The process of initiation of a new phase in a supercooled (for liquid) or supersaturated (for solution or vapor) environment; the initiation of a phase change of a substance to a lower thermodynamic e
nergy state (vapor to liquid condensation, vapor to solid deposition, liquid to solid freezing). In nature, heterogeneous nucleation is the more common where such a change takes place on small particles of different composition and structure. Homogeneous nucleation occurs when the change of state centers upon embryos that exist in the same initial state as the changing substance. In this case, the nucleation system contains only one component, and it is termed homogeneous nucleation. In meteorology, particularly in cloud physics, a number of types of nucleation are of interest. The process by which cloud condensation nuclei initiate the phase change from vapor to liquid is important in all cloud formation problems. The physical nature of freezing nuclei that may be responsible for the conversion of drops of supercooled water into ice crystals is critically important in precipitation theory, as is the clarification of the role of homogeneous nucleation near -40C. Thermodynamically, all nucleation processes involve free energy decrease associated with the bulk phase change and the free energy increase associated with the creation of new interfaces between phases.
The process by which nuclear reactions at very high temperatures and pressures produce the various chemical elements of the periodic table, either in the Big Bang or in stellar interiors.
The positively charged core of an atom about which its electrons orbit. 1. Almost all the mass of an atom resides in its nucleus, the diameter of which is about 10^4 times smaller than that of the ato
m. The nucleus of an (electrically neutral) atom is made up of protons, equal in number to its electrons, and neutrons bound together by nuclear forces. 2. In physical meteorology, a particle of any nature that initiates a phase transition in an environment supersaturated or supercooled with respect to a phase with lower chemical (Gibbs) potential, for example, a solid or liquid particle or gas/vapor bubble in a supercooled/supersaturated environment.
In physical meteorology, a particle of any nature upon which molecules of water or ice accumulate as a result of a phase change to a more condensed state.