Question

Snow in the winter can slowly disappear even when the temperature is consistently below
freezing. How do you account for this observation? (Hint: this phenomenon also
explains why ice cubes appear to “shrink” in the freezer)

Answer 1

The temperature can vary greatly as it is measure at different levels. The temperature may be well above freezing at ground level, but the temperatures above the ground, where snowflakes form, can easily be below freezing. If the snow does not melt on the way down, it will fall as snow, regardless of what the ground temperature is.

There can be many variables as to why snow would still melt even below the freezing point. Here are some examples:

If the sun is out, the energy of the sunlight can be sufficient to raise the temperature of the parcel of snow to above freezing, despite the ambient air temperature, especially if the snow is on other objects such as pavement or roofing which readily absorb solar energy. This effect typically doesn’t occur if it is colder than about 0 to 10 degrees Fahrenheit.

If the snow falls on a heated building, the heat escaping through the roof is enough to cause the snow to melt.

The ground itself may still be too warm, depending on what the previous weather was like. The earth traps a fair amount of heat and it would take weeks of sub-freezing weather for this effect to not be a factor.

The snow itself undergoes a settling process after it is on the ground or another object for a while - the air pockets that are caught between the flakes slowly escape and the snow “compresses”… typically as much as 3 inches per foot of snow in a days’ time. This could cause it to appear to have melted, as there isn’t as much snow there today, for instance, as there was yesterday.