Snow and ice will blanket much of the United States this week as a winter storm pounds the South and Northeast. This wintry weather leads us to wonder why we end up with icy roads on one bone-chilling day and snow -- or sleet -- on another?

Most precipitation begins as snow high in the atmosphere, where the temperature stays below 0 degrees Celsius (32 degrees Fahrenheit), the freezing point of water. As that snow falls, the temperature of the air and land determine whether the precipitation remains as snow, melts into rain, melts then refreezes into sleet, or covers the land in a treacherous layer of ice.

If the temperature from cloud to ground stays below zero, snowflakes drift to Earth. However, if a large enough band of warm air lies between the snowstorm and the ground, the snow melts into rain.

PHOTOS: Intense Winter Storm Blasts U.S. States: Photos

The situation gets complicated when a layer of air near the surface is above zero, while the ground remains below freezing, according to the Weather Underground. In this case the high altitude snow melts into rain, then refreezes into a slick layer of ice once the water hits the ground.

NEWS: Holy Snow Rollers! Strange Snowballs Invade US

Meteorologists call this nasty phenomenon freezing rain, while commuters call it misery.

Sleet is another possible fate for the high-altitude snow. Tiny ice pellets form when the airborne snow melts into rain, then refreezes. For sleet to form, the layer of above-freezing air must be either thinner or warmer than in the conditions that create freezing rain.

Photo: iStock