Stratospheric air temperature (radiance) fluctuations over the Andes observed by the Upper Atmosphere Research Satellite (UARS) Microwave Limb Sounder (MLS) are presented. The MLS radiance variances show strong annual variability over the Andes mountains in South America, which is closely correlated to the background wind conditions associated with mountain wave generation and propagation. The variances are significantly larger in southern hemispheric winter when the winds in the troposphere and stratosphere over the Andes are both westerly and mountain wave critical levels (zero-wind lines) are absent. The annual variation of MLS radiance variance agrees well with data from radiosondes and output from the Naval Research Laboratory (NRL) Mountain Wave Forecast Model (MWFM) over the same region and period. The amplitude of the radiance variances seems to correlate well with the intensity of surface wind at upstream positions westward of the Andes, which is also related to the meridional temperature gradient in the region. Horizontal scale analysis suggests that mountain waves over the Andes might have two preferential horizontal wavelengths at ~110 and ~400 km.