Nimbostratus cloud to a surface observer. Nimbostratus usually has a thickness of about 2000 to 4000 m. Though found worldwide, nimbostratus occurs more commonly in the middle latitudes. It is coded C2 on the SYNOP report. Nimbostratus occurs along a warm front or occluded front where the slowly rising warm air mass creates nimbostratus along with shallower stratus clouds producing less rain, these clouds being preceded by higher-level clouds such as cirrostratus and altostratus. Often, when an altostratus cloud thickens and descends into lower altitudes, it will become nimbostratus. Nimbostratus, unlike cumulonimbus, is not associated with thunderstorms, however at an unusually unstable
Wall cloud the wall cloud, as it is also an inflow feature. Most movement is horizontal, but some rising motion is often apparent as well. Some wall clouds also have a band of cloud fragments encircling the top of the wall cloud where it meets the ambient cloud base; this feature is a collar cloud. Another accessory cloud is the flumen, commonly known as the beaver's tail. It is formed by the warm, humid inflow of a strong thunderstorm, and is often mistaken for tornadoes. Although the presence of a flumen is associated with tornado risk, the flumen, similar to scud clouds,