The Characteristics of Natural Hailstones and Their Interpretation

Abstract

Present knowledge of the characteristics of natural hailstones and their interpretation is reviewed. The theory of hailstone growth is first presented as this defines the terminology used in discussing the growth of hail. Broadly, the growth of the various hailstone layers may be described as “dry” or “wet” depending on whether or not all of the accreted supercooled droplets can be frozen by the forced ventilation processes of heat conduction and evaporation from the hailstone surface. The nature of the ice deposited depends on the particular regime in which the layer was formed. In the wet growth regime, it is usual for the excess unfrozen water to be incorporated in the ice structure to form a “spongy” deposit. The characteristics of hailstones are then discussed. These include their shape and size, the nature of the embryos, their layer and lobe structure, and the isotopic composition and particulate content of the layers. Sections on the aerodynamic behaviour and the growth parameters (density, collection efficiency, and drag and heat coefficients) of hailstones are also included. These are important because they affect the growth and heat balance equations in the theoretical treatment. Finally, methods of hailstone analysis and the interpretation of the data so obtained are described. The current main methods of analysis are determinations of the isotopic composition, the crystal size and orientation distributions, and the air bubble concentrations and size distributions in the individual hailstone layers. These indicate the ambient temperatures, liquid water concentrations and, to a lesser degree, the cloud droplet sizes at and from which the layers were formed. The analyses to date suggest that large hailstones remain balanced in their respective updrafts between about the −20 and −30°C levels for most of their growth history. This means that the updrafts increase with time or, alternatively, that the hailstones move around the main cores of the updrafts in such a way that they encounter increasing updraft speeds. Variations in the opacity of hailstone layers are due to fluctuations in the liquid water concentration, by as much as 30 percent. On the assumption that the median volume radius of the cloud droplets is ~ 10 μm, the liquid water concentrations in the updrafts are approximately the adiabatic values. It is pointed out, however, that there are assumptions underlying the analytical techniques used in the analyses and these have yet to be fully confirmed.