Understanding the Physical Processes that Create Precipitation

Illustration of research on the affects of continental versus maritime atmospheric conditions, and clear versus polluted air, on the formation of various forms of precipitation in different cloud types. Understanding the very small (microphysical) precipitation processes such as droplet formation, furthers our understanding of weather and climate systems, and improves weather forecasting. It is well known that the structure and evolution of precipitating weather systems depend strongly on the microphysics and, in particular, on the conversion of water to ice and vice versa. Such microphysical processes affect the dynamics of systems through their influence on the strength of updrafts, downdrafts, and cold outflows; they also directly affect important forecast parameters such as precipitation type and amount. Quantitative precipitation forecasts, which are a critical societal requirement, are highly sensitive to these microphysical properties and processes. Despite this, precipitation formation processes are currently not adequately represented in both weather and climate models. Especially uncertain is the treatment of water and ice phases and precipitation development. Physically based improvements to the model physics must be developed, particularly for the ice formation that accounts for much of the deficiency.

How do hailstones form?

A thin slice of hailstone under polarized light produces this kaleidoscope of color. These crystal patterns were photographed in a cold room at the National Center for Atmospheric Research. Hail is created by strong updrafts, typically in thunderstorms. Small hailstones have a soft center and a single outer coat of ice. Large hailstones usually alternate between hard, clear layers (made of solid ice, these layers form when the hailstone encounters moisture that spreads across the stone before freezing) and softer, milky layers (these form in a colder environment as moisture freezes on contact, trapping tiny air bubbles as it freezes). Hailstones are generally spherical or oblong, although agglomerations of hailstones often have a spiky appearance. Most hail is small, but some hailstones grow to a diameter of 5 inches (13 centimeters) or more. Hail causes much damage to agriculture and property and can injure or kill livestock and people.