A blizzard is a period of strong wind and falling or blowing snow that sharply cuts visibility for hours. The National Weather Service defines it by wind, visibility and duration, not by how much snow falls, so a blizzard can happen with little or no new snow.
The working definition
The NWS standard is sustained wind or frequent gusts of 35 mph (about 30 knots) or more, with falling or blowing snow frequently reducing visibility to less than a quarter mile, for three hours or longer. Those same numbers trigger a Blizzard Warning.
Older definitions also included cold temperatures, around 20°F (-7°C) or lower, and 10°F (-12°C) or lower for a severe blizzard. The current criteria leave temperature out.
Ground blizzards
Some blizzards have no falling snow at all. Wind picks up snow that fell earlier and drives it across the landscape, and this is called a ground blizzard.
Blowing snow can be falling snow, loose snow lifted from the ground, or both, and it can also pile into significant drifts.
Where blizzards happen
In the United States, blizzards are common in the upper Midwest and the Great Plains. They occur in most of the country, except along the Gulf Coast and the California coast.
Forecasters look for two ingredients together: wind strong enough to keep snow moving, and a supply of snow, either falling or lying loose on the ground.
How it relates to nearby hazards and alerts
Ahead of time, the possibility shows up in a Winter Storm Watch, which reads "Blizzard conditions possible" when blizzard conditions are the only concern. Once the criteria are expected, the local office issues a Blizzard Warning.
A snow squall can produce similar whiteouts, but usually for 30 to 60 minutes rather than hours. A heavy snowstorm with light wind is not a blizzard, however deep the snow gets.
The cold that often comes with a blizzard is handled separately. Wind chill alerts now fall under the NWS Extreme Cold products rather than the winter storm products.