Freezing drizzle is
drizzle
Drizzle is a light precipitation consisting of liquid water drops smaller than those of rain – generally smaller than in diameter. Drizzle is normally produced by low stratiform clouds and stratocumulus clouds. Precipitation rates from dri ...
that freezes on contact with the ground or an object at or near the surface. Its
METAR
METAR is a format for reporting weather information. A METAR weather report is predominantly used by aircraft pilots, and by meteorologists, who use aggregated METAR information to assist in weather forecasting.
Raw METAR is the most common fo ...
code is FZDZ.
Formation
Although freezing drizzle and
freezing rain
Freezing rain is rain maintained at temperatures below freezing by the ambient air mass that causes freezing on contact with surfaces. Unlike a mixture of rain and snow or ice pellets, freezing rain is made entirely of liquid droplets. The rain ...
are similar in that they both involve liquid
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
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 the ''supercooled warm-rain process'', in which cloud droplets coalesce until they become heavy enough to fall out of the cloud, but in subfreezing conditions.
Despite this process taking place in a subfreezing environment, the liquid water will not freeze if the environmental temperature is above , via
supercooling
Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid or a gas below its melting point without it becoming a solid. It achieves this in the absence of a seed crystal or nucleus around which a crystal ...
.
If
ice crystals
Ice crystals are solid ice exhibiting atomic ordering on various length scales and include hexagonal columns, hexagonal plates, dendritic crystals, and diamond dust.
Formation
The hugely symmetric shapes are due to depositional growth, n ...
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-level
stratus-type clouds where air saturation occurs entirely below the layer in which ice crystals can develop and grow.
Effects
When freezing drizzle accumulates on land, it creates an icy layer of
glaze. Freezing drizzle alone does not generally result in significant
ice accumulations due to its light, low-intensity nature unlike its rain counterpart. However, even thin layers of slick ice deposited on roads as
black ice
Black ice, sometimes called clear ice, is a thin coating of glaze ice on a surface, especially on streets. The ice itself is not black, but visually transparent, allowing the often black road below to be seen through it. The typically low level ...
can be very slippery and cause extremely hazardous conditions resulting in vehicle crashes.
Freezing drizzle is extremely dangerous to aircraft in
icing conditions
In aviation, icing conditions are atmospheric conditions that can lead to the formation of water ice on an aircraft. Ice accretion and accumulation can affect the external surfaces of an aircraft – in which case it is referred to as ''airframe ...
, as the supercooled water droplets will freeze onto the airframe, degrading aircraft performance considerably. The loss and accident of
American Eagle Flight 4184
American Eagle Flight 4184, officially operating as Simmons Airlines Flight 4184, was a scheduled domestic passenger flight from Indianapolis, Indiana to Chicago, Illinois, United States. On , 1994, the performing this route flew into severe ...
on October 31, 1994, has been attributed to ice buildup due to freezing drizzle aloft.
See also
*
Black ice
Black ice, sometimes called clear ice, is a thin coating of glaze ice on a surface, especially on streets. The ice itself is not black, but visually transparent, allowing the often black road below to be seen through it. The typically low level ...
*
Freezing rain
Freezing rain is rain maintained at temperatures below freezing by the ambient air mass that causes freezing on contact with surfaces. Unlike a mixture of rain and snow or ice pellets, freezing rain is made entirely of liquid droplets. The rain ...
References
Precipitation
Weather hazards
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