A visible, dense mass of water droplets and/or ice crystals, suspended in the air, and generally forming when air is forced to rise: at a front, over mountains, or because of convection. Clouds mirror atmospheric processes; the approach and passage of a warm front, for example, often follows the sequence: cirrus, cirro-stratus, alto-stratus, nimbo-stratus. At active ana-fronts these clouds may take on a more cumulus form. Atmospheric convection currents are generally indicated by the presence of cumulus or cumulo-nimbus clouds. A cumulus cloud will often form over a heated surface and then shift with the wind, so that further cumulus is formed over the same spot; see cloud street. Turbulence is a common cause of stratus cloud, which is often trapped beneath an inversion; it also creates a nearly continuous sheet of strato-cumulus cloud. See C. D. Ahrens (2000).