Many times people wonder, "If the temperature is in the 20s, how can it be raining?" For the answer we need to look to the clouds.
When a developing winter storm is approaching an area, the wind at heights of 3,000 to 10,000 feet above the ground are often from the south. This allows mild air, with temperatures as warm as the upper 30s to lower 50s, to move over an area. However, east winds near the ground keep a flow of cooler and drier air moving into the region.
This allows ice crystals (snow flakes) which form higher in the cloud to completely melt into a rain drop several thousand feet above the ground. (See the image above) The temperatures above the ground stay warm enough to keep the raindrop from re-freezing before it hits the ground. Remember, though, that the temperature of the ground and other objects (such as trees, power lines, and cars) is below freezing.
This causes the rain drop to instantly freeze and create layers of ice. This is what we call FREEZING RAIN. Ice accumulations of 1/4" or greater of freezing rain can cause dangerous travel conditions, damage to trees and power lines and extended power outages. The National Weather Service (NWS) issues an ICE STORM WARNING in these cases.
For ice accumulations less than 1/4" the NWS issues a FREEZING RAIN ADVISORY - which means that the weather conditions will be hazardous for travelers and pedestrians on the icy surfaces.