Sonic boom minimization exploits the shaping of nonasymptotic ‘‘mid-field’’ signatures of large aircraft [F. E. McLean, NASA TN D-2877 (1965)]. This is assisted by the ‘‘freezing’’ effect of downward propagation in the real atmosphere [W. D. Hayes and H. L. Runyan, Jr., J. Acoust. Soc. Am. 51, 695–701 (1971)]. Cleveland and Blackstock [J. Acoust. Soc. Am. 92, 2331 (A) (1992)] have recently shown that true freezing occurs for flight altitudes in excess of 80 000 ft. Recent studies of boom minimization, however, show greater success at much lower altitudes, and that shaping is increasingly more difficult at higher flight altitudes. Parametric analysis of the age parameter with regard to altitude and distance shows that successful minimized booms are not close to asymptotic freezing, and that ‘‘freezing’’ is not necessarily the best perspective. There is concern that increasing flight altitude as fuel load decreases might put a low-boom aircraft design into an off-design condition of higher boom. It is shown that, for realistic flight profiles, the increased age parameter is more than offset by reduced weight, so that sonic boom loudness will decrease throughout supersonic cruise.