(I know it's not astronomy, but this is the best science board I know of...)

We were having a discussion in chemistry class today, and I wanted to see what you guys thought about it:

Temperatures lower than twenty-eight degrees F damage grape vines. Grape growers often spray water on their plants before a night when the temperature is expected to drop below freezing to keep the grape plants from being damaged, and the technique works.

Why?

We've agreed that the water on the outside of the grape will freeze before the grapes themselves will. I hold that because of this, the grape will maintain a temperature higher than freezing for a longer period of time: the cold takes longer to get to it since it has to freeze the water outside the grape first.

I'm told that I'm not totally correct. In order for ice to melt, it must absorb a heat of fusion. Thus, when ice forms, heat must be released. While some of this heat travels to the cold air outside, some of that heat, I'm told, travels inside to warm the grape.

I have a lot of difficulty buying that. As I understand, when the water freezes, heat has to be removed from it; the difference between removing and giving off heat is important. When something radiates heat, that heat travels in all directions, so in this case, if the water radiated heat when it froze, it'd certainly warm the grape. However, since the heat is just removed, the heat is transferred only to the cold air outside. By the definition of "cooling something off," the heat must be transfered, not radiated, and so if the air "cools off" the water, then shouldn't the heat only go to the cold air? And isn't that part of the second law of thermodynamics, anyway?

So what's correct? More importantly, can anyone give me something authoritative that explains whatever's correct so I can finally put this argument to rest? We've been going back and forth, and without some authoritative source, I don't think we'll get anywhere.

Thanks a lot...
-Starwiz

When water freezes into ice, heat is released, through the heat of fusion. 1gallon of water releases about 300kcal of heat as it freezes. However, water is also evaporating in the atmosphere. This evaporation of water results in a loss of close to 2,300 kcal per gallon of water evaporated. Therefore, to provide heat to the vineyard, more water must freeze than evaporates. Therefore, 7.5 times more water must freeze than that which evaporates.

For vineyards that use sprinklers, during sprinkler activation, the ambient temperature can actually be decreased through evaporative cooling. So without getting too detailed into agricultural meteorology, sprinkler activation must be timed correctly, with regard to other environmental factors such as relative humidity.

Hope this helps a bit.
Josh.

You're dead on actually. Heat flows from hot to cold and so will flow from the cold water to the colder air and not to the relatively warm plant. You're also right that's not why the technique works. You're overlooking something simple.

Let me ask you this then: You now have a three layered system: air -> water -> vine. What role is the water going to play? Or put another way, what temperature is the water going to freeze at and is its freezing instantaneous? In either case, where does that leave the temperature at the water/vine interface?

Don't have an authoritative source handy - but if it helps, your grasp of thermo has given you all the pieces you need.

@Taibak: The water/ice plays the role of an insulator, right?

Its temperature should always be at or below the temperature of the vine, since in order to form ice, not only does the water need to be cooled to 0 degrees, but it needs to have the heat of fusion taken out before it can freeze. During that whole process, the vine is only exposed to the slowly decreasing temperature of the ice (and during the whole freezing process, that temperature doesn't even decrease), rather than the cold air, which could be much colder than freezing. Plus, once the ice is frozen, heat must again be taken out of it before that temperature drop can reach the grape vine.

I, too, am not a metorologist, but I did take a metoeorology class in college. :-)

There is a good website the BA sometimes frequents, www.madsci.org, where people ask questions like this and a scientist in that field answers to the best of their ability based on the inquisitor's age and education.

My answer is that americans use the Fehrenheit system. However, most vinifera wine-type grapes come from france where the celsius/centrigrade system is used. So, 32 degrees is cold to us, but to the grape it's relatively balmy. 8)

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"Oh no no no I'm a rocket man Rocket man burning out his fuse up here alone." -- Sir Elton John

J Pax

Bingo. You're always going to lose heat into the air (2nd Law). But as long as enough heat remains trapped inside the ice you should be fine.

The example came out of my chem book, and it specified degrees Fahrenheit. Nice try, though.

As a meteorology student, I stick by my previous reply. :wink:

Doesn't mean it's right, but hey, I'm a meteorology student. I don't have to be right all of the time. hehe

Hey: this is why I'm asking in the first place...

Why do you think the other answers are given are wrong, though?

Heat always flows from hot to cold. The best anyone can ever hope to do is to control the rate of heat flow, not the flow of heat.

That's what the ice is doing around the grape: limiting the heat flow from the grape to the ambient air.

In any mechanical refrigeration system, the idea is to maximize heat flow by extending the surface area of the substance to be heated (or cooled) by effectively adding surface area to the medium being heated (or cooled).

That's why a radiator in your car typically has a huge surface area: it's made when thin metal (thermally conductive) sheets are crimped onto the tubes that carry water or refrigerant (the fins). Since the fins are very thin, made of a very thermally conductive material, and have a fan blowing air across them, the fins maximize a substances surface area against what is used to cool (or heat) the substance.

The exact opposite happens when trying to insulate an object: you try to minimize the surface area. That's what the ice around a grape is doing.

It's not that the grape gets "heat" from the water turning into ice; the grape loses heat while water turns to ice.

But after that, the ice will slow (insulate) the heat that can be removed from the grape from the outside, because the surface area of the grape (now covered with ice, which has to be melted) has been minimized.

Doug. (I'd bet almost a dollar that I'm right...)

I didn't say the other answers are wrong, I just believe my answer is correct too.

I have done some additional research:

This information was obtained from
http://cati.csufresno.edu/VERC/rese/96/960803/.

Considering they do this stuff for a living, I'd go with their answer. Just a suggestion, though.

Josh.

Freezing can be a good thing - I like icewine.

Typical extra class Ham! Heavy details!

The numbers are no doubt correct. Consider though that the heat loss from evaporation comes from the water, not necessarily from the grape. The heat was in the water when it came from the sprinkler. If the water does not freeze, it gives up some heat to become vapor (again, not heat loss from the grape). If the water freezes, it will have given up it's latent heat at little expense (some conduction probably) of the grape and now acts as an insulator.

I'd be curious if timing really is a problem in sprinkling.

N5PLQ

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

But the point is that the energy lss in evaporation is greater than the heat of fusion. If the evaporation ratio is above the threshold, the water will freeze and more energy is lost - thus the temperature of the ice drops below freezing, and there is no barrier to keep it going lower. The grape will tend toward equilibrium with the lower ice temperature.

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"I have a cunning plan that cannot fail."
S. Baldrick

Yes. But what I see is that the heat loss is from the water and not so much the grape. Since the sprinklers stay on through the night, the liquid (non-frozen) water is a heat supply to the grape surface. The 2300kcal is applicable only if it evaporates and only if it does so from the grape surface. Also, freezing air temperatures are poor environments to see much evaporative rates. I would guess most the water will go to the ground with little evaporation.

I want to say the air temperature will rise as well but this would be likely only a minor benefit.

I'm pretty rusty on my thermo. The early post regarding heat from hot to cold reminded me of my favorite quote of the 2nd law...

"Heat won't flow from a cooler to a hotter. You can try if you like, but you far better notter!" [Simmang]

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

The air is cooling the ice...I think we all agree on that. Since it's cooling the ice, the air has to get warmer; the ice transfers some of its heat to the air. So I'm in agreement: the ice will get hotter.

As for evaporation, it could definitely affect the system, but that wasn't what we were really arguing about. That article says that the water releases heat when it freezes, and I don't think any of us are disputing that. Rather, aren't we considering whether that heat goes to the warmer grape or the colder air...or have we all agreed that the heat only goes to the air around it? Either way, I'm not sure how the whole evaporation thing proves or disproves that.

@George: I'll remember that quote...that's funny!

Heh! I've had my foot in my mouth so many times that while I don't especially enjoy it, I am getting used to it.

What could one more time hurt? Soooo...

I'm still going to say that you cannot raise the temperature of substance "A" by bringing it into contact with a colder substance "B".

The heat source is the sprayed water (if the water isn't above 32°F, it's not rain, it's, well, snow...). That's what warms up the grape because the grape should be at or very close to ambient (25° F). The grape isn't warmer: it's colder than the water. We're warming the grape up by spraying warmer water onto it.

The question of evaporation comes into play only if and when conditions are right for an insulating layer of ice to form around the grape (from evaporative cooling) before a low ambient air temperature in and of itself causes harm to the grape.

Say we wetted the grape before ambient dropped below freezing and the grape stayed wet while the temp continued to drop. When ambient air temp goes below freezing, we'd get a nice layer of ice around a frozen grape, right?

But if we wait to spray the grape with warm(er) water until after the ambient temp is below freezing but before the grape has been harmed, the water will heat the grape; freeze into an insulating layer of ice around the grape (provided more water freezes than evaporates); and let the grape survive a colder night than it would have because of the insulating ice layer.

Doug (He who changes socks regularly...)

Grapes?

I have no experience with this sort of thing, but why must the grape be at a temperature greater than freezing? If it is a degree or so below freezing, the ice will release heat to it as the ice freezes. Maybe that is what your friend is talking about.

If you have a freezer, and put a grape in it, the grape will quickly freeze, being small. If you put a large glass of water at the same temperature as the grape in the freezer, it will take longer to freeze. If you put the grape in the glass of water, the grape will take longer than the glass of water--because it has to go to an even lower temperature.

Ice can release a lot of heat before it freezes. That tends to raise the temperature of the surrounding air, even if the grapes are not coated with ice.

We have some friends that grow strawberries on their farm. They do not worry about preventing the plants from getting frost. What they do is spray the plants with water in the early morning hours before the sun comes up. They say as long as the sunlight doesn't strike the frosted leaves, the plant will be ok.