Atmospheric conditions affect how snow crystals form and what happens to them as they fall to the ground. Snow may fall as symmetrical, six-sided snowflakes, or it may fall as larger clumps of flakes. Similarly, once snow is on the ground, the snowpack may assume different qualities depending on local temperature changes, whether winds blow the snow around, or how long the snow has been on the ground. For instance, a fresh snowfall may be loose and powdery, but snow that has been on the ground throughout the winter may have dense, crusted layers caused by melting and refreezing. Scientists and meteorologists have classified types of snowfall, snowpack, and snow formations.