A blizzard is a severe snowstorm caused by strong sustained winds of at least 56 km/h (35 mph) and lasting for a prolonged period of time – typically three hours or more. A ground blizzard is a weather condition where snow is not falling but loose snow on the ground is lifted and blown by strong winds. The difference between a blizzard and a snowstorm is the strength of the wind, not the amount of snow. To be a blizzard, a snow storm must have sustained winds or frequent gusts that are greater than or equal to 56 km/h (35 mph) with blowing or drifting snow which reduces visibility to 400 m or 0.25 mi or less and must last for a prolonged period of time – typically three hours or more.