Snow is precipitation in the form of flakes of crystalline water ice that falls from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft, white, and fluffy structure, unless subjected to external pressure. Snowflakes come in a variety of sizes and shapes. Types that fall in the form of a ball due to melting and refreezing, rather than a flake, are hail, ice pellets or snow grains. The process of precipitating snow is called snowfall. Snowfall tends to form within regions of upward movement of air around a type of low-pressure system known as an extratropical cyclone. Snow can fall poleward of these systems' associated warm fronts and within their comma head precipitation patterns (called such due to the comma-like shape of the cloud and precipitation pattern around the poleward and west sides of extratropical cyclones). Where relatively warm water bodies are present, for example because of water evaporation from lakes, lake-effect snowfall becomes a concern downwind of the warm lakes within the cold cyclonic flow around the backside of extratropical cyclones. Lake-effect snowfall can be heavy locally. Thundersnow is possible within a cyclone's comma head and within lake effect precipitation bands. In mountainous areas, heavy snow is possible where upslope flow is maximized within windward sides of the terrain at elevation, if the atmosphere is cold enough. Snowfall amount and its related liquid equivalent precipitation amount are measured using a variety of different rain gauges.