Illinois State Water Survey - University of Illinois at Urbana-Champaign

State Climatologist Office for Illinois

Glossary of Winter Weather Terms

Dr. Jim Angel, State Climatologist

Here are the meanings of some common terms used by the National Weather Service in referring to winter weather.

Heavy Snow - refers to either the intensity or amount of snow. When used in reference to intensity, heavy snow is that which reduces visibility to less than 5/16 statute mile (about 1/4 mile). The National Weather Service issues a heavy snow warning if amounts of 6 or more inches are expected in a 12-hour period, or 8 or more inches in a 24-hour period. In areas where this amount of snow is infrequent, a snowfall of 2 or 3 inches would qualify as heavy snow.

Blizzard - a winter storm characterized by winds of at least 35 mph and accompanied by considerable falling and/or blowing snow that reduces visibilities to less than a 1/4 mile. These conditions must prevail for 3 or more hours.  

Wind Chill Index - the effective temperature on the human body of the combination of air temperature and wind, also called wind chill temperature. It is an approximation only for the human body, and has no meaning for cars, buildings, or other objects. See also the Frequently Asked Questions on windchill.

Wind Chill Warning - wind chill values drop to -50 degrees or below.

Wind Chill Advisory - wind chill values drop to between -30 and -49 degrees.

Snow Flurries - snow falling for short durations at intermittent periods, and resulting in generally small or no accumulation. Flurries may at times be heavy enough to greatly reduce visibility for brief periods.

Snow Squall - a brief, intense fall of snow of a showery nature, usually accompanied by gusty surface winds.

Blowing Snow - visibility is intermittently 1/4 mile or less with sustained winds of 25 to 30 mph.

Drifting Snow - strong winds blowing snow that is falling or loose snow into significant drifts.

Freezing Rain - rain falling in a liquid form through a shallow below-freezing layer of air near the ground. The rain (or drizzle) freezes on impact with the ground or other objects, resulting in a clear coat of ice known as glaze.

Ice Storm - freezing rain resulting in a substantial accumulation of ice, usually 1/4 inch or more.

Sleet - also called ice pellets. Sleet is formed when raindrops or melted snowflakes freeze as they pass through a below-freezing layer of air near the earth's surface. Sleet does not normally stick to trees and wires, and usually bounces on hitting the ground. An accumulation of sleet often has the consistency of dry sand.

Cold Wave - a rapid fall of temperature in a 24-hour period, usually signifying the beginning of a period of very cold weather.