Question for chemist, physicist, or maybe meteorologist.
 

Last week I went on a ride with the air temperature in the mid-/high-20s (F). Like other times I've ridden in cold temps...my water bottle froze. Earlier today I went on a ride and the temp was 10*F but my water bottle didn't freeze. Why did it freeze at 25* but not at 10*?

Dan

Interesting question. Plain water both times?

Ride length? Speed? Wind? Insulated bottle?

Sun?

You've probably experienced supercooling of the water. If there is a lack of nucleation sites in the water, you can cool the water below its freezing point without the liquid freezing. This usually requires very pure water in very clean vessels. This video shows how it can be done


The agitation of riding should have prevented the supercooling but you may have just gotten lucky.

On a side note, if you want to put in a ° (degrees) symbol, you can use shift/option/8 on a Mac keyboard or use ALT + 0176 on the number keypad on your keyboard. You can find all kinds of other alt keys here

The temperature was actually in °C.

Most likely, the water had not yet reached thermal equilibrium with the surroundings.

My guess: Your high-20's rides are longer than your 10-deg rides, enough so that the extra time has more effect than the lower ambient in getting the bottle from the starting temp to below freezing.

- Mark

To answer the above observations;
The quoted temps were both Fahrenheit, and it was just regular tap water (filtered from the fridge dispenser). It could be ride length...BUT...The ride when the temp was in the high 20s was a 25 mile ride. The ride when the temp was 10 degrees was only a 15 mile ride (that was all I could handle). BUT...when it froze in the 20s ride I noticed that it was frozen about halfway through that ride. In both cases there was little wind. I had thought about the sun factor. But the sun was also about the same in both cases.

Dan

Well, hard to say, but there are no mysterious forces or phenomena going on here - it's simple thermodynamics - so for whatever reasons, your shorter/colder ride didn't get the bottle as cold as your longer/warmer ride. Exactly why is hard to say, but probably a combo of different starting bottle temps, length of ride, effect of head/tailwinds (tail would reduce the apparent wind on the bottle and keep it warmer longer), crosswind angle and leg shielding, variations in ambient with terrain, sun intensity/angle-to-bottle, how often you used the bottle, etc.

Significant supercooling isn't possible in this scenario.

- Mark

If there water supercooled (granted a big "if"), the filtering may be the key. The water needs to be pure to supercool which is why the guy uses bottled water in the video. Maybe your water bottle on the first ride was slightly dirtier than the second ride.

I wouldn't say that supercooling is impossible, just highly unlikely.

The length of the ride isn't going to be significantly different. A 15 mile ride is still going to be about an hour while a 25 mile ride is about an hour and a half. At those temperatures, the bottle would be exposed to a low enough temperature that I would suspect both to be frozen within about 30 minutes.

Side or tailwinds are going to have little effect on how quickly both bottle freeze since they are both subjected to similar wind from the bike moving through the air. Having a headwind or tailwind doesn't really have a significant effect on airflow over the bike.

You may be on to something with "how often you used the bottle", however. A full bottle has more heat mass and would take longer to freeze. If...and, again, it's a big "if"...the bottle was supercooling, a full bottle will likely have less turbulence and less reason to crystalize than a partially empty bottle that is sloshing around.

The likely answer is humidity. Humid air is more effective at heat transfer than bone dry air. So the humidity of the warmer day was probably significantly higher. Which makes sense. Very low temperatures tend to coincide with lower humidity.

This is a more complicated area of thermo, but I doubt the convective heat transfer from a closed water bottle to the surrounding air is affected very much by ambient humidity.

- Mark

Cross-winds could make a difference. I originally suspected radiant energy from sunlight, but not enough information for that.

Humidity, it seems to me that it could increase heat transfer from the skin of the bottle.

As the OP...I'm going with this. On my ride today the temp was 29°F when I started...25°F when I finished...and the water froze again at around mile 15. And, there was significantly more humidity...around 80%...while on the 15°F day humidity was around 50%.

Dan

OK, I'm getting the big guns out and calling my retired chemist father lol. There's no doubt that humidity DOES impact heat transfer, but I'm not nearly smart enough to say how much, and I couldn't find a simple equation online.

The humidity aspect is surprisingly complicated, but a key thing to keep in mind is the amount of water vapor in air is never that much - maybe a few percent depending on temp. Air is around 78% nitrogen and 19% oxygen with the remaining 3% or so can be a lot of trace gases and water vapor. But regardless of the change in 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. The fact that the literature on changes in convective cooling due to humidity is so sparse would lead me to believe that the affect is generally small enough to be ignored.

That's not to say that water vapor doesn't have a huge affect on how the atmosphere behaves, but this is due to the relatively large amounts of energy involved where water phase changes between liquid and gas - that's not a factor in the convective cooling of a water bottle in an air stream which is more/less determined by the gross properties of the stream (e.g.,its density).

Actually, come to think of it, that's another factor which might be playing a bigger role than you might think - was the bottle externally wet before the ride started or did the bottle become wet during the ride? Evaporative cooling is a pretty big deal.

- Mark

The answers are no, and no. It wasn't, and didn't get wet. 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.

Dan

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

Yes. It's not convective, but conduction. Also water molecules contacting and moving away from the surface will have higher thermal conductivity than dry air. Sucking the warmth out faster.

Full disclosure, I'm not 100% certain that air with water vapor has higher thermal conduction at low temperatures.

We're just talking terminology here, but it is definitely convection we're talking about here, not conduction, at least as the terms are commonly used. Conduction is the transfer of heat through solids; convection is from a object to a fluid, in this case from the solid surface of the water bottle to the air fluid.

- Mark

Yep we have different definitions. I use the term when there is movement of the fluid carrying the thermal energy away. In a layer next to the surface, without fluid movement or without much fluid movement, I call the transfer between the solid and fluid conduction.

ie, convection requires conduction if you zoom in enough. Your earlier explanation sounded like there could be little or no thermal transfer from the water in the bottle to air, because there is no "convective" interface, which is what I reacted to there.

Maybe you could call it conduction at the microscopic level, but I always thought conduction was a solid to solid thing, not between the surface of the water bottle and the individual air molecules. In any event, convection is carrying the warmed-up air molecules away from the surface of the bottle so I'm pretty sure this would be called convective cooling.

Regardless of the terminology, we know what we're talking about here and yes, humid air might have a percent or two of gaseous water vapor that dry air doesn't have with somewhat different heat transfer characteristics than the nitrogen and oxygen in the dry air. But I don't think there is any mechanism for this to have a large affect - even if the water vapor were dramatically different, the heat transfer characteristics of the mixture would be dominated by the main constituents, not trace gases.

For the OP's observation to be caused by humidity differences, you'd have to assume that humid air was perhaps 50% more efficient in air-cooling the bottle. If this were the case, I'd think we'd have a lot of literature discussing how air-cooled engines (as one example) in dry climates are much more prone to overheating than in wet climates (or vice versa - I'm still haven't seen something that says water vapor is better/worse than transferring heat compared to nitrogen/oxygen). Other than some fringe esoteric papers, I can't find anything on the net that talks about atmospheric humidity having a big role convective heat transfer. Humidity matters to creatures that sweat, but it doesn't matter a bit to things that don't (e.g., engines or closed water bottles).

All in all, the idea that a difference in humidity is the cause of the OP's observation just doesn't pass the common sense test.

- Mark