Freezing rain

Mechanism

Freezing rain often forms with the approach of a warm front, where subfreezing air is trapped near the surface while warm air moves aloft. This occurs, for example, when a low pressure system moves from the Mississippi River Valley toward the Appalachian Mountains and the Saint Lawrence River Valley during the cold season, with a strong high pressure system to the east—a setup known as cold-air damming. Falling snow encounters a warm layer aloft, typically around the 800 mbar level, melting into rain.

As the rain falls through a shallow subfreezing layer just above the surface, it cools below freezing but does not freeze before impact. If the subfreezing layer is deeper, the drops may freeze into ice pellets (sleet) instead. A supercooled warm-rain process also exists, where liquid droplets remain unfrozen until contact with surfaces at or below 0 °C. Upon impact, nucleation causes instant freezing, forming a thin ice film.

Observations

Surface observations from staffed or automatic stations provide the only direct confirmation of freezing rain. Weather radar cannot directly see freezing rain, rain, or snow, but can estimate the area indirectly. Radar reflectivity is proportional to precipitation form and diameter; rain has stronger reflectivity than snow, but its diameter is smaller, so reflectivity from melted snow is only slightly higher.

In the melting layer, wet snowflakes coated with water produce much stronger returns, creating a brightband that indicates a warm layer aloft where snow can melt. The brightband's height and slope help determine the extent of melting, and combining this with surface observations and numerical models allows precipitation type mapping. Dual-polarized radar, which sends vertical and horizontal waves, can deduce target shape and state mixture, indirectly identifying precipitation type.

Effects

Freezing rain causes major power outages by forming glaze ice. Light, short-lived freezing rain produces thin ice and minor damage, such as relieving trees of dead branches. Heavy accumulation is one of the most dangerous winter hazards: when ice exceeds approximately 0.25 inches, tree limbs break under the weight and fall onto power lines. Wind and lightning worsen the damage.

Power lines become extremely heavy, breaking support poles, insulators, and lines. Roadways become treacherous because wet ice offers almost no traction, causing vehicles to slide even on gentle slopes. Unlike sleet, freezing rain conforms to surfaces, creating a thick layer of glaze. Large-scale freezing rain and glaze ice is called an ice storm.

Plants suffer severely, as they cannot support the ice weight; dormant trees may snap, and pine needles catch ice but cannot bear it. In February 1994, a severe ice storm caused over $1 billion in damage in the Southern United States, primarily in Mississippi, Tennessee, Alabama, and Western North Carolina. The North American ice storm of 1998 affected eastern Canada and northern parts of New York and New England. Freezing rain is extremely hazardous to aircraft, causing rapid structural icing.

Most helicopters and small airplanes lack deicing equipment for freezing rain, and heavy freezing rain can overwhelm large aircraft deicing systems. Ice changes airfoil shape, reducing lift and increasing drag, raising stalling speed and impairing climb or altitude maintenance.