like not melting to water but an ice cube left out side will slowly get smaller even when its super cold out.
In: 5
This depends on a lot of things.
But generally, if you’re seeing snow and ice melt below freezing either salt has been dispensed and that lowers the freezing temp of H20, making it liquify and run off.
Sunlight can also make snow and ice melt as it raises the temp of the material at the surface even if the surrounding air temp in lower.
Also if the surrounding air temp is higher than freezing for some reason (near a building or roadway where excess heat might radiate), then the temp might be high enough in the immediate area to melt the snow/ice but the overall, aggregate temp of the entire surrounding area might be below freezing.
Those are just a few examples.
You’re probably witnessing *sublimation*. That is the state change where solids skip melting and turn directly into a gas. This happens for the same reason that puddles will evaporate without boiling.
Temperature is a measure of the *average* energy of the molecules in the sample. Some will be above that average and some will be below it. As the molecules jiggle around, they trade some of their energy back and forth, knocking into each other like billiard balls. Sometimes, randomly, some of them will knock into each other in such a way that one molecule ends up with a lot more energy.
If that molecule is inside the sample, nothing much happens. It will bang into another molecule or molecules and lose that energy and that’s it. If, however, that molecule is on the surface it might be given enough energy to be knocked away. If the energy is enough to loosen the molecule and it “melts” out of the solid structure, but doesn’t fly off as a gas, then it won’t get very far before it bumps into one of the frozen molecules and trade that energy back in, and get trapped again. If it has enough to fly off as a gas, it will take that energy with it and just like that your ice cube is one molecule smaller.
Gaseous molecules can also bang into the solid, lose their energy, and become trapped. If there is a lot of moisture in the air, some of those water molecules will run into the ice and become trapped, and the ice cube is one molecule *bigger*. In static conditions, this will lead to an equilibrium where there are just as many molecules getting trapped as there are molecules becoming freed. If the air is very dry, then it will take a long time to reach that equilibrium and the ice will shrink a lot. If the air has a high relative humidity, the ice won’t shrink, or it may even grow a bit.
The equilibrium also depends on the overall temperature of the sample and the air. If everything is very cold, very few molecules will end up with enough energy to leave, and it will be more likely for gaseous molecules to get stuck. If it’s not *super* cold, the equilibrium will shift more towards the ice sublimating rather than the gas condensing.
Freezers tend to be cold, but also *very* dry. Moisture in the air tends to condense onto the coils that cool the air in the freezer, and the system is designed to remove that moisture 1) because it preserves your food better, since bacteria like moisture, and 2) condensation is bad for the coils. Since your refrigerator stays closed most of the time, no new moisture can get in so the freezer stays very dry, shifting the equilibrium so that ice tends to sublimate and shrink. This also causes “freezer burn” where foods tend to become dry and tough, as the water sublimates away.
Water turning to ice doesn’t stop the process of evaporation, but it does slow it down. Assuming the ice is not placed on a surface that collects and amplifies the sun it will sublimate to gaseous water. Sublimation is when a solid gains enough energy for the atoms to skip the liquid phase and jump straight to gas.
[https://www.kxnet.com/weather/weather-whys/weather-whys-why-snow-melts-or-disappears-in-subfreezing-temperatures/](https://www.kxnet.com/weather/weather-whys/weather-whys-why-snow-melts-or-disappears-in-subfreezing-temperatures/)
This happens best for ice and snow on sunny, dry days when the sun gives the solid water enough energy to go to gaseous water.
The other way snow and ice melts in subfreezing temperatures is the sun heating the ice and the surface it is on (the darker the surface, the warmer it can get) which allows the ice to become liquid and flow even though air temperatures are below freezing.
There is a misconception that people have that below the freezing point ALL the water will be a solid. Above this and below the boiling point ALL water will be liquid. Above the boiling point ALL water will be gaseous. This is incorrect (I’m assuming just water here, but it works for all the other substances too).
If you look at an ice cube its just a bunch of H2O molecules that on average have such low energy that they pack themselves for the MOST part into a crystal structure. Some of the H2O molecules on the surface may get hit by molecules in the liquid or air surrounding the ice. These molecules may gain enough energy from these collisions to escape their neighbors and become part of a liquid or gas surrounding the ice cube. Some H2O molecules may also hit the ice cube and stick to it. Pretty much all ice (even well below freezing) will have a very thin layer of liquid water around it. And if the ice cube is in a gas, there will be some H2O in gaseous form around it.
What you are describing with the ice cube getting smaller even when it is super cold is not really melting, but it is sublimation. Sublimation is a phase transition going directly from a solid to a gaseous form (dry ice is famous for this). Sublimation like evaporation or freezing is a macroscopic description of what is actually going on. If you zoomed in enough you’d more likely see the ice becoming part of the thing liquid film surrounding the ice cube and then becoming gaseous.
A great description of what is going on can be found in [Chapter 1 of Feynman Lectures on Physics](https://www.feynmanlectures.caltech.edu/I_01.html#Ch1-S1).
EDIT:
Although it doesn’t match the text in your description, you can also melt ice below its freezing point by putting it under pressure. This is what happens when you are skiing. Your weight pushing down on the snow causes a little bit of it to melt and creates a layer of water between your skis and the snow. This reduces the friction allowing you to easily ski down the slope.
Temperature is the average amount of energy in a system, so of all the really large number of water molecules in an ice cube, some small fraction of them will be above freezing temperature. For the same reason, you could set a glass of liquid water outside when it’s above freezing and you’d see some evaporating over time even though the outside temperature is well below boiling point.
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