A principal cloud type (cloud genus), gray colored and often dark, rendered diffuse by more or less continuously falling rain, snow, sleet, etc. , of the ordinary varieties and not accompanied by lightning, thunder, or hail. In most cases the precipitation reaches the ground, but not necessarily. Nimbostratus is composed of suspended water droplets, sometimes supercooled, and of falling raindrops and/or snow crystals or snowflakes. It occupies a layer of large horizontal and vertical extent. The great density and thickness (usually many thousands of feet) of this cloud prevent observation of the sun; this, plus the absence of small droplets in its lower portion, gives nimbostratus the appearance of dim and uniform lighting from within. It also follows that nimbostratus has no well-defined base, but rather a deep zone of visibility attenuation. Frequently a false base may appear at the level where snow melts into rain. Nimbostratus usually results from the thickening of altostratus to the point where the sun becomes totally indiscernible (Ns altostratomutatus); this point in time usually coincides with the beginning of relatively continuous precipitation. Rarely, it may evolve in like manner from stratocumulus or altocumulus (Ns stratocumulomutatus or Ns altocumulomutatus). Nimbostratus sometimes forms by the spreading of cumulonimbus or cumulus congestus when these clouds produce rainfall (Ns cumulonimbogenitus or Ns cumulogenitus). By definition, nimbostratus is always accompanied by the complementary features praecipitatio or virga. The accessory cloud, pannus, also is a common feature. At first the pannus consists of separate units, but later they may merge into a continuous layer and extend upward into the nimbostratus. Nimbostratus is most easily confused, in identification, with thick masses of altostratus, stratus, or stratocumulus. Altostratus, however, is lighter in color, appears less uniform from below, and does not completely hide the sun. In case of further doubt, a cloud is called nimbostratus if precipitation from it reaches the ground. Stratus also may have precipitation, but only of very small-sized particles. Stratocumulus shows clear relief and a well-defined limit of its base. See cloud classification.