SI units to he used in AMS journals 1. Introduction guidelines for its application will be helpful to all in Usage of the International System of Units (Le Systeme making the changeover with a minimum of confusion. International—SI) will become standard in all AMS The purpose of the remainder of this note is to pro- publications according to a decision by the AMS Publi- vide an interpretation of the AMS policy and guide- cations Commission at its meeting on 10 October 1973. lines on usage of SI (section 2), information on the The editors of the journals are planning to carry out major features of the SI (section 3), a list of permissible this new policy beginning with the 1975 issues. Because non-SI units (section 4), and some special considerations manuscripts received during the remainder of 1974 are on usage of units in AMS publications (second 5). unlikely to be published until 1975, the editors urge 2. AMS policy and guidelines authors to use SI units in all manuscripts submitted henceforth. If manuscripts have to be returned to the The AMS policy is that SI will be the standard for usage authors for conversion of units to SI, publication may in all AMS publications. Departures from this standard be delayed. should be made only for deliberate and good cause. For example, exception may be justified under cir- With the rapid increase in interdisciplinary and cumstances where: 1) the use of non-SI units is multi-national uses of atmospheric and oceanic science deemed by the editors to be essential for effective com- information, now seems to be an appropriate time for munication with the intended audiences for the publi- the AMS to join the many national and most foreign cations; 2) widespread professional practice or inter- scientific publishers already using this well-established national agreement dictates the use of non-SI units for international metric standard as an aide to communica- several quantities; and 3) data used solely for descrip- tion. Among the reasons for adopting the SI system of tive purposes—e.g., description of arrangements, non- metric units are: critical dimensions, and shapes of apparatus—are ex- 1) SI, as a recognized international standard already pressed in non-SI units. The AMS does not intend that widely adopted and used in scientific work through- these occasional exceptions will be used as a basis for out the world, seems to be the only system of units perpetuating non-SI usages. upon which all fields of science can agree and thus The following sections on SI units, permissible non-SI join in uniform practice. units, and special considerations provide further guide- 2) SI units are well defined and documented in readily lines for carrying out the AMS policy. available publications and guidelines of such well known standards groups as the International 3. The international system of units Organization for Standardization, the International SI units are somewhat arbitrarily divided into three Bureau of Weights and Measures, the National classes: 1) base units, 2) derived units, and 3) supple- Physical Laboratory (UK), the National Bureau of mentary units. Seven well defined units, regarded as Standards, and the American National Standards TABLE 1. SI Base units. Institute. 3) SI units form a coherent set of units; use of the Quantity Name Symbol units in a physical equation ensures a computed result that will also be in the system. length meter* m mass kilogram kg Because most authors and readers of papers in AMS's time second s journals are scientists who, through training, reading, electric current ampere A thermodynamic temperature** kelvin K publishing, and other professional practices, are thor- amount of substance mole mol oughly familiar with metric units, the adoption of SI luminous intensity candela cd immediately, instead of setting a lengthy transition period, is not expected to cause any major difficulty. * In some guides to SI usage, the spelling "metre" is specified, Nevertheless, because the system has some flexibility and but the AMS editors prefer "meter"; see item 7 of Special Con- siderations (section 5). permits some choices and exceptions to standard usage, ** With regard to use of the Celsius degree (°C) of temperature, an interpretation of the new AMS policy and some see Special Considerations (section 5). 926 Vol. 55, No. <9, August 1974 Unauthenticated | Downloaded 10/05/21 09:11 PM UTC Bulletin American Meteorological Society dimensionally independent, were chosen as base units. linking the corresponding quantities. Several of these These are listed with their names and symbols in algebraic expressions of derived units can be replaced by Table 1. The definitions of the base units, as defined by special names and symbols which can themselves be international convention, are given in several of the used to form other derived units. Some of the derived references (e.g., Page and Vigoureux, 1972). The gen- units, with their names and symbols expressed in terms eral principle of type selection for printing the unit of base units and special names (if any), are given in symbol is: Roman type, in general lower case, is used for Table 2. The special names given in the table usually symbols of units; if however, the symbols are derived will be preferred in place of derived units expressed in from proper names, capital roman type is used. Unit terms of the basic units. Although derived units can be symbols do not change in the plural, nor are they fol- expressed in several equivalent ways by using names of lowed by a period. base units and special names of derived units, certain Derived units are units that can be formed by com- combinations or special names are preferred for some bining base units according to the algebraic relations quantities. For example, the hertz is often used in prefer- TABLE 2. Examples of SI derived units. Here and in Tables 4 and 8, the raised dot is used to indicate the product of units, and negative powers to indicate division. However, a space may be used instead of a dot and a solidus (/) may be used for division if no ambiguity results. Symbol For In terms of SI special In terms of Quantity Name base units name other units 2 area square meter m — — 3 volume cubic meter m — — _1 speed, velocity meter per second m-s — — 2 acceleration meter per second squared m-s" — — 1 divergence per second s" — — 1 vorticity per second s" — — 1 wavenumber 1 per meter m" — — 2 -2 geopotential; dynamic height meter squared per second squared m • s — — 3 density kilogram per cubic meter kg • m~ — — 3 1 specific volume cubic meter per kilogram m • kg" — — -2 luminance candela per square meter cd • m — — 1 frequency hertz s- Hz — -2 force newton m • kg • s N -- 2 pressure pascal m-i. kg. s-2 Pa N-m- energy joule m2 • kg • s-2 N-m 2 3 J power watt m • kg • s~ W J-s"1 electric charge coulomb s-A C As 2 3 -1 1 electric potential volt m • kg • s~ • A V WA" 2 -1 4 2 1 capacitance farad m~ • kg • s • A F C-V" 2 -3 -2 1 electric resistance ohm m • kg • s • A n V-A" 2 -1 3 2 1 conductance siemens m~ • kg • s • A s A-V" 2 2 -1 magnetic flux weber m • kg • s~ • A Wb V-s 2 -1 -2 magnetic flux density tesla kg • s~ • A T Wb • m 2 2 -1 1 inductance henry m • kg • s~ • A H Wb-A" lumen cd-sr lm — luminous flux -2 illuminance lux m - cd-sr lx — -i. kg. -i pascal second m s — Pa-s dynamic viscosity 2 2 meter newton m - kg • s~ — N-m moment of force 2 1 surface tension newtcn per meter kg • s~ — N-m" 3 2 watt per square meter kg-s~ — W-m- heat flux density 2 _2 _1 entropy joule per kelvin m -kg-s -K — J-K-i 2 -2 -1 gas constant, universal joule per kelvin m • kg • s • K — J-K-i 2. -2. -i -1 _1 joule per kilogram kelvin m s K — J • kg • K specific heat capacity 2 2 1 joule per kilogram m -s" — J-kg" specific energy 3 -1 -1 -1 thermal conductivity watt per meter kelvin m • kg • s~~ • K — W • m • K -1 2 3 joule per cubic meter m • kg • s~ — J-m- energy density 3 -1 1 electric field strength volt per meter m • kg • s~ • A — V-nr 3 3 coulomb per cubic meter m~ -s-A — C-m~~ electric charge density 2 2 coulomb per square meter m~ -s-A — C-m~ electric flux density 2 ampere per square meter A-m~ — — current density 1 ampere per meter A-m" — — magnetic field strength -2 -2 1 henry per meter m • kg • s • A — H-nr permeability 2 -2 -1 1 joule per mole m •kg • s • mol — J-mol" molar energy 2 -2 -1 -1 -1 -1 molar entropy joule per mole kel vin m •kg• s • K • mol J • mol • K 927 Unauthenticated | Downloaded 10/05/21 09:11 PM UTC Vol. 55, No. 8, August 1974 TABLE 3. SI supplementary units. —and preferably chosen so that the exponent of 10 is divisible by 3 (i.e., . , 10"6, 10~3, 103, 106, . .). How- Quantity Name Symbol ever, it is generally better to use the same prefix for all plane angle radian rad values of a quantity that appear in the same context, solid angle steradian sr for example in a table, even if some fall outside the range 0.1 to 1000.