Why it takes a long time to melt snow

When does snow melt? When does snow thaw?

This is how the snow disappears

Melting, thawing, sublimating - what is what?

Some can't get enough of snow, others find it stupid. So it is fair that sooner or later - in spring at the latest - the snow will hit the collar again. But how does snow actually disappear?

Snow collapses

A fresh blanket of quite dry snow contains a lot of air. Over time, the loose, fluffy snow sags into the cavities under its own weight. The total snow depth is reduced, even if there was permafrost all the time. The snowpack is now not so nice and fresh to look at and lower.

Sublimation: Snow or ice become gaseous

If the air around the snow can still absorb moisture, i.e. is not yet saturated with water vapor, snow and ice can change directly from a solid to a gaseous state. Regardless of the temperature. This phenomenon can be observed especially on sunny ice days.

In this process, however, energy is applied and the rest of the snow cools down. It can therefore take a long time until a blanket of snow is gone through sublimation.

Melting: It's about more than the 0-degree limit

So that snow melts, so that it passes into both the gaseous and the liquid phase, the so-called wetness temperature must be above 0 degrees, but the dew point temperature must be below that.

The wet temperature is the temperature measured with a wet thermometer. It is lower than the air temperature - unless the air has a humidity of 100%.

Just try it out:
Hold the thermometer briefly in the water and let it dry. It will usually display a temperature that is below the air temperature. Only when the water film on the surface has completely evaporated does the value equalize to that of the air temperature. The phenomenon can also be observed on the temperature display in the car after it has been in the car wash.

Melting can be faster than subliming, but there is a more effective method.

Thawing: the snow turns into water

Now the dew point temperature comes into play. The dew point temperature is the temperature at which the humid air would be saturated with water vapor and therefore water is released from it in the form of droplets, e.g. B. as fog or dew, can settle.

If the dew point is also above 0 degrees, the snow thaws - an even more effective process than melting. The snow then only changes into the liquid phase. This is the worst way for the snow to hold up.

Approximate limits for thawing, melting, sublimation

Assuming an average relative humidity of 50%, snow sublimes below a temperature of +3.5 degrees, melts at 3.5 to 10 degrees and thaws above 10 degrees.

Snow killer and melt accelerator

Sure, the higher the temperatures, the faster the snow disappears. When rain falls on snow, it says goodbye even faster. Wind also speeds up the process. Because it blows away the cold layer of air above the snow cover.

The most merciless "snow killer" of all, however, is humans, who apply salt or salt solutions to streets and paths. This lowers the freezing point of water and snow can no longer hold in solid form.