Before revision: A riometer ( relative ionospheric opacity meter ) (30 MHz) is an instrument used to quantify the amount of electromagnetic wave ionospheric absorption in the atmosphere.
After revision: A riometer ( relative ionospheric opacity meter , although originally: Relative Ionospheric Opacity Meter for Extra-terrestrial Emissions of Radio noise) is an instrument used to quantify the amount of electromagnetic-wavecommonly ionospheric absorption in the atmosphere.