The distinction between a snowstorm and a blizzard is often unclear, despite meteorologists using precise, technical definitions for these severe winter weather events. While both involve snowfall, the official classification hinges on more than just the amount of snow that falls on the ground. Understanding the difference is important because it dictates the level of danger and the necessary public safety response. Specific criteria involving wind speed, visibility, and duration separate a heavy snow event from a truly life-threatening blizzard.
Characteristics of a Typical Snowstorm
A snowstorm is generally characterized by a significant accumulation of frozen precipitation over a period of time. This type of storm is categorized by the amount of snow expected, which triggers advisories or warnings based on local thresholds. For example, a Winter Storm Warning is often issued when a region anticipates heavy snowfall, sometimes defined as six inches or more within a 12-hour period, or eight inches or more in 24 hours.
While a snowstorm can involve wind, velocity is not the defining factor for its classification. The primary hazard is the volume of snow, which can cause power outages and make travel difficult due to slick conditions and deep drifts. Snowstorms are less severe than blizzards because they lack the sustained, violent wind component that creates whiteout conditions required for a blizzard.
The Three Key Requirements for a Blizzard
A blizzard is not simply a heavy snowstorm; it is defined by a simultaneous combination of three specific weather criteria that must be met for a defined duration. The first requirement is a sustained wind speed or frequent gusts of 35 miles per hour or greater. This wind velocity is enough to violently lift and move snow, regardless of whether it is currently falling or already on the ground.
The second criterion is severely reduced visibility, which must be a quarter mile or less. This condition, often referred to as a whiteout, is caused by falling snow or, more often, by the high winds blowing existing snow, a phenomenon known as a ground blizzard. The third condition is that these intense wind and visibility criteria must persist for a minimum of three hours.
A blizzard does not technically require new snowfall to be occurring. A “ground blizzard” occurs when strong winds pick up snow that has already accumulated, meeting the wind and visibility criteria. The National Weather Service uses these strict, simultaneous measures to distinguish the most severe winter weather events.
Practical Implications of Official Weather Alerts
The difference between a snowstorm and a blizzard has direct, practical consequences for public safety and governmental response, which are communicated through official alerts. A Winter Storm Warning signals a significant weather event that will make travel difficult and possibly cause power issues. This warning encourages caution and preparation for challenging conditions.
A Blizzard Warning is a higher-level alert that signifies life-threatening conditions are imminent or already occurring. The extreme combination of wind and near-zero visibility leads to whiteout conditions, making travel virtually impossible and extremely dangerous. A Blizzard Warning often triggers formal recommendations to avoid all travel and may lead to governmental actions like travel bans, preparation of emergency shelters, and high-alert status for emergency services. The severity of a blizzard necessitates a much more urgent and restrictive public response than a heavy snowstorm.