How do icicles under a porch form?
After the snow, while the roof is still covered with quite thick white snow, one can often notice that there are columns of coarsely scattered columns of ice in the dark shade under the porch. How did the icicles form?
On a clear day, after the snow falls, the accumulated snow absorbs the energy of sunlight that begins to melt. But then the air temperature could still be below 0 ° C. People often feel that “the snow is not cold, the snow is melting” because of that.
If the air temperature is 1-2 ° C, snow accumulated on the roof facing away from the Sun can directly receive the Sun’s irradiation, which immediately melts. When the melting snow along the eaves flows down to the porch’s dark shade due to turning away from the Sun, the surrounding air temperature may still be below the freezing point; the falling snow will naturally freeze again. Before the water could fall to the ground, it was already frozen. One drop, two drops, three drops … the next water non-stop clumping together, forming a small icicle hanging under the porch.