1. In general, a broad cloud formation that is not more than four times as long as it is wide. 2. In synoptic meteorology, the principal cloud structure of a typical wave cyclone, that is, the cloud forms found on the cold-air side of the frontal system. The maximum areal coverage is usually found over the region in advance of the warm front, and the minimum behind the cold front. Within the area of the cloud shield, there is an idealized but smaller precipitation shield.