Most astronomical observations are conducted by measuring photons (electromagnetic waves) which originate beyond the sky.
The molecules in the Earth's atmosphere, however, absorb and emit their own light, especially in the visible and near-IR portion of the spectrum, and any ground-based observation is subject to contamination from these telluric (earth-originating) sources.
Contamination by water vapor was particularly pronounced in the Mount Wilson solar Doppler measurements.
[1] Many scientific telescopes have spectrographs, which measure photons as a function of wavelength or frequency, with typical resolution on the order of a nanometer of visible light.
Unless they are corrected for, telluric contamination can produce errors or reduce precision in such data.