An old scientific instrument designed to measure or observe weather conditions and atmospheric phenomena in the sky.
From Greek aithrios (αἴθριος) meaning 'of the air or sky' combined with -scope (from skopein, to observe). The term was created to describe tools that help scientists examine the atmosphere.
This is one of those wonderfully obscure old scientific words—people actually tried to build machines to directly 'see' and measure the invisible properties of the air, which is basically what modern weather stations do, just with electricity instead of mechanical parts.
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