The level at which a parcel of descending air becomes cooler than its surroundings. On a thermodynamic diagram the level of free sink is given by the point of intersection of the process curve representing the parcel's descent and the sounding curve representing the environment. The descent can be moist- or dry-adiabatic, depending on the availability of water substance to evaporate. From the level of free sink to the point where the parcel either reaches the surface or again becomes warmer than its surroundings represents a region of negative buoyancy and a descending parcel will be accelerated according to parcel theory. Compare level of free convection.