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Have you ever caught yourself wondering where all the snow went? Most of us assume snow has to melt to disappear, but there’s actually a phenomenon that can occur which causes snow to evaporate right in front of our eyes.
Snow levels can decrease from two different phase changes. Melting is a well-known occurrence and the main way snow is changed to liquid water. Sublimation is a second phase where solid water is converted to its gas phase without becoming liquid. This can cause snow to essentially disappear.
How Does Snow Change It’s State?
Most of us know that when conditions are right, snow will melt from solid ice back into its natural state, liquid water. The term “natural” in this case simply means what water molecules are most stable as in the standard temperature and pressure in our atmosphere.
Melting is a result of a phase change of water. This is when a solid phase changes to a liquid phase. The driving factor behind a phase change is energy, known as latent heat. The amount of latent heat required is determined by a number of factors like pressure.
When the right conditions are met, snow will typically change phase back to water as it melts. That does not answer how snow seems to sometimes disappear without melting though, right? Well, there is actually more to this than meets the eye.
Can Snow Evaporate Without Melting?
So we know that when a solid phase changes liquid phase this is called melting. We also know that when a liquid changes to a gas, it is called evaporation, but what about if a solid changes directly to a gas without changing to a liquid first?
This is called sublimation, and it can be the cause of snow disappearing without melting first.
The easiest way to tell if sublimation or melting is occurring is by looking for water, most likely in the form of run-off or puddles. If the levels of snow are decreasing without much water being visible, particularly in areas where it can’t seep into the ground, sublimation is probably happening.
What is Sublimation?
As mentioned, sublimation is when a solid phase changes directly to its gas phase, completely skipping over the liquid phase.
Sublimation is made possible by the right combinations of temperature, pressure, humidity, and wind. The sublimation process requires a lot more energy than melting does, so these conditions are essential for it to occur.
One of the main conditions for sublimation to occur instead of evaporation is surface temperature must be below 0 °C or 32 °F.
Dry ice is one of the simplest ways to picture the sublimation phenomenon. Frozen carbon dioxide (dry ice) lets off a fog when exposed directly to air. This is because gaseous carbon dioxide is the most stable form of the chemical compound.
Does Snow Melt Into the Ground or Evaporate?
Snow melts from top to bottom because it is rather well insulated by itself. Snow has a lot of little air pockets inside its structure, and this is very good at stopping energy, in the form of heat, from traveling through the snow and melting it.
This means only the top layer is really exposed to melting factors like sunlight, humidity, and air temperature. With enough energy transfer, the top layer of snow will melt. The liquid water will then seep through the other snow with the help of gravity.
The type of surface the snow is on now comes into play. If it is on a porous surface like soil or grass, it will seep into the earth, essentially disappearing. If the snow has landed on a roof, road, or car, or any other non-porous material it naturally will not melt into the ground. It will actually become visible run-off water.
At What Temperature Does Snow Evaporate?
The answer here is more complicated than most of us would think. It is well known that at temperatures at or below 0 °C or 32 °F liquid water will change to solid ice. On the other end of the spectrum, at temperatures at or above 100 °C or 212 °F liquid water will change to steam.
In between these two critical points, you will typically find a mix of liquid water and water vapor. Water vapor is usually present in the form of humidity. The closer to boiling you get, the more water vapor. The closer to freezing, the less water vapor.
This makes it impossible to determine a universal temperature by which sublimation will occur. There’s a lot of thermodynamics going on behind the scenes that are a bit too far out of scope for us.
Does Snow Dry Out the Air?
In short, yes, snow does dry out the air.
Snow is a form of precipitation, and precipitation occurs as a result of an excess of water vapor in the atmosphere. When enough of this water vapor is present, it will snow, rain, hail, or one of the other types of precipitates.
As these precipitates form, they pull water vapor out air. This then drys out the air rather significantly.
Most of us have experienced dryer air in winter. These are usually things like dry lips and skin. Some of us, myself included, use a humidifier in the colder months because of just how dry it can get.
It is inevitable as seasons change that once plentiful snow will disappear in lieu of warmer temperatures and sunshine. This usually happens via the process of melting where snow is converted back to liquid water, but there is a secondary process that causes snow to evaporate.
Sublimation can occur under the right conditions. The surface temperature below the snow mass has been cited as the determining factor between sublimation and melting. If snow levels are decreasing without significant water present, it is most likely because of sublimation.