Introduction

The formation of hailstones is an intriguing aspect of precipitation development studies owing to the unique cloud systems in which hailstones form. As equal amounts are added of hailstones, for a given density the shells of equal mass will be of different thickness (Fig. 10.1). In particular, the formation of very large hailstones, some of which are greater than 51 mm in diameter, is of great interest. Models of hail growth can be very simple or imply very detailed processes (Takahashi 1976 and Fig. 10.2).

Hailstones are typically defined as solid or nearly solid ice particles that are greater than 5 mm in diameter. The National Weather Service in the United States classifies hailstones as constituting severe hail if they are larger than 19 mm (3/4″) in diameter, and constituting very severe hail if the hailstones are larger than 51 mm (2″). These larger hailstones develop most frequently from rapid riming of higher-density graupel particles and/or large frozen drops. Studies suggest that high-plains storms produce most of their hail from graupel, and Southern Plains storms produce most from frozen drops (Fig. 10.3). It is not known exactly why this is so, but some have speculated that high-plains storms do not produce large water drops above the freezing level because they have cold cloud bases cooler than 5 °C (278.15 K) (Fig. 10.3) and the 500 mb temperature in the updraft core is perhaps 260.15 to 263.15 K. Thus, many of the hailstone embryos are perhaps formed from graupel.