Atmospheric transmission in the 2.8-5.5-microm region: description of the Fourier interferometric transmissometer and typical result at low temperatures.

A study of moderate resolution (1 cm(-1)) atmospheric transmission measurements made over a 5.7-km path and under a wide range of temperatures and humidities is presented. A description of the Fourier interferometric transmissometer (FIT) used for this study and a detailed analysis of the experimental protocol developed for that type of instrument are given. Results for the 2.8-5.5-microm spectral window are compared to calculations using the LOWTRANG and FASCOD2 transmission codes. We examine the accuracy with which these codes predict transmittance in spectral domains (1800-2000 and 3200-3500 cm(-1)) strongly affected by water vapor concentration. Preliminary analysis, indicates that although LOWTRAN6 predicts well the summer transmittance (+30.3 degrees C) there are significant divergences for the winter case (-21.4 degrees C). The comparison of FASCOD2 with experimental results shows much closer agreement than LOWTRANG for both summer and winter cases.