Originally Posted by
_ForceD_
BUT...you explain first that moisture isn't THAT big of a factor to impact the change in the temp of the water bottle -- "... water vapor content between bone dry air and saturated air, it's not going to swing the general heat convective properties of the gross fluid THAT much." But then say that the moisture will make and impact if the bottle is wet. But if the bottle is dry, and the air is very humid...why can't the moist air conduct the heat exchange as the bike/bottle moves through it? It's not evaporation, but the warm temp of the bottle can be absorbed my the moisture in the air and then carried away as the bike/bottle move...sort of like evaporation.
Because water evaporation (a phase change) is a completely different energy animal from simple convective transfer of heat.
Certainly water vapor in the air, like the nitrogen and oxygen, will convectively transfer heat from the bottle to the air. But that's different from the heat transfer involved when water evaporates and changes phases from a liquid to a gas. Much larger amounts of heat are transferred during this phase change. If the bottle is dry, there is no phase change as the bottle transfers heat to the water vapor in the air - the water in the air doesn't change from a liquid to a gas.
The term "water vapor" tends to make people think in terms of a liquid aerosol entrained in air like fog. This is incorrect. Water vapor is completely gaseous, colorless, transparent, etc. just like the nitrogen and oxygen in the air. It does have somewhat different heat transfer properties compared to the nitrogen and oxygen, but not dramatically different and as I said earlier, it is, at most, a few percent of the air. So the bulk heat transfer properties of saturated air and dry air can't be drastically different. I'm arm waving here a bit as I'm not a thermodynamicist but I am an engineer with a couple courses in thermo so I'm reasonably certain of this.
Another aspect of this which throws layman is that from our body experiences, we think that moist air makes heat seem hotter - the old "it's not the heat, it's the humidity" saying. But that again has to do with the fact we sweat and the evaporation of the sweat carries away large amounts of heat. A water bottle doesn't sweat so there is no mechanism like this involved, at least not unless the bottle gets wet due to external factors. This was why I was asking whether the bottle might have started externally wet.
- Mark