Fundamental Quantities and Units

Abstract

The international system of units (système international d’unités, SI) has now largely replaced all earlier systems of units used to describe physical quantities. SI is built upon seven base quantities each having its own dimension: length, mass, time, thermodynamic temperature, amount of substance (chemical amount), electric current, and luminous intensity (Bureau International des Poids et Mesures 1991). All other quantities are derived quantities that acquire dimensions derived ­algebraically from the seven base quantities by multiplication and division. The possibility of combining SI units with prefixes designating decimal multiples or submultiples of units provides additional flexibility when dealing with quantities that range over many orders of magnitude, a feature which makes SI especially suitable for use in the atmospheric sciences. Accordingly, SI units are strongly recommended for use in atmospheric chemistry. However, some non-SI units are still required, and they will remain in use along with SI. Examples are time periods such as minute, hour, day and year, which can be expressed in terms of the second, but which defy decimalization. The tables that follow provide an overview on SI, on units that are not part of SI but are used along with it, and on a variety of non-SI units still in use together with the conversion factors to the corresponding SI units. For the correct treatment of symbols and units the reader should consult the manual Quantities, Units and Symbols in Physical Chemistry by Mills et al. (1993).