Snow - Types

Types

Types of snow can be designated by the shape of the flakes, the rate of accumulation, and the way the snow collects on the ground. Types which fall in the form of a ball due to melting and refreezing cycles, rather than a flake, are known as graupel, with ice pellets and snow pellets as types of graupel associated with wintry precipitation. Once on the ground, snow can be categorized as powdery when fluffy, granular when it begins the cycle of melting and refreezing, and eventually ice once it packs down into a dense drift after multiple melting and refreezing cycles. When powdery, snow drifts with the wind from the location where it originally fell, forming deposits with a depth of several meters in isolated locations. Snow fences are constructed in order to help control snow drifting in the vicinity of roads, to improve highway safety. After attaching to hillsides, blown snow can evolve into a snow slab, which is an avalanche hazard on steep slopes. A frozen equivalent of dew known as hoar frost forms on a snow pack when winds are light and there is ample low-level moisture over the snow pack.

Snowfall's intensity is determined by visibility. When the visibility is over 1 kilometer (0.62 mi), snow is considered light. Moderate snow describes snowfall with visibility restrictions between 0.5 and 1 km. Heavy snowfall describes conditions when visibility is less than 0.5 km. Steady snows of significant intensity are often referred to as "snowstorms". When snow is of variable intensity and short duration, it is described as a "snow shower". The term snow flurry is used to describe the lightest form of a snow shower.

A blizzard is a weather condition involving snow which has varying definitions in different parts of the world. In the United States, a blizzard is occurring when two conditions are met for a period of three hours or more: A sustained wind or frequent gusts to 35 miles per hour (56 km/h), and sufficient snow in the air to reduce visibility to less than 0.4 kilometers (0.25 mi). In Canada and the United Kingdom, the criteria are similar. While heavy snowfall often occurs during blizzard conditions, falling snow is not a requirement, as blowing snow can create a ground blizzard.