We cannot really measure a snowfall. When we say, “Four inches of snow fell,” what we really mean is, “Four inches of snow accumulated,” and that is something entirely different. 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 settles. How deeply snow accumulates is partly a matter of how much the snow crystals are broken as the snow settles onto the ground. Furthermore, if the snow falls on a windy day, the wind can break up the crystals in the air. Finally, conditions within the clouds can affect the size and shape of the snow crystals as they are forming. All of this impacts the accumulation of snow on the ground. Broken crystals pack together densely and a lower snow measurement is the result. 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 which can accumulate readily.
