What kind of cloud is this? A roll cloud. These rare long clouds may form near advancing cold fronts. In particular, a downdraft from an advancing storm front can cause moist warm air to rise, cool below its dew point, and so form a cloud. When this happens uniformly along an extended front, a roll cloud may form. Roll clouds may actually have air circulating along the long horizontal axis of the cloud. A roll cloud is not thought to be able to morph into a tornado. Unlike a similar shelf cloud, a roll cloud, a type of Arcus cloud, is completely detached from their parent cumulonimbus cloud. Pictured above, a roll cloud extends far into the distance in 2009 January above Las Olas Beach in Maldonado, Uruguay.
Jody Xiong of DDB China in conjunction with the China Environmental Protection Foundation created this wonderful outdoor campaign to create a subtle visual reminder of the environmental benefits of walking versus driving. Enormous white canvases with a bare tree were placed across 132 crosswalks in 15 Chinese cities. As pedestrians crossed their shoe soles were imprinted with a small amount of green paint, leaving behind a trail of leaf-like footprints. BBD estimated that nearly 3,920,000 people passed through the installations, and the final posters were eventually hung has billboards in several urban locations. Awesome!