Snow drifts

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Sections of railways or highways running in hollows will be covered with snow, even if there is no snowfall. Why is this happening? At first glance, the answer is clear: snow is carried by the wind. However, to understand in detail the mechanism of this process, a whole study was required.

In 1936, the English physicist Bangold studied the transport of sand by wind in a wind tunnel.

It turned out that if the wind speed is less than some speed $v_1$, then the sand does not move. At wind speeds greater than $v_1$, but less than some other speeds $v_2$, sand can also remain at rest. But if at the same time a grain of sand falls from somewhere into a motionless mass of sand, several other grains of sand jump up from its impact. These grains of sand are carried away by the wind, fall, set other grains of sand in motion, and as a result, the sand is carried along the wind. When the wind speed is greater than $v_2$, the grains of sand rise in the wind, forming a sand-air flow that is quite significant, but the density decreases upward. figure 1 gives an idea of ​​the trajectories of grains of sand.

Now we can explain why snow fills the recesses in windy weather. At the point of deepening, the flow expands (this immediately becomes visible from the picture of linear figure 2), and therefore its speed decreases. As a result, the balance between those carried by the wind and those falling -