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Freezing drizzle isdrizzle that freezes on contact with the ground or an object at or near the surface. ItsMETAR code isFZDZ.[1]
Although freezing drizzle andfreezing rain are similar in that they both involve liquidprecipitation above the surface in subfreezing temperatures and freeze on the surface, the mechanisms leading to their development are entirely different. Where freezing rain forms when frozen precipitation falls through a melting layer and turns liquid, freezing drizzle forms via thesupercooled warm-rain process, in which cloud droplets coalesce until they become heavy enough to fall out of the cloud, but in subfreezing conditions.[2] Despite this process taking place in a subfreezing environment, the liquid water will not freeze if the environmental temperature is above 18 °F (−8 °C), viasupercooling.[3] Ifice crystals are already present in this environment, the liquid droplets will freeze onto these crystals and be effectively removed before they can grow large enough to fall out of the cloud. As a result, freezing drizzle develops in shallow low-levelstratus-type clouds where air saturation occurs entirely below the layer in which ice crystals can develop and grow.[2]
When freezing drizzle accumulates on land, it creates an icy layer ofglaze. Freezing drizzle alone does not generally result in significantice accumulations due to its light, low-intensity nature unlike its rain counterpart. However, even thin layers of slick ice deposited on roads asblack ice can be very slippery and cause extremely hazardous conditions resulting in vehicle crashes.
Freezing drizzle is extremely dangerous to aircraft inicing conditions, as the supercooled water droplets will freeze onto the airframe, degrading aircraft performance considerably. The loss and accident ofAmerican Eagle Flight 4184 on October 31, 1994, has been attributed to ice buildup due to freezing drizzle aloft.[2]
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