The type of precipitation that a winter storm produces is dependent on the vertical temperature profile of the atmosphere over a given area.
This temperature profile can be affected by the advection, or horizontal movement, of air into or out of a region. Warm advection at high levels of the atmosphere and cold advection at lower levels can set the stage for frozen precipitation over a region.
There are several kinds of precipitation that may occur during the winter. They are rain, freezing rain, snow, and sleet.
In order for significant amounts of precipitation to fall, there must be an adequate amount of moisture in the atmosphere. There also must be a way to lift this moisture so that it can turn into precipitation.
Both these conditions are met in the storm systems that commonly form in the Plains or along the East coast in winter, as well as in storm systems that move onshore along the West Coast.
Also, warm advection aloft can carry moisture northward over cold air. This northward moving air can rise and produce heavy precipitation even in the absence of a strong area of low pressure.
Rain is water droplets that form in clouds and fall to the ground. Wintertime rain is generally produced by nimbostratus or stratus clouds.
All precipitation starts out as ice or snow crystals at cloud-level. When this frozen precipitation falls into a layer of sufficiently warmer air (with temperatures above freezing) it melts into rain. If this warm air extends all the way to the surface of the earth, rain will fall at ground-level.
Elevation plays an important role in determining precipitation type. For example, in the Pacific Northwest, cool moist air sweeps in from the ocean along and ahead of a cold front. Cities at low elevations near the coast, like Portland, Oregon, and Seattle, Washington, receive rain.
In the Cascade and Olympic Ranges, however, higher locations will be above the "snow level" - the elevation at which falling ice crystals and snow melt into rain.
Geography is also crucial along the East Coast in determining whether rain or snow will fall at a given location. The Atlantic ocean is a source of moisture - and mild air. When storm systems sweep up the coast, the counterclockwise flow of air around the low pressure center results in an easterly component of wind at low levels of the atmosphere.
Even if the air starts out well below freezing along and near the coast, this easterly wind can advect milder air inland, causing frozen precipitation to change to rain.
Rain droplets that fall into a shallow layer of cold air near the earth's surface can freeze upon contact with the ground, leaving a coating of glaze. This is known as freezing rain.
Freezing rain most often occurs when mild, moist air is advected over a cold polar or Arctic air mass near the Earth's surface. Lower elevations are often vulnerable to ice storms - significant and damaging accumulations of ice - since cold, dense air will naturally settle into lower elevations.
For example, it is quite typical for the Connecticut River Valley in New England to receive an ice storm when cold air in the valley is "overridden" by milder, moist air from the Atlantic.
Freezing rain causes dangerous weather conditions for travelers. Rain can freeze on anything it contacts, including roads. It is extremely difficult to drive on a road glazed over with ice. Bridges and overpasses, which typically freeze quicker than other surfaces, are particularly hazardous to drivers.
Power outages are also common in an ice storm. The weight of ice and freezing rain often causes downed power lines and tree limbs, leaving thousands in the affected area without electricity, sometimes for days at a time.
Snow is frozen precipitation in the form of a six-sided ice crystal. Snow requires temperatures to be below freezing in all or most of the atmosphere from the surface up to cloud level.
Snow can fall when surface temperatures are above freezing in a relatively shallow layer. In situations like this, the snow will not have enough time to melt before reaching the ground - though it will be quite wet with large flakes, the result of wet snowflakes sticking to one another.
Generally, ten inches of snow will melt into one inch of water. Sometimes the snow-liquid ratio may be much higher - on the order of 20:1 or 30:1. This commonly happens when snow falls into a very cold airmass, with temperatures of 20 degrees or less at ground-level.
Sleet is frozen precipitation falling as ice pellets. Ice pellets occur when snowflakes melt into raindrops as they pass through a
thin layer of warmer air. The raindrops then refreeze into particles of ice when they fall into a layer of sub-freezing air near the surface of the earth.
Notice that this is the same type of situation that produces freezing rain. Freezing rain occurs when raindrops fall into subfreezing air that is so shallow the raindrops do not have time to refreeze into ice until they make contact with the ground.
Sleet occurs when raindrops fall into subfreezing air thick enough that the raindrops refreeze into ice before hitting the ground.
Sleet is different from hail. Sleet is a wintertime phenomena; hail falls from convective clouds (usually thunderstorms) under completely different atmospheric conditions - and often during the warm spring and summer months.