John Wheeler: The accumulation of snow is a complex process

FARGO — When we say, “Four inches of snow fell,” what we really mean is, “Four inches of snow accumulated.” We can measure a rainfall easily enough by capturing a sample in a rain gauge, but snow accumulation is different because as snow accumulates, it compacts. How deeply snow accumulates is largely a matter of how densely the flakes pack as they settle.

Varying conditions within a cloud create different shaped crystals, from narrow needles that pack densely to broad, airy flakes that settle with a lot of air. If the snow falls on a windy day, the wind can break up the crystals in the air. Small or broken crystals will pack tightly together and a lower snow measurement is the result.



Any melting upon impact will also reduce the accumulation. However, when conditions are cold and relatively windless, the snowflakes can be quite large and airy and break up very little on impact, resulting in a very fluffy snow that can accumulate readily even with a small amount of precipitation..