Absolutely knocked off my axis finding out about a 19th century meteorological instrument called the “rain band spectrometer”, which is exactly what you might guess: a spectrometer designed to focus on the wavelengths of visible light which water vapor readily absorbs. And they work! If you point one at the sky and see distinct dark bands around two specific wavelengths (roughly 0.59μm green-yellow and 0.67μm deep red), there’s a lot of water vapor in the column overhead absorbing that light. They never caught on because there was, at the time, no real way of objectively recording these observations—their usefulness was limited almost exclusively to warning of storms on those occasions when air aloft had become “deeply” (throughout a great range of altitudes) wet without influencing the ground level. This basic observation is the basis for satellite observations of water vapor (using sensors tuned to absorption lines in the infrared rather than visible spectrum)—but I think we should bring these little guys back, we should all carry one in our pocket so that whenever it’s time to do some small talk about the weather we can pull out our spectroscopes and argue about how dark the absorption lines are.
You can find out more in Jill Austin’s "A Forgotten Meteorological Instrument: The Rainband Spectroscope”, which is behind a paywall but can be found through other methods here undisclosed. Photo from here of the spectrometer once used at the Blue Hill Observatory outside of the Boston metro area.
