It is lighter in color than nimbostratus and darker than high cirrostratus. Frontal precipitation typically falls out of nimbostratus clouds. Clouds of the genus nimbostratus tend to bring constant precipitation and low visibility. "Nimbostratus" and "Altostratus", which cannot originate from human activity. Fractonimbus may eventually merge completely with overlying nimbostratus clouds. These include nimbostratus, towering cumulus congestus, and cumulonimbus. The top of a nimbostratus deck is usually in the middle level of the troposphere. The next set of clouds are the rain-bearing nimbostratus clouds. Nimbostratus clouds are stratiform layers with sufficient vertical extent to produce significant precipitation. This can also occur on nimbostratus clouds (stratus nimbostratogenitus) and on cumulonimbus clouds (stratus cumulonimbogenitus). Nimbostratus usually produces precipitation over a wide area. However, an advancing nimbostratus cloud could have a diffuse precipitation leading edge. Nimbostratus usually has a thickness of about 2000 to 4000 m. These are the nimbostratus clouds, and the cumulonimbus clouds, and are classified as pannus clouds. Though found worldwide, nimbostratus occurs more commonly in the middle latitudes.