According to Accuweather, for a snowstorm to be classified a blizzard, it needs winds of at least 35 miles per hour, and blowing or falling snow must reduce visibility to less than a quarter of a mile. If the storm sustains these features for at least three hours, it is officially categorized as a blizzard.
Unofficially, the term blizzard often applies to any heavy snowstorm, especially when snow is already present on the ground. Technically, no new snowfall has to occur to create a blizzard, as high winds can create blizzard conditions by blowing snow that is already present. Extreme cold conditions that prevent fallen snow from melting or solidifying increase the risk of blizzards, as the dry, powdery crystals are more likely to be picked up by the wind and reduce visibility.
Blizzards can be extremely dangerous due to the unpredictable visibility. While a blizzard may restrict visibility to less than a quarter of a mile, in many cases local conditions can reduce visibility even further. This presents a major hazard while driving, and even travelers on foot may become disoriented by the blowing snow. The high winds increase the wind-chill factor, increasing the likelihood of hypothermia or other cold-related injuries.