  Extinction coefficients and sensitivity values in the JHKL band-passes
measured on nearly 200 nights of observing between 1978 and 1992 on the
1.5-meter and the Blanco 4- meter telescopes at Cerro Tololo Inter-American
Observatory are presented and discussed. Analysis of these data shows the
following: There are seasonal variations in both the extinction coefficients
and sensitivity values that are qualitatively consistent with expected
variations in the amount of H2O in the atmosphere - relatively high in the
summer months, lower in the winter months. The linear correlation coefficients
between most of these quantities are statistically significant. The yearly mean
values of these quantities also show significant variability of a few
hundredths of a magnitude. The correlations between these yearly means are
again consistent with variations in the H2O content of the atmosphere. At least
some of these longer term variations are closely correlated with quantitative
measures of the strength of the atmospheric and oceanic El Nin~o/Southern
Oscillation phenomenon.
