Rime is a type of ice deposit that appears as a white, opaque layer on surfaces. It forms when specific atmospheric conditions combine to create a freezing process. This granular ice typically builds up on objects exposed to the elements, such as tree branches, power lines, or aircraft surfaces. Its distinctive appearance is recognizable in cold, misty environments.
How Rime Forms
Rime forms through accretion, where supercooled water droplets freeze upon impact with a surface. These droplets remain liquid even at temperatures below freezing, found in fog or mist. When they encounter an object at or below 0°C (32°F), the impact causes them to instantly freeze. This rapid freezing traps tiny air bubbles within the ice, giving rime its characteristic white, opaque, and rough appearance.
Rime accretion occurs in temperatures from 0°C to -40°C, most frequently between -10°C and -20°C. Supercooled liquid water droplets are a fundamental requirement. They are common in stratiform and cumuliform clouds. The rate of ice accumulation is directly proportional to the amount of supercooled liquid water present. The object’s surface temperature must remain below freezing for continued formation.
Types of Rime
Rime primarily manifests in two forms: soft rime and hard rime, distinguished by physical characteristics and formation conditions. Soft rime appears as a milky, feathery, or spongy deposit that is fragile and easily dislodged. It forms in calm or light wind conditions where supercooled droplets are small and freeze slowly. This type of rime builds up on the windward side of objects, resembling hoarfrost in its crystalline structure.
Hard rime is denser and more granular, appearing as a rough, opaque deposit. It forms under moderate wind speeds and air temperatures between -2°C and -10°C. Higher wind speeds cause the supercooled droplets to freeze rapidly and individually upon impact, trapping air within the ice and creating a rough, uneven surface. This form is observed on trees and structures on mountain ridges enveloped by freezing fog, where strong winds push droplets against exposed surfaces.
Rime Compared to Other Ice Formations
Rime is often confused with other ice formations like frost and glaze ice, but their formation and appearance differ significantly.
Frost, specifically hoarfrost, forms when water vapor directly deposits as ice crystals onto a surface below freezing. Unlike rime, which requires liquid water droplets, hoarfrost bypasses the liquid phase, forming from water vapor. This direct sublimation results in a white, feathery, crystalline deposit that forms on clear, cold nights.
Glaze ice, also known as clear ice, forms from larger supercooled water droplets that freeze relatively slowly upon impact with a surface. These droplets spread out before freezing, allowing air bubbles to escape, which results in a transparent, smooth, and dense layer of ice. Glaze ice occurs at warmer temperatures, between 0°C and -10°C, and is associated with freezing rain or large supercooled droplets in clouds. The slow freezing process allows the ice to conform to the object’s shape.
Rime’s opaque, rough texture, caused by trapped air bubbles from rapid freezing, distinguishes it from the clear, smooth appearance of glaze ice. Key differences lie in supercooled droplet size and freezing speed: small droplets freeze instantly for rime, versus larger droplets spreading and freezing slowly for glaze ice. Rime forms from supercooled liquid droplets, while frost forms directly from water vapor.