Atmospheric convection can occur whenever an air parcel or air layer is heated so that it expands, becomes less dense than its surroundings, and begins to move upward by buoyancy. The large-scale (synoptic) conditions determine the buoyancy structure and stability of the atmosphere, the distribution of moisture, and overall motion patterns of the air, and thereby set the preconditions for the type and intensity of convective motion. Initiation of actual convection occurs on the local scale by small-scale processes, most commonly by local solar heating or terrain-induced lifting of air.