Actually, that is not correct. Air is moving into the smoker at all times and leaving with more water than it started with. Cool air coming in is much smaller than warm, wet air leaving and far more dense.
Wet bulb versus dry bulb temperatures are nothing more than a measure of the Relative Humidity. I assume your suggestion is to raise the RH to prevent evaporation of water. The problem is you have to evaporate water to raise the RH. The theory is that reduces the rate the water CAN evaporate from the product. It will not evaporate as rapidly and will cook with the lower evaporation rate.
Elevated RH leads to water droplets on the product which trap smoke particles and often lead to "hot spots" and such in flavor... It is not a good thing in fish smoking unless the smoke is filtered well.
Please excuse the dydactic nature of this next example but I think it will be much more clear if spelled out this way:
Wet air is lighter than dry air.
Warm air is lighter than dry air.
Warm, wet air is far lighter than cool, dry air.
In the combustion area liquids and solids are being converted by heat to gases and oxidizing (burning), or reacting to other components, or perhaps just remaining themselves and wafting away.
Those elements that oxidize release heat.
Those elements that react with other elements may release heat, consume heat, or do neither... Almost all release heat.
In all cases conbustion gases become huge compared to their former physical size as wood. As gases they now have the ability to carry away water vapor with them. The water that evaporates from the wood is of course part of that water, but far more can be held by the combustion gases.
As the gases leave the combustion area they cool rapidly (Juul-Thompson effect among many others) and increase their water carrying abilities.
All of which contribute to draft.
Trying to keep up with the amount of water it takes to keep RH high enough to inhibit evaporation means a lot of water has to evaporate somewhere else. It takes more than 15 thousand calories to evaporate an ounce of water... which explains exactly why evaporative cooling is so effective as the heat required to evaporate the water leaves with the water vapor, cooling the substrate it leaves behind.
Why does this mean anything to fish smoking? Pretty simple and obvious once you look at it. The pellicle is water soluble and will go away in high humidity. All the attempts to control the stall with wraps and such speak to the lack of good crust in the end. With fish smoking the pellicle is critical...
Actually, commercial smokers have been discussing humidity control and understood evaporative cooling as the culprit decades ago. In the lumber industry wood in solar dry kilns was noted to cool during the heat of the day as RH rose and circulation increased in my "Dry Kiln Operators Manual" dated 1961 and they reference work from the late 1800s.
(almost deleted this bit of bits but decided to just set it aside in parenthesis for those looking for wandering details... sorry...
If there is not enough air available to maintain fire you have to have electric heat so we will leave that out of the discussion.
Propane of course generates a ton of water to produce heat; one propane molecule and five oxygen molecules produce four water molecules and three carbon dioxide molecules. That is more than a pound and a half of water vapor produced for every pound of propane burned.
Ethanethiol is a whole other subject on propane, and I do not want to eat it...)
