How does an ice storm form?

Answer 1

http://snrs.unl.edu/amet451/bartlett/icestorm.htm
The url above has a nice illustration of the conditions that give rain, ice, sleet, and snow. An ice storm is freezing rain.
A layer of air above the ground is warm enough that falling water is in the form of rain instead of snow or hail. However, before the rain hits the ground, it goes through a cold layer of air just above the surface of the ground. The raindrops are rapidly cooled, actually to below freezing, and when they hit the ground or anything solid, they freeze, instantly, forming a thin film of ice.