1. In meteorology, the condition existing in a given portion of the atmosphere (or other space) when the relative humidity is greater than 100%, that is, when it contains more water vapor than is needed to produce saturation with respect to a plane surface of pure water or pure ice. Such supersaturation does develop because frequently there is no “plane surface of pure water (or ice)” available. In the absence of water surfaces and in the absence of condensation nuclei or any wettable surfaces, phase change from vapor to liquid cannot occur due to the free energy barrier imposed by the surface free energy of the embryonic droplets that would then have to form by spontaneous nucleation. Humid air, purified of all foreign nuclei, can be expanded in cloud chambers to relative humidities of the order of 400% without any condensation taking place. Cloud condensation occurs in our atmosphere at relative humidities near 100% only because nature provides an abundance of condensation nuclei. 2. In physical chemistry, the condition existing in a solution when it contains more solute than is needed to cause saturation. Thermodynamically, this type of supersaturation is closely allied to supersaturation of a vapor since the solute cannot crystallize out in solutions free from impurities or seed crystals of the solute.