I have just experienced a snowfall and I am not so clear on how it works.
Three days after a short day of snowfall, and having 2 min | 17 max degrees Celsius, full sunny scarcely cloudy each day, there is still some snow persisting in shadow and dark places.
This is contrary to my intuition: I would've expected all the snow to have melted and disappeared after the first sunny day, or after the second. Yet we are on the third day and still some snowman heads are alive.
Is it because the snow contains salt? Or does the snow create low temperature air around itself? Or does the daily morning humidity turn the snow into ice blocks that are harder to melt and more solid to scatter sun rays?
Best Answer
Just as a complement to Ziggurat's answer: you can try to estimate the time required for the sun to melt a certain quantity of snow by yourself.
The energy required to melt a mass $m$ of snow is $$Q=L m$$ where $L$ is the latent heat of fusion. For ice, $L=334$ kJ/kg.
The density of snow $\rho$ ranges from $100$ to $800$ kg/m$^3$
If the surface exposed to sunlight is $S$, the absorbed energy in the time interval $\Delta t$ will be
$$E_{in}=(1-A) IS \Delta t$$
If $V$ is the snow volume, the energy required to melt it will be
$$E_{melt} =L \rho V$$
Equating these two expressions we get
$$\Delta t = \frac{L \rho V}{(1-A)IS}$$
Assuming $A=0.9$, $\rho=300$ kg/m$^3$ and $I=200$ W/m$^2$, we get, for a sheet of snow of surface $1$ m$^2$ and thickness $1$ cm, $\Delta t \simeq 5 \cdot 10^4$ s, i.e. $\simeq 14$ hours.
This is a very rough estimate that doesn't consider conduction processes. But anyway, you can see that even if we assume a pretty high irradiance we need a considerably long time to melt a modest quantity of snow. If the snow is in the shade, the value of $I$ will be less. Also, for snowmen, since we would be talking about compressed snow, the value of $\rho$ could be $2-2.5$ times larger.