Emission lines from water vapor as a function of wavelength. The excitation temperature increases from the top to the bottom panels. Different observatories cover different wavelengths; this plot confirms that water lines detected by JWST come from high excitation temperatures (≳1000 K, i.e., hot (or warm) water). A future FIR space observatory would be the most sensitive to cold water vapor. A spectral resolution of R approximately equal to a few thousand appears to be sufficient for detections.