Graupel is a type of frozen precipitation. Southern Wisconsin experienced this on Thursday afternoon.
Most clouds outside the tropical regions have temperatures that are below freezing somewhere in the cloud. These cold clouds are likely to have frozen cloud particles. They are also likely to include supercooled water drops, drops made of water that are below the freezing point.
Collisions between particles inside a cloud help create precipitation in cold clouds. When an ice crystal falls through a cloud it may collide with and collect supercooled water drops. This process of ice crystal growth by sweeping up supercooled water drops is called accretion, which can be thought of as a riming of the crystals. When ice crystals collide with supercooled drops, the drops freeze almost instantly. Accretion thus provides a mechanism for the particle to grow quickly, and when large enough fall out the bottom of the cloud.
An ice particle produced by the accretion process that has a size between 1 and 5 millimeters (0.04 to 0.2 inches) and no discernible crystal habit is called graupel (plural, graupeln). On collision and freezing, the supercooled water often traps air bubbles. Because of this trapped air, the density of a graupel is low, and it can easily be crushed, unlike a hailstone.
Hail is precipitation in the form of large balls or lumps of ice. Hail develops in the complex air motions inside a towering cumulonimbus cloud.
Aggregation is the process by which ice crystals collide and form a single larger ice particle. The probability that two crystals will stick together depends on the shape of the crystals and the temperature. A snowflake is an individual ice crystal or more likely an aggregate of ice crystals.
Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at firstname.lastname@example.org or email@example.com.