1802, from barometer + -ic. Barometrical is recorded from 1660s.
1660s, from Greek baros "weight" (from barys "heavy;" see grave (adj.)) + -meter. Probably coined (and certainly popularized) by English scientist Robert Boyle (1627-1691).
An instrument that measures atmospheric pressure.
Note: In general, when the barometer falls in response to a drop in pressure, bad weather is approaching; when the barometer rises because of an increase in pressure, good weather will follow.