Riming

Riming is a microphysical process in atmospheric science in which supercooled liquid water droplets collide with existing ice crystals or snowflakes and freeze on contact. The added frozen droplets glaze the particle, making it more opaque and often rounded in shape. Riming is a form of accretion and a key mechanism for growing ice particles in clouds, distinct from aggregation (particles sticking together) and deposition (water vapor turning directly into ice).

Riming occurs in clouds that contain supercooled droplets, typically at temperatures near or below freezing. The process requires sufficient liquid water content and lifelike air motions, such as updrafts, which keep droplets suspended long enough for multiple collisions with ice crystals. The extent of riming depends on temperature, humidity, and the intensity of updrafts.

The most common outcomes of riming are graupel and hail. Heavily rimed snow crystals become graupel, soft, rounded ice pellets that resemble small snowballs. If riming persists in strong updrafts and the particle remains within the cloud long enough, hailstones can form through repeated riming and layering, often accompanied by rapid growth and internal structure that records the storm’s history.

Riming influences cloud microphysics, precipitation types, and radar signatures. By transforming delicate snowflakes into denser, more opaque particles, it can alter snowfall rates and the potential for ice pellets or hail. In weather modeling, riming is represented in microphysical schemes to predict precipitation composition and intensity, particularly in developing convective systems and mixed-phase clouds.