I finally got a chance to do a leaf curl spray today. I noticed something I’ve not seen before while spraying. Ice was forming in the area of the fan and nozzle exhaust. The combination of the spray expanding out of the nozzles, and the compressed air expanding from the fan formed ice around the end of the sprayer. We all know an expanding gas/liquid absorbs heat. Apparently, the expanding spray/air absorbed so much heat from the back of the sprayer, water condensed and froze on the metal. I looked at the official temp on my phone when I took the pic. It was 37F. I thought it interesting.
In a closed container that is true, but expanding gas or liquids in a container open to the air would lose heat. I would think that when you engage the sprayer you are now in an open container. Given an expanded volume, molecules have fewer heat-generating collisions, therefore the temperature of the volume would drop. The walls of the tank drop in temperature as their heat is transferred to the volume inside. The cold tank walls would freeze the water in the outside air right next to its surface producing frost.
Maybe I’ve got it wrong…need a physics major to chime in!
I’m certainly not physics major, but this is how I understand it (in layman’s terms, because I’m a layman). When water goes through a phase change from ice to water and from water to a gas, it absorbs a lot of heat. That’s why it takes salt to make home made ice cream. The salt quickly causes the ice to change phase into liquid (without adding heat). The phase change absorbs a lot more heat, than if there wasn’t a change from ice to water. The ice/water bath absorbs heat from the yummy sugar, milk, vanilla ingredients and turns it into a fluffy ice desert (fluffy because air is mixed in by the churning).
Likewise when a gas expands it absorbs heat. That’s why when you let air out of your tires with your fingernail, your finger gets cold. The expanding gas is absorbing heat from your finger.
As I understand it, when a liquid turns to a gas as a result of pressure differences, a lot more heat is absorbed by the gas from changing phases, vs just a gas expanding.
I may not be saying all this quite right, but I think I have the concept correct?
Maybe you and I are trying to say the same thing in a different way?
Actually the frost/ice isn’t on the tank portion where the water inside the tank is. The water inside the tank is on the left hand side of the picture, whereas the ice is formed on the fan shroud on the right. I’m assuming the fan pulling air in the shroud forces the air to expand because of the vacuum created by the fan (thereby absorbing heat from the shroud). At the same time the spray nozzles, under pressure from the pump, release the spray into a “semi” gaseous state (part gas and part water droplets) and hit the shroud causing it to cool further till ice forms on the shroud.
Anyway, that’s my theory for what is going on. But again, I’m not a chem/physics major either.
Not that good at physics myself, but here’s one way I have of looking at it.
Heat pumps work because of the same principle. When you compress a gas you are taking a set number of molecules with a given amount of energy contained in them, and packing them in a much, much smaller space with the same energy. Hence, more energy per unit space equals higher temperature. After that hot, now liquid gas has had a chance to cool by exposure to ambient temperatures it can be released in to the larger volume. Now the same thing happens but in the opposite direction - not very much energy being released in a much larger volume equals lower temperature. So refrigeration systems have a high pressure side and a low pressure side; in heat pumps each side can be interchanged with the other. Or something along those lines - I think.
I think so! Your tire analogy is a good example of an open vs closed container. When filing your tire with air you are increasing the # of collisions in a fixed volume, raising the temperature in the tire. When you release this warm air you are reducing the # of collisions in an expanded volume (the atmosphere), dropping its temperature.
I am thinking that your tank and sprayer work on the same principle as releasing the compressed air from a tire.
It’s possible the temp was below freezing when I sprayed, but highly unlikely. I was going off the temp on my phone, which said 37F. It was sunny in the afternoon and near the expected high for the day. And there was no other ice on anything I could see (except for old ice, which hadn’t melted yet, on the ground).
In terms of freezing plants on the ground. Most of what I’ve read says that frost/ice really can’t form on plants above 32. If one does an internet search (i.e. Can frost form above 32?) the answers say something like even if the thermometer says it’s above 32, it’s really not above 32 where the frost is forming (cold pockets can be several degrees colder than the air temp where the thermometer may be placed).
In the case of the airblast sprayer, since I didn’t see any ice forming anywhere except around the area of the fan hood, I really think the ice forming there had something to do with pressure differentials, like Mark’s heat pump explanation.
I’ve had ice form on the drain valve while draining an air compressor in very warm weather (well above freezing).
And now for something completely different….
I put some store bought bottled water in the freezer , to use as ice for a cooler. After several days I opened the freezer to take it out.
Picking up a bottle, it was still liquid. Looked at the thermometer in the freezer , it showed 5 F.
Shook the bottle a little, and it froze instantly in my hand.
Flicking the other bottles with my finger , they instantly froze.
Apparently pure water from reverse osmosis won’t freeze at 32F.
Called super cooled water ! It’s a thing .
Amazed me !
That is pretty cool. I’ve not seen it personally, but seen videos of it. It would freak me out a little bit to have bottles freeze in my hand, if I wasn’t expecting it.
Actually this same phenomena is used to try to keep frost off blooms. Frost/ice can be more damaging to flowers than water, below freezing. Dew doesn’t contain a lot of minerals, and doesn’t readily want to crystalize into frost/ice below freezing. This is where ice nucleating bacteria come in. They are somewhat ubiquitous in the environment. They allow water/dew to easily crystalize, forming frost.
Kocide 3000 is labeled to kill the ice nucleating bacteria and allow the dew to remain dew and not to freeze below 32F. I’ve sprayed it before in anticipation of a predicted frost event.
Did it work? Nope. If there was dew, it formed frost anyway.
We regularly see frost on windshields and grass and garden stuff… when my two mercury thomometers on the back porch are showing 1-4 dsgrees above freezing.
Even though my house and fields are on ridge top… it is just a short distance in almost any direction to hollows where the elevation drops 150 ft in elevation quickly.
Air flow up from those deep hollows can easily cross my fields and yard… perhaps it does not so freely make it to my back porch which is sort of a inset cove.
Would be nice to have a thermometer out in the orchard… with a wireless feed back to the house.
Any of you have something like that you could recommend ?
OK, I’ll put my oar in. As it turns out, solids radiate heat more effectively than gases do. Thus, on a clear, calm, cold night, the ground cools by radiation to deep space more quickly than the air above it does, so you can have frost on the ground even when the air remains above freezing. It’s a paradox.
I’ve read that, like with blades of grass. Does that mean the actual temperature right next to the grass is still above freezing, and the actual blade of grass is below freezing?
In a case where intuition fails, it’s risky for the layman to hold forth for fear of contradiction by those who have studied the topic. … but that doesn’t stop me.
I’d say the ground cools the air next to it at night. There is no inversion. The lower atmosphere is stable. In this situation, cooling proceeds, but, as there is no mixing of the air, there is perceptible difference in temperature over the altitude of a few feet. If your thermometer is six feet off the ground, it will measure above freezing even while frost is condensing.
In northern wooded areas you can see differences in vegetation where woody plants are mostly killed by cold air that collects in “frost hollows.”
Here is how I reason it: At night, downwelling solar radiation is nil. The ground cools from upwelling long-wave radiation (infrared radiative cooling). The air is warmed by this, but it is cooled by its own upwelling long-wave radiation to deep space. It think the notion is that both the land and the sky cool, but the land cools quicker. The air is poor at capturing and holding heat, and it’s poor at radiating it, too. It doesn’t matter that the land holds vastly more heat than the sky; the sky can still be warmer. Does that make sense?
Maybe. As I understand what you are saying is that the ground can be cooler than the air. If that’s what your are saying, that makes sense to me. Air is fickle (as you say, it doesn’t store, or absorb nearly the amount of energy as a solid). I suppose I could see a thin solid (like a blade of grass) lose heat faster than the air, since solids radiate heat more than air.
It would be interesting to take close thermometer measurements right next to the blade of grass when it freezes.
### Nocturnal ice making in early India and Iran[edit]
*Before the invention of artificial refrigeration technology, ice making by nocturnal cooling was common in both India and Iran. Such apparatus consisted of a shallow ceramic tray with a thin layer of water, placed outdoors with a clear exposure to the night sky. The bottom and sides were insulated with a thick layer of hay. On a clear night the water would lose heat by radiation upwards. Provided the air was calm and not too far above freezing, heat gain from the surrounding air by convection was low enough to allow the water to freeze.[27][28][3]
