The variation of the infrared solar spectrum in the region from 2$\cdot $2 to 13 $\mu $m with altitude has been observed during a series of balloon flights. The primary objective of these flights has been to obtain data concerning the transmittance of the Earth's atmosphere in this wavelength region particularly at altitudes from 13 to 31 km. The transmittance observed during these flights are compared with the transmittances calculated using a line-by-line integration technique. It is shown that it is possible to get good agreement between the observed and calculated results provided the distribution of the absorbing molecule is known. In addition to the flights during which only relative data were obtained, two flights were made during which absolute spectral data were obtained. From these data the spectral radiance of the Sun has been determined without the complication of correcting for atmospheric effects. The results are in good agreement with the results of other investigators in the wavelength regions where such measurements have been made from the ground.