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Precipitation typically forms high in the atmosphere where the temperature is below freezing. As ice crystals form aloft and fall toward the surface, they collect each other to form large snowflakes. If ground temperature is above 32 F, the freezing level must be located somewhere above the ground. As the falling snow passes through the freezing level into the warmer air, the flakes melt and collapse into raindrops. During the summer months, it is not uncommon for the freezing level to be found at a level above cloud base. When the air temperature at the ground is less than 32 F, the snowflakes do not melt on the way down and therefore reach the ground as snow. Occasionally, we observe snow reaching the ground even though the outside temperature is above freezing. This occurs when a very thin layer of warm air is found near the surface. In the case of a cold front, a colder, denser air mass lifts the warm, moist air ahead of it. As the air rises, it cools and its moisture condenses to produce clouds and precipitation. Due to the steep slope of a cold front, vigorous rising motion is often produced, leading to the development of showers and occasionally severe thunderstorms. In the case of a warm front, the warm, less dense air rises up and over the colder air ahead of the front. Again, the air cools as it rises and its moisture condenses to produce clouds and precipitation. Warm fronts have a gentler slope and generally move more slowly than cold fronts, so the rising motion along warm fronts is much more gradual. Precipitation that develops in advance of a surface warm front is typically steady and more widespread than precipitation associated with a cold front. Freezing Rain supercooled droplets freezing on impact Ice storms can be the most devastating of winter weather phenomena and are often the cause of automobile accidents, power outages and personal injury. Ice storms result from the accumulation of freezing rain, which is rain that becomes supercooled and freezes upon impact with cold surfaces. Freezing rain is most commonly found in a narrow band on the cold side of a warm front, where surface temperatures are at or just below freezing. The diagram below shows a typical temperature profile for freezing rain with the red line indicating the atmosphere's temperature at any given altitude. The vertical line in the center of the diagram is the freezing line. Temperatures to the left of this line are below freezing, while temperatures to the right are above freezing. Freezing rain develops as falling snow encounters a layer of warm air deep enough for the snow to completely melt and become rain. As the rain continues to fall, it passes through a thin layer of cold air just above the surface and cools to a temperature below freezing. However, the drops themselves do not freeze, a phenomena called supercooling (or forming "supercooled drops"). When the supercooled drops strike the frozen ground (power lines, or tree branches), they instantly freeze, forming a thin film of ice, hence freezing rain. Freezing rain is dangerous because it is almost invisible on smooth surfaces and consequently, people are often unaware of its presence. Sidewalks become extremely slick when covered with freezing rain, increasing the likelihood of someone slipping and injuring themselves. Automobile accidents are more likely to occur during an ice storm because of the icy roads. The weight of ice can damage telephone poles and wires, cutting power and lines of communication to millions of people. During one severe ice storm in New England in 1921, ice that accumulated on the wires between two telephone poles was estimated to weigh over 4 tons. Ice can accumulate up to 4-6 inches deep during the most intense events, forcing businesses to shut down and greatly restricting commuters due to the ice-covered roads. During the 1921 New England ice storm mentioned earlier, Worcester Parks Recreation Department estimated that 7,500 to 8,000 trees were completely destroyed and that an additional 5,000 to 7,000 were going to die from severe damage. Devastation to a forest by an ice storm can be as severe as the damage caused by large tornadoes. Ice storms also have devastating effects on livestock and birds. Grazing areas covered with ice can cause many livestock to slip and fall, while ice build-up on their nostrils can cause them to suffocate. Birds have been found suffocated, their beaks and nostrils having been frozen shut. Birds have also been found frozen to trees or unable to fly due to the weight of the ice on their wings. In most cases, freezing rain results from the process of warm moist air "overrunning" colder air. Perhaps the most common overrunning scenario occurs as warm moist air flows up and over a warm front associated with a midlatitude cyclone. The rising air cools, the water vapor condenses, producing a narrow band of freezing rain ahead of the front. This band is typically less than 50 kilometers (30 miles) wide and is represented by region #1 (shaded in orange) in the diagram below. This band is often wrapped around and behind the low pressure center by counterclockwise winds flowing around the cyclone. Some of the most devastating ice storms occur in association with this narrow band of freezing rain. A second area of freezing rain is typically found behind the cold front, (region #2 shaded in orange in the diagram above). Freezing rain develops as southerly winds at upper levels push warm moist air up and over the cold front, producing precipitation that falls into the colder air. Freezing rain associated with the cold front is usually very light and scattered, and in rare cases, even observed ahead of the front. Forecasting Freezing Rain the importance of temperature profiles Freezing rain is one of the most difficult events to forecast. The smallest variations in temperature (even only tenths of a degree) can mean the difference between rain, freezing rain, sleet or snow. Freezing rain occurs less frequently than other winter weather events and falls in very narrow bands, usually not more than 50 kilometers wide. When attempting to forecast a freezing rain event, sounding data is very useful for examining vertical temperature profiles of the atmosphere, which are indicative of what type of precipitation (if any) will likely occur. There are four types of soundings associated with the four different types of precipitation (mentioned above). In the following diagrams, the blue line represents the temperature profile of the atmosphere and the black line represents the 0C isotherm (a line of equal temperature). When the blue line is to the right of the black line, it means the atmospheric temperature is warmer than 0C, but when the blue line is to the left of the black line, it means the atmospheric temperature is colder than 0 degrees C. The entire temperature profile near the ground is above freezing so all ice particles completely melt and reach the ground as rain A shallow layer of cold air lies below a layer of warmer air, which completely melts all ice particles as they pass through. When the raindrops enter the shallow layer of cold air, they supercool and freeze instantly on contact. The warm layer is very shallow so ice crystals only partially melt as they pass through. Once they enter the cold layer below, they freeze again and strike the ground as ice pellets, or sleet. The entire sounding is completely below freezing so the precipitation reaches the ground as snow. Upper Air Soundings useful when forecasting for freezing rain Soundings are the most important tool for identifying potential freezing rain regimes. Three types of soundings can lead to freezing rain and the most common consists of a shallow layer of cold air at the surface with a depth of about 600 meters (1,800 feet). A Sleet is more difficult to forecast than freezing rain because it develops under more specialized atmospheric conditions. It is very similar to freezing rain in that it causes surfaces to become very slick, but is different because its easily visible.