Hollow columns, needles, solid plates and stellar dendrites are all types of snow crystals. Snow crystals are not frozen drops of water. The formation of snow crystals most commonly occurs by a process known as heterogeneous nucleation. This process which happens when clouds are supercooled (temparatures at or below -40 degrees C, in Farenheit this temperature is also -40 degrees), results when very small particles cause
the formation of ice, generally by the freezing of water vapor in the cloud onto the surface of the particle. Silicate particles (dust)
are one of the most common nucleating agents - snow crystals typically begin to form from these particles. There needs to be a nucleus of some kind to form a snow crystal. Particles can also arise from industrial plants or many other sources, even a broken fragment of another snow crystal. Falling snow
crystals are divided into eight major categories based on their shape: columns, needles, plates, dendrites (most often seen in Christmas decorations) , irregular crystals, graupel, hail, and ice pellets. These snow crystals
all have six sides. The oxygen and hydrogen atoms in water molecules always link together to form hexagons. However, through a magnifying lens, some cystals may have a pencil shape (columns), the shape of a dinner plate, or a fern-like appearance (dendrites). The different shapes are determined by the temperature at which the crystal starts to grow. If the temperature of the air is relatively warm (0 to about -4 C), plates predominate, if the temperature is slightly cooler, then needles are more common, and at colder temperatures (-10 C to -20 C), dendrites are more numerous. As the crystals fall toward the ground, they encounter layers of different
humidity and temperaure. These often subtle changes cause the crystals to grow at different rates as they fall and are responsible for the intricate patterns
and delicate forms that we associate with falling snow.
When two or more crystals combine as they fall, a snowflake is born. Snowflakes may form from many combinations of falling crystals, but because they have abundant radiating arms, the dendritic crystals tend to aggregate more readily than do other types. Once a snowflake or snow crystal falls to the surface it is quickly broken or rounded by friction with other snowflakes.
Hollow columns, needles, solid plates and stellar dendrites refer to what? What do two or more of these form?