Understanding Hail in the Earth System

The processes leading to the development of hail and the distribution of these events worldwide are reviewed. Microphysical and physical characteristics of hail development are described to provide context of the notable gaps in our understanding of what drives hail to grow large, or what determines how it falls to the ground. Distributional characteristics of hail are explored, utilizing both surface observations of hailstones and remotely sensed observational data sets to identify opportunities and needs for new observations. These observational deficiencies contribute to our limited capacity to both forecast hail or its expected size and reduce the effectiveness of using favorable conditions for hail development as a proxy to frequency where observations are unavailable. Given the substantive influences of both climate variability and the changing Earth system on hail, the latest understanding of their contributions to risk are addressed. Applying this understanding of the distribution and physical characteristics of hail, the damage by hail to agriculture and insured property is assessed. Much remains unknown about the processes leading to hail growth and environmental controls on hail occurrence, size, and magnitude, particularly outside of the United States and Europe. A better understanding of the global occurrence of hail is also needed to better anticipate the hazard and associated impacts. Plain Language Summary The processes leading to the development of hail and the resulting distribution of these events worldwide are reviewed. Hail forms from small frozen embryos that are lofted into the updraft of a thunderstorm, subsequently encountering a region of supercooled water, and growing by either riming or accumulation. Contrary to popular belief, the suggestion that hailstones that take multiple up-and-down excursions through an updraft are not observed. Hail can grow to large sizes on any continent, and stones as large as 200 mm have been recorded. Damages associated with hail reach more than 10 billion U.S. dollars a year in North America alone. In this review a range of topics are covered, including characteristics of hail growth and development, surface observations of hail, remotely sensed observations, the approaches to forecasting hail and estimating its frequency where observations are unavailable, the influences of climate variability and a warming earth system and the impacts on insured property. Much remains unknown about the processes leading to hail growth and environmental controls on hail occurrence, size, and magnitude, particularly outside of the United States and Europe. A better understanding of the global occurrence of hail is also needed to better anticipate the hazard and associated impacts.