File:Twilight photometer measurements of the Raikoke volcano aerosol cloud over Geronimo Creek Observatory, Texas, on 27 July 2019. The blue trace shows the atmospheric profile on a very clear evening nearly the same day a year before.jpg

English: In 2013 I began using twilight photometry of the zenith sky to measure the altitude of aerosol layers. I described my basic instrument in Make magazine at https://makezine.com/projects/twilight-photometer/ To date the atmospheric profiles detected by my twilight instruments include smoke, dust and pollution layers in the troposphere, volcanic aerosols in the stratosphere and meteor smoke from 70 km to 140 km. My instruments are pointed at the zenith sky for an hour after sunset (or before sunrise). They employ no external optics and use standard LEDs as spectrally selective photodiodes. The amplifier is a simple op amp with a feedback resistance of from 10 to 20 gigohms. The amplified signals is stored in one channel of a 4-channel, 16-bit Onset data logger. While most measurements are at Geronimo Creek Observatory in Central Texas, I have also made twilight measurements at Hawaii's Mauna Loa Observatory during my annual visits to calibrate my atmospheric instruments.