A sleet storm is proof that layers of air with differing temperatures lie overhead. This sort of precipitation starts off as snow at cloud level, melts as it falls through warmer air, then refreezes when it passes through cold air on its way to the ground. Freezing rain also shows that different temperatures can be found at different altitudes. The falling icy crystals melt but stay liquid as they descend through cooler air, supercooling as they go. The supercooled drops freeze as soon as they hit something, such as the cold ground or a chilly airplane wing.
Snow shows that the air overhead is cold, all the way to cloud level.
The first constraint on form is set by the electrical bonds between water molecules. That, in a nutshell, is why all ice crystals show some kind of six-sided arrangement. Hollow hexagonal columns appear at temperatures between 21 o F and 14 o F and again when the temperature is -8 o F and lower. Six-sided needles appear at temperatures in the low twenties, and flat hexagonal plates show up nearer the freezing point.