Nimbostratus is a low-to-middle altitude cloud that has considerable vertical and horizontal extent and produces precipitation over a wide area. "Nimbo" is from the Latin word "nimbus", which denotes precipitation. It is a multilevel stratiform layer with a diffuse cloud base generally found anywhere from near surface and about 10000 ft (3000 m). This cloud typically forms from altostratus in the middle altitude range, but it tends to thicken into the low altitude range during precipitation. Although usually dark at its base, it often appears illuminated from within to a surface observer.