When the weather outside is frightful during winter, that may mean a blizzard is blasting a region. The National Weather Service defines a blizzard as a storm featuring large amounts of snow, or blowing snow, with winds of more than 35 mph, and visibility of less than a quarter of a mile for at least three hours. Blizzards often develop from the difference in high and low pressure systems. This is called a tight pressure gradient, which in turn produces high winds that generate snowdrifts and blowing snow that reduces visibility.
Blizzard activity in the U.S. has steadily increased in frequency over the last several decades. According to a recent study published in the January 2017 issue of the Journal of Applied Meteorology and Climate, the number of blizzards has climbed to about 22 as of 2014 from an average of six per season in 1959. The study also found that the area with the most blizzard activity is the northern Great Plains. North Dakota, sections of northern South Dakota and northwestern Minnesota have a more than a 60% chance of getting at least one blizzard a year.
Unlike hurricanes and earthquakes, there is no widely used index for assessing the impact of snowstorms. In recent years, however, the meteorological community has made several successful attempts to establish a standard for measuring the impact of extreme snow events and their historical importance. The Regional Snowfall Index, introduced in 2014, ranks snowstorm impacts on a scale of 1 to 5 using data on a storm’s area of snowfall, the amount of snowfall, and the number of people living in the storm. The RSI has since been used to retroactively classify nearly 600 snowstorms that occurred between 1900 and 2013.
To determine the worst blizzards of all time, 24/7 Wall St. ranked snowstorms based on their Regional Snowfall Index values, published by the National Oceanic and Atmospheric Administration’s National Centers for Environmental Information. We included the 25 snowstorms designated as Category 5 since 1900. Data on duration, region, affected area, and affected population also came from the NOAA. Data on affected area and population for Category 5 storms that spanned multiple regions were combined and considered one event. Storm names, as well as measures of snowfall in the affected areas, came from various news and media sources.