Formation

The formation of an ice storm relies entirely on the storms ability to create conditions which produce freezing rain.  These conditions are very precise and if are not just so, the freezing rain will alter its molecular state into something more common, such as snow, sleet or simply rain.

This delicate dance between air temperature and moisture begins with the advancing of moist, warm air in the form of a warm front.  As this warm front smashes into a mass of cold air it produces a classic vertical temperature inversion with warm air above cold.  As the warm air rises over the cold air it begins to cool and condense.  When this condensed air reaches the upper levels of the atmosphere it has now reached temperatures well below freezing (Figure 1), forming crystallized ice which quickly grow into snowflakes.  The snowflakes, too heavy to stay in the atmosphere, fall to earth.  At this point, if the temperature of the air remains consistently below freezing, the snow will fall to earth as snow.  Nevertheless, if there is a warm enough and deep enough layer of air the snow will melt and continue to the ground as rain.  For the raindrops to reach the ground as freezing rain they must be supercooled – where the temperature of a water droplet must drop below 0 C (32 F) without freezing.  This temperature not constant, but is specific for each droplets size and particulate matter content.  The thickness and temperature, as with the warm air mass, is critical in the cold air mass for the formation of freezing rain (Figure 2).  If the cold air mass is too thick or too cold it could alter the rain droplets into sleet or ice pellets.  If the cold air mass is too warm or too thin the rain could continue to the ground as rain, and remain that way unless it comes in contact with a surface temperature well below freezing.For freezing rain to occur, warm air must be lifted, cooled and crystallized into snowflakes. 

These snowflakes then fall to earth, but not before experiencing two more temporal changes.  The flakes will pass through a warm layer of air that will melt them into rain droplets, then finally pass through a cold layer of air that will need to cool the droplets to freezing 0 C (32 F) without freezing them (Figure 3).  Then and only then is freezing rain produced.  This fragile coordination of air temperatures and moisture is critical for the creation of an ice storm.