A somewhat dated or technical term for a barometer; in modern everyday Japanese, 気圧計 (きあつけい) is more common.
See also: 気圧計 (きあつけい)
In old weather forecasts, barometers were often used.
aneroid barometer
Compound of 晴 (clear weather), 雨 (rain), and 計 (meter), literally a 'clear-rain meter'. The exact historical derivation is uncertain, but the spelling is conventionally associated with measuring atmospheric pressure to predict weather.