A practical but simplified atmospheric noise model in universal time for 1-MHz Faro usable in estimating system signal-to-noise ratios in microminicomputer-based HF propagation prediction systems is presented. Numerical mapping techniques were used to represent worldwide atmospheric noise in 24 numerical maps: one for each of six 4-hour UT time blocks and each of four 3-month periods. This representation results in a minimum reduction by one fifth in the number of coefficients necessary in computer memory over that of an earlier model in which the map represented an entire 24-hour day. The simpler expression for the median noise levels (Faro) results in significant savings in computer code and time. Several versions were produced, each with a different number of coefficients (number of harmonics) in the Fourier representation for each time block. The number of coefficients ranged from 192 coefficients per time block for the most accurate versions to 35 coefficients for the least accurate. The accuracies were determined by calculating the rms residual between values of Faro used to develop the model and the corresponding values computed from each model. The most accurate model had an average rms error for all times and seasons of 1.3 dB, whereas the least accurate had arms errol of 4.5 dB. During summer, the worst season, the most accurate model had arms error of 1.4 dB, whereas the least accurate had arms error of 5.6 dB. The International Radio Consultative Committee noise data used to generate the models had an average standard deviation of the error at 1 MHz of 5.4 dB and an error of 6.7 dB during summer; the numerical map in use in current HF prediction programs has an average rms error of 1.7 dB and an error of 2.0 dB during summer. The locations of the maximum absolute deviations of the models were confined to a small region in central Africa. several types. The most usual types are of atmospheric, galactic, and man-made origin. Below about 30 MHz, atmospheric noise usually predominates. It may change over wide limits as a function of location, frequency, bandwidth, time of day, season, and azimuthal direction. Although in the presence of local storms, atmospheric noise may be an important factor at almost any frequency, it is the ability of noise from distant thunderstorms to propagate over This paper is not subject to U.S. copyright. Published in 1983 by the American Geophysical Union. Paper number 3S0548. long distances that makes it so important at HF and below. Because ionospheric absorption is high during the daytime, the contribution from distant sources is reduced, and local sources become important. Because of the strength of propagated noise from distant storms at night, the diurnal maximum occurs at night, even for locations in the earth's major source regions. The internationally accepted method of predicting atmospheric noise is outlined by the International Radio Consultative Committee (CCIR) [1964] in CCIR Report 322. The purpose of this paper is to describe the development of a practical but...