Measurements of downwelling infrared spectral radiance at Mauna Loa, Hawaii during April and May of 1998 indicate unusually high infrared atmospheric transmittance. This is linked to large-scale drying of the upper troposphere and stratosphere in the subtropical Pacific, which appears to be associated with the extreme El Ni¿o-Southern Oscillation (ENSO) event of 1997--1998. During the driest periods, transmission windows opened in the wavenumber range of 500--600 cm-1, a water vapor rotational band that normally has high opacity. These data demonstrate the potential value of long-term mountain-top measurements for monitoring upper tropospheric water vapor and for validating satellite measurements of upper-tropospheric water vapor. ¿ 1999 American Geophysical Union