The first prerequisite for a blizzard is snowfall. When the jet stream dips to the south, it can carry warm, moisture-laden air northward, where it encounters the cold, dry air that can trigger a winter storm. A significant amount of snow is required for a blizzard, but the snow does not have to fall during the storm. A ground blizzard occurs when sustained winds blow already-fallen snow, reducing visibility and causing a potential whiteout condition without requiring new precipitation.
When warm air rises above a cold air mass, it creates a pressure differential as it cools and falls back down to earth. This difference in pressure can trigger powerful winds, blowing falling snow or redistributing snow already on the ground. In addition to reducing visibility, these powerful winds can create a dangerous wind chill, exacerbating the effect of already-freezing temperatures. Blizzard-scale winds can drop the effective temperature of the air by 20 degrees Celsius (30 degrees Fahrenheit) or more, punishing anyone with exposed skin or inadequate winter gear.
The official National Weather Service definition of a blizzard requires a winter storm with sustained or gusting winds above 56 kilometers per hour (35 miles per hour) and visibilities of less than one-quarter of a mile for at least three hours. If winds reach 72 kilometers per hour (45 miles per hour) and air temperatures drop below negative 12 degrees Celsius (10 degrees Fahrenheit), the storm becomes a severe blizzard, and visibility may drop to near zero at times during these powerful storms.
One particularly powerful type of blizzard forms during a nor’easter. These cyclonic storms strike the Atlantic coast, and their name comes from how the counterclockwise rotation pushes strong winds onto land from the northeast quadrant of the storm. When a nor’easter forms in the depths of winter, the rotation can pull warm, moist air from southern waters, wrapping it around the storm’s eye where it meets the cold, polar air from the north. This clash of temperatures and high moisture levels can create the perfect conditions for a blizzard, and these storms can be as intense as a hurricane. In 1888, a particularly powerful nor’easter blizzard dropped 40 inches of snow throughout New England, paralyzing New York City and killing 400 people.