The technical threshold for a blizzard requires surface visibility to be a quarter-mile or less due to snow or blowing snow along with temperatures below 20 degrees Fahrenheit and sustained winds of more than 35 mph for three consecutive hours.
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When a low stratiform cloud contacts the ground, it is called fog if the prevailing surface visibility is less than 1 kilometer, although radiation and advection types of fog tend to form in clear air rather than from stratus layers.
