From AMS Glossary
A layer in the middle or upper troposphere in widespread precipitation in which ice crystals form in small convective cells and fall to lower altitudes.
Such layers are thought to be characterized by convective instability. Small convective cells called generating cells that develop within the layer may produce ice crystals that then settle into lower altitudes. The base of the convectively unstable layer is called the snow-generating level. On time–height displays from vertically pointing radars, generating cells are typically observed to extend about 1–2 km above the snow-generating level. Snow trails, or streamers, are terms used to describe the trails of precipitation that subsequently emerge from the base of the generating cells.