The change occurs after dew formation and then the temperature falls below freezing. Heating Degree Day - A form of degree day used to estimate the required energy for heating. One heating degree day occurs for each degree the daily mean temperature is below 65 degrees Fahrenheit. Also issued when snowfall totaling 8 inches or more in 24 hours or less is expected. Ice Crystals - A barely visible crystalline form of ice that has the shape of needles, columns or plates.
Ice crystals are so small that they seem to be suspended in air.
Ice crystals occur at very low temperatures in a stable atmosphere. Ice Fog - Fog composed of ice crystals instead of water droplets. Ice fog forms when clear and calm conditions prevail in extremely cold arctic air. Ice Storm Warning - Issued when damaging accumulations of ice are expected during a freezing rain event. Snow - Precipitation composed of white or translucent ice crystals.