Radiative cooling is an important ingredient in hydrodynamical models invol
ving evolution of high-temperature plasmas. Unfortunately, calculating an a
ccurate cooling coefficient generally requires the solution of over 100 dif
ferential equations to follow the ionization. We discuss here a simple two-
parameter approximation for the cooling coefficient due to elements heavier
than H and He, for the temperature range T = 10(4)-10(8) K. Tests of the m
ethod show that it successfully tracks the ionization level in severe dynam
ical environments and accurately approximates the nonequilibrium cooling co
efficient of the trace elements, usually to within 10% in all cases for whi
ch cooling is actually important. The error is large only when the temperat
ure is dropping so rapidly due to expansion that radiative cooling is negli
gible, but even in this situation, the ionization level is followed suffici
ently accurately. The current approximation is fully implemented in publicl
y available FORTRAN code. A second paper will discuss general approaches to
approximation methods of this type, other realizations that could be even
more accurate, and the potential for extension to calculations of nonequili
brium spectra.