least partly made on the spot; the drawings by Fra Bartolomeo also seem clearly sketched from nature. Dürer's finished works seem generally to use invented landscapes, although the spectacular bird's-eye view in his engraving Nemesis shows an actual view in the Alps, with additional elements. Several landscapists are known to have made drawings and watercolour sketches from nature, but the evidence for early oil painting being done outside is limited. The Pre-Raphaelite Brotherhood made special efforts in this direction, but it was not until the introduction of ready-mixed oil paints in tubes in the 1870s, followed by the portable "box easel", that painting en plein air became widely practiced. A curtain of mountains at the back of the landscape is standard in wide Roman views and even more so in Chinese landscapes. Relatively little space is given to the sky in early works in either tradition; the Chinese often used mist or clouds between mountains, and also sometimes show clouds in the sky far earlier than Western artists, who initially mainly use clouds as supports or covers for divine figures or heaven. Both panel paintings and miniatures in manuscripts usually had a patterned or gold "sky" or background above the horizon until about 1400, but frescos by Giotto and other Italian artists had long shown plain blue skies. The single surviving altarpiece from Melchior Broederlam, completed for Champmol in 1399, has a gold sky populated not only by God and angels, but also a flying bird. A coastal scene in the Turin-Milan Hours has a sky overcast with carefully observed clouds. In woodcuts a large blank space can cause the paper to sag during printing, so Dürer and other artists often include clouds or squiggles representing birds to avoid this. The monochrome Chinese tradition has used ink on silk or paper since its inception, with a great emphasis on the individual brushstroke to define the ts'un or "wrinkles" in mountain-sides, and the other features of the landscape. Western watercolour is a more tonal medium, even with underdrawing visible. Related -scapes Traditionally, landscape art depicts the surface of the Earth, but there are other sorts of landscapes, such as moonscapes. Skyscapes or Cloudscapes are depictions of clouds, weatherforms, and atmospheric conditions. Moonscapes show the landscape of a moon. Seascapes depict oceans or beaches. Riverscapes depict rivers or creeks. Cityscapes or townscapes depict cities (urban landscapes). Battle scenes are a subdivision of