entirely and dry exclusively in the basement. Whatever...
Skyline (on a couple of recent threads) has done an outstanding job of taking this idea into a real-world outdoor drying situation. His curves are very similar in shape to the ones I generated, even though they have a lesser slope because he isn't drying his wood at the 20-30% RH I had in my basement. By taking accurate weights at regular times and using these weights to determine a rough constant of proportionality, you should be able estimate within a month or two when your wood will be at any desired MC in your area simply by using an exponential decay formula.
BTW nice score with the black birch. It dries fast and burns very hot for a good long time. Your stove will think you fed it candy.