Freezing rain
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Freezing rain begins as snow, falling from a cloud towards earth and melts completely on its way down through a layer of relatively warm (above freezing) air. As it falls toward the earth's surface, it encounters a layer at lower level of colder air and becomes supercooled. This water will then freeze upon impact of any object it then encounters.[1] The ice can accumulate to a thickness of several centimetres. The METAR code for freezing rain is FZRA.[2] (see Freezing drizzle for another way of forming ice accretion)
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[edit] Mechanism
Usually freezing rain is associated with the approach of a warm front when cold air, at or below freezing temperature, is trapped in the lower levels of the atmosphere as warmth streams in aloft.[3] This happens, for instance, when a low pressure system moves from the Mississippi River Valley toward the Appalachian mountains and the Saint Lawrence River Valley of North America, in the cold season, and there is a strong high pressure system sitting further East. The warm air from the Gulf of Mexico is often the fuel for freezing precipitation.
The warm air is then forced aloft where it dramatically alters the temperature in the middle layer, around 800 mb (800 hPa). If the advection is strong enough to warm a thin layer several degrees above 0˚C for a brief period or a larger one slightly above 0˚C for a long period, the falling snow into this layer will melt and become rain droplets. These will freeze on contact when they hit the ground, which is still at or below 0˚C.
[edit] Observations
Surface observations by manned or automatic stations are the only direct confirmation of freezing rain. One can never see directly freezing rain, rain or snow on weather radars, Doppler or conventional. However, it is possible to estimate the area covered by freezing rain with radars indirectly.
The intensity of the radar echoes (reflectivity) is proportional to the form (water or ice) of the precipitation and its diameter. In fact, rain has much stronger reflective power than snow but its diameter is much smaller. So the reflectivity of rain coming from melted snow is only slightly higher. However, in the layer where the snow is melting, the wet flakes still have a large diameter and are coated with water so the returns to the radar is much stronger.
The presence of this brightband indicates that there is a warm layer above ground where snow melts. This could be producing rain on the ground or the possibility of freezing rain if the temperature is below freezing. This artifact can be located, like on the image at left, with a cross-section through radar data. The height and slope of the brightband will give clues to the extent of the region where melting occurs. Then it is possible to associate this clue with surface observations and numerical models prediction to produce output such as the ones seen on television weather programs that divide radar echoes into rain, mixed and snow precipitations.
[edit] Effects
Freezing rain often causes major power outages. Power lines coated with ice become extremely heavy, causing support poles, insulators and lines to break. Tree limbs with branches heavily coated in ice also can break off under the enormous weight and fall onto power lines. The ice that forms on roadways makes vehicle travel dangerous. Unlike snow, wet ice provides almost no traction, and vehicles will slide even on gentle slopes. Because freezing rain does not hit the ground as an ice pellet and is still a rain droplet when it makes contact with the ground, the freezing rain conforms to the shape of the ground, making one thick layer of ice, often called glaze. Because sleet is in pellet form it can be easily moved around, unlike freezing rain which is a continuous layer of ice and cannot be moved around.
Freezing rain and glaze measured on a large scale is called an ice storm. Effects on plants can be severe, as they cannot support the weight of the ice. Trees may snap as they are dormant and fragile during winter weather. Pine trees are also victims of ice storms as their needles will catch the ice, but not be able to support the weight.
One particularly severe ice storm struck eastern Canada in 1998; for details see 1998 Ice Storm.
