Propagation of aircraft en route noise for flights above 18,000 feet above ground level (AGL) (5.49 km AGL) is not currently included in the FAA's community noise prediction tools, but nevertheless important for estimating noise impact in U.S. National Parks and other quiet areas. One key to the accurate estimation of noise impact is atmospheric absorption. In the atmosphere, the absorption coefficient is dependent on altitude as well as frequency. The altitude dependent parameters include mean pressure, temperature, and relative humidity as well as the concentration of molecular species. This altitude effect can be important for certain frequency bands, and this point will be emphasized in this paper. In addition, an improved atmospheric absorption model of Sutherland and Bass [J. Acoust. Soc. Am., Vol. 115, No. 3, 1012-1032 (2004)] shows different values of atmospheric absorption in critical frequency bands for aviation noise at altitude compared to values determined using the ANSI Standard S1.26-1995 (R 2009). For certain situations, the updated atmospheric absorption coefficient could produce significantly different en route noise predictions on the ground.