[al-toh-strey-tuh
s, -strat-uhs] 
| a cloud of a class characterized by a generally uniform gray sheet or layer, lighter in color than nimbostratus and darker than cirrostratus: of medium altitude, about 8000–20,000 ft. (2450–6100 m). |
| al·to·stra·tus (āl'tō-strā'təs, -strāt'əs) n. An extended cloud formation of bluish or gray sheets or layers. [Latin altus, high; see al-2 in Indo-European roots + stratus.] |