Winter storms: snowstorms, ice storms, and blizzards
The winter has its share of severe weather, just as the summer does, and although this is often rather less spectacular, it can have a long-lasting impact on local conditions and human activity as well. Anyone who has been forced to postpone a journey by a blizzard -- or has slid off the road during one and been forced to wait for hours for help to get back on the highway, who has found their driveway blocked by deep drifts of snow, or who has lost their power for hours or days on end thanks to ice dragging trees over onto the power lines has experienced winter's version of severe weather.
Tornadoes, hail, lightning, and damaging winds can all happen in the winter as well as the summer, but they are much more rare in the winter season. This is because winter air is more stable than summer air -- there is always less moisture, meaning that condensation is less intense, and heat is nowhere near as high, limiting the power of updrafts which are needed to create the most violent kinds of weather.
Most of winter's weather phenomena -- ordinary or severe -- come in the form of snow or ice. Snow storms, blizzards, and ice storms are the three most common kinds of winter weather, and of the two, blizzards and ice storms are the most likely to be severe.
Snowstorms are the ordinary storms of the winter season, equivalent to a summer rainstorm and often occurring, as summer rains do, along frontal boundaries. Snow accumulates during the course of a snowstorm and can add up to anywhere from a light dusting to several feet of new snow, depending on the length of the storm and the speed at which the snow falls.
Snow forms in clouds where the air has reached saturation, meaning that the air is no longer able to hold the moisture it contains as water vapor and this condenses out into water droplets. The droplets, in the case of snow, freeze into ice crystals, and other droplets then freeze onto them, building up a delicate lattice of ice crystals that becomes a snowflake. Snowflakes are six-sided because ice crystals are naturally hexagonal, and the crystals tend to build at the corners of the 'core crystal' most heavily, growing into the branching arms of a snowflake.
Snowflakes are light and airy, although they can fall with a slight hissing sound when they are numerous enough or fall into long, dry grass. Due to this 'fluffy' character, 13 inches of snow is equivalent to 1 inch of rain. From this, it can be seen that most snowstorms have fairly light precipitation compared to rainstorms, which is due to the dry air of winter. A snowstorm can last anywhere from a few minutes (when it is either a flurry or a squall, depending on whether the snow is light or heavy) to several days, depending on weather conditions aloft.
A blizzard is a snowstorm whose snowfall and winds are intense enough to make it severe weather that has the potential to threaten damage and possibly cost lives. A blizzard will almost certainly impede or halt travel in the area it passes through until the snowfall can be cleared from roads by plow trucks, graders, and other snow clearing equipment.
Officially, a blizzard is a snowstorm with steady winds of 35 miles per hour or more, and enough blowing snow to reduce visibility to less than a quarter mile. If a snowstorm like this lasts less than three hours, it is an exceptionally heavy squall. If it lasts 3 hours or more, then it is considered a blizzard by American meteorologists. Other countries have different criteria to determine if a snowstorm is actually a blizzard.
Blizzards are dangerous because they block roads, cause vehicles to slide off into ditches or run into one another because drivers can't see other cars or trucks ahead, and bring dangerous wind-chills. People who are stranded by a blizzard can easily freeze to death, especially if they abandon their car, which both gives them shelter from the outside conditions and makes it far easier for rescuers to find them, since it is a large object that can't wander away from the road.
Blizzards also shut down emergency services, since fire trucks, ambulances, and the police may be simply unable to reach a location where they are needed. A blizzard may interfere with travel and normal life for days after it has passed, until the deep snow has been properly cleared.
Ice storms produce conditions of glittering, magical beauty, when the storm has passed and the sun sparkles on a world that has been coated with a thick layer of clear, crystal-like ice. They are also one of the more destructive types of severe winter weather, since the coating of ice is heavy and can break power lines, branches, and even whole trees on occasion. It can also make the ground surface slick, causing cars to slide out of control and crash, or people to fall and possibly injure themselves. Since it often downs power lines, an ice storm can cut electrical power to thousands of people -- many of whom need electricity to keep warm.
Ice storms occur when a layer of warm air is sandwiched between a cold storm aloft and cold air near the ground. Snow falling from the clouds melts into rain in the warm air layer, then becomes supercooled in the layer of cold air -- meaning that it is below the freezing point of water (32 F) but is still liquid. A peculiarity of supercooled water is that as soon as it touches an object, it instantly freezes. Thus, when the rain touches the ground, trees, or power lines, it adheres to them as ice.
Ice can be a thin glazing, or it can build up to be an inch or more thick. The thickest ice accumulation known in the United States from a single storm is 8 inches. This ice is dense and heavy, and can easily snap weak branches or bring down entire trees. If the ice storm covers a large area -- and many of them do -- then thousands or even millions of people can lose their electric power and the cleanup and repair effort after the storm becomes colossal.
Since warm air is needed for ice storm formation, these severe weather events tend to happen in the early or late winter when warmer air is present in the atmosphere. However, they can happen at any time of year in the southern part of the temperate zone, where temperatures never fall far enough to eliminate the chance of ice.