The US weather map is in a real state of flux. Warm conditions in the South but winter in the Plains and Northeast. I a woke to 0 degrees this morning and no heat wave in sight. The Midwest is experiencing ice conditions that remind me of the Great Northeast Ice Storm of 1998 that knocked out power to 75% of Maine residents for up to 14 days. So, here is some insights into what, where and why of ice.
So, What Is Ice?
Simply defined, ice is the solid form of water. Water freezes at 32° Fahrenheit (0° Celsius). But ice won¹t form on its own between 32°F and -40°F. Liquid water and water vapor need specks of dust, dirt, or other debris to serve as a nucleus around which the hexagonal (six-sided) crystals of ice can form. This is why water droplets can be ³supercooled² and remain liquid below 32°F.
Types of Icy Weather
Icy weather can occur in many forms; as snow, sleet, hail, or freezing rain. Ice can deposit on objects as hoar frost, rime, or glaze. Ice fog, or freezing fog, is yet another type ice.
Snow
Snow forms when water vapor in a cloud condenses and creates crystals of ice. Snowflakes can be made up of just one crystal or of many crystals. When the ice crystals grow to be heavier than air, they fall to earth. The largest snowflakes on record occurred during a snowstorm on January 28, 1887, in Ft. Keough, Montana where snowflakes 15 inches in diameter were observed!
Snowflakes, like their parent ice crystals, are six-sided. The beauty, diversity, and complexity of snowflakes are breathtaking.
Sleet and Hail
The biggest confusion people have, when it comes to icy weather, is between hail and sleet, says Matt Noyes, a good friend and meteorologist with New England Cable News (NECN). Hail tends to be observed during the summertime, and occurs during a thunderstorm. A hailstone is basically a ball of layered ice, very much like an onion. If you were to cut a hailstone open you¹d see layers inside much like you’d see if you cut open an onion.
Strong winds lift raindrops upwards, where they freeze, attract more moisture and become ice pellets. This process continues as the pellets come into contact with more raindrops and eventually the ball of ice becomes heavier than the wind can support: that’s when you have a hailstone that finally falls to the ground.
A sleet pellet and notice that instead of a “hail-stone”usually falls during the wintertime. Sleet occurs in the winter when there¹s a warm layer of air above a freezing layer near the ground.
Freezing Rain
Freezing rain is just rain that freezes on contact.
Basically, it¹s warm above your head it may even be warm 10 feet off the ground but the ground temperature is below freezing. You get regular raindrops that fall from the sky, but when they hit the very cold ground, they freeze on contact. Sometimes with freezing rain, the air temperature can be above freezing, but if the ground itself is below freezing, the rain still freezes on contact‹on objects like power lines and tree limbs.² The smooth, icy glaze formed by freezing rain coats trees, bushes, fences, telephone wires, and streets. It¹s beautiful to look at, but it can break tree limbs, down power lines, and create treacherous roads.
Glaze
Glaze is an accumulation of freezing rain. If you have freezing rain that falls long enough, you end up accumulating a glaze on cold surfaces.
Glaze is a smooth, clear, mostly transparent ice that forms as a frozen film on objects. It¹s usually less than an inch thick, but can be much thicker; an ice storm in January 1961 encased wires in northern Idaho in eight inches of glaze.
Ice Storms
Meteorologists issue an ice storm warning if a coating of ice a half inch thick or more is expected. That’s because once the ice builds up that much, there¹s enough stress on wires and tree branches to actually break them. There¹s enough weight from the ice that it can cause things to collapse and you can have structural damage.
While ice storms are most commonly caused by freezing rain, they can also be caused by sleet. Sometimes we see a sleet storm in which the sleet, because it¹s basically ice pellets, collects on the ground almost like balls of ice.That¹s why we see problems with road conditions not only from freezing rain causing a glaze, but also from sleet.
As we are seeing into he Midwest, ice storms can be among the most dangerous and disruptive of storms. Glaze on wires causes electrical, telephone, and cable TV outages. Slip-and-fall injuries are common due to icy steps and sidewalks. Driving is especially hazardous over 85 percent of the fatalities in ice storms result from traffic accidents.
One of the worst ice storms in recent memory took place in 1998, with devastating effects in Quebec, Ontario and New Brunswick, Canada, and the northeastern U.S. The storm caused 56 deaths, primarily from hypothermia or carbon monoxide poisoning, and damages of more than $5.4 billion. Over 4.5 million Canadians were without power, as were 56,000 New Hampshire residents and 80 percent of Maine¹s residents. In places, it took more than three weeks to restore the electricity. In Montreal, traffic lights were out, the subway system closed, and radio and television stations had no broadcast signals. The storm that has blanketed the Midwest will certainly rival the ‘98 storm in terms of intensity and damage.
Ice fog and freezing fog
Fog occurs when clouds form near the ground. Clouds are usually made up of water droplets, but they can also be made of ice crystals, if it¹s very cold. Ice fog forms when droplets of water freeze into ice crystals in midair.
Rime and Hoar Frost
Rime is the often sugary-looking ice that forms on objects as the result of a freezing fog, a phenomenon that regularly occurs on mountaintops. The Mount Washington Observatory in New Hampshire, known as the ³Home of the world¹s worst weather,² has spectacular formations of rime ice, or ³frozen fog,² since freezing fog frequently cloaks the summit. Super cooled water droplets in clouds create the feathery rime‹up to a foot an hour‹on the windward sides of buildings and weather instruments.
How can you see rime ice without taking a snow tractor to the top of Mount Washington? Look for rime formations along the banks of rivers‹on tree branches, for example, as fog often occurs above rivers.
Hoar frost is the ice that grows on solid surfaces from the water vapor in air. If it¹s freezing outside and warm inside, water vapor on a windowpane sublimes directly into ice, forming frost. Frost differs from rime and glaze in that frost develops from the vapor, or gas, phase into a solid (sublimation), and never goes through the liquid phase (condensation). Rime condenses to the liquid phase before it freezes.
Since objects may be colder than the air, frost may appear even when the temperature outside is above freezing.
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by pgeiger
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