An ice crystal can grow if the air around it has a relative humidity near 100%. The ice particle grows by water vapor deposition. Growth by deposition is generally slow. If you find nicely shaped snowflakes, they likely were produced by vapor deposition. A snowflake can be an individual ice crystal or an aggregate of ice crystals.
There are four basic shapes of ice crystals: the hexagonal plate, the needle, the column and the dendrite. The dendrites are hexagonal with elongated branches, or fingers, of ice; they most closely resemble what we think of as snowflakes. The temperature at which the crystal grows determines the shape.
Aggregation is the process by which ice crystals collide, get entangled or stick together and form a single larger ice particle. The probability that two crystals will stick together depends on the shape of the crystals. If two dendrites collide, it is likely that their branches will become entangled and the two crystals will stick together. When two plates collide there is a good chance that they will simply bounce off one another. Temperature also plays a role in aggregation. If the temperature of one crystal is slightly above freezing, it may be encased in a thin film of liquid water. If this particle collides with another crystal, the thin film of water may freeze at the point of contact and bond the two particles into one.
The record size for an aggregate snowflake occurred in January 1887 in Fort Keough, Montana, when some flakes were measured at 15 inches in diameter. That is about the size of a family sized pizza pie!
The world’s largest solitary ice crystal measured 10 millimeters, or 0.394 inches, from tip to tip. This dendritic-shaped crystal was photographed by Kenneth G Libbrecht on Dec. 30, 2003, during a gentle snowfall in Cochrane, Ontario.
Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.
