3rd March 2006
Freezing Rain
Source: Excerpt from The Book " Weather "
Distribution: Common in regions that experience
winter snows.Cause:
Water droplets
falling through
subfreezing air or encountering frozen ground.Associated Weather: Ice deposits at ground level. Hazard Warning: Slippery surfaces, ice accretion on aircraft and boats. In wintry conditions, when temperatures at cloud level are below zero, any water droplets that fall from clouds will be supercooled. This means that they are likely to freeze as soon as they encounter a colder layer of air or a surface whose temperature is below 32° F (0° C). Precipitation that freezes in either of these ways is known as freezing rain.
In the former case, the rain turns into tiny pellets of ice in mid-air. In the United States, this type of frozen rain is known as sleet. In Australia and the United Kingdom, it is referred to as ice pellets, while sleet is used to describe a fall of partially melted snow.
Probably the most important distinction to be made between the various types of frozen precipitation is the difference between the ice pellet form of frozen rain and hail. The latter only forms in a thundercloud, while freezing rain may fall from any cloud that can produce rain, provided that the air is cold enough.
Surface Conditions
When large supercooled droplets strike subfreezing ground, they tend to spread out on impact before freezing, coating surfaces with a layer of clear ice known as glaze (as shown in the picture on right). This type of ice can produce hazardous conditions, making it extremely difficult to drive or even walk. A heavy downpour in these conditions is known as an ice storm. The accumulation of glaze on exposed objects as a result of an ice storm can cause significant structural damage --- it has been known to bring down overhead wires and tree branches.
When very small supercooled droplets strike subfreezing ground, they tend to freeze immediately on impact, trapping air between them. This produces an opaque, granulated coating of ice, known as rime, which is not as slippery as glaze. (Picture on the right shows supercooled droplets which have frozen on impact with the ground).
Ice pellets normally shatter on impact, scattering ice debris across the ground. However, if the ice pellets have not completely frozen through, water from inside the pellets may spread across the ground, forming a glaze as it freezes. If carried by a strong wind, ice pellets can sting exposed skin, causing great discomfort.
Once glaze has formed, it normally thaws in a few hours. However, there have been occasions when glaze has persisted for days. The most extreme case on record was during the winter of 1969, in Connecticut, in the United States, when glaze remained on trees for six weeks.
While freezing rain can be very inconvenient, the greatest associated hazard is ice accretion on aircraft and boats. If an airplane flies through a supercooled cloud, ice will quickly form on its body and wings, altering its speed and aerodynamics. A substantial build-up of ice on the masts of a sailing boat at sea can cause the vessel to capsize.
Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.
