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An anemometer measures air motion. In a cup anemometer, moving air exerts different drag on the two sides of the cups, turning a shaft; a calibration converts its rotation rate into wind speed. A separate directional measurement is needed to recover a horizontal velocity vector. A sonic anemometer instead compares sound travel times along different paths and can operate without rotating parts.^[ref]

The instrument measures the flow at its own location, so obstacles, mounting height and gust averaging affect the result. The reported wind is therefore not simply an intrinsic property of an entire town or atmospheric layer. Comparing observations requires a consistent exposure and a stated sampling procedure.

The Met Office is the United Kingdom's national meteorological service, producing observations, forecasts and climate information. Its scientific work connects instruments and numerical models: measurements constrain the present atmospheric state, while fluid and thermodynamic equations predict its evolution.

A concrete example is wind observation. The Met Office describes how cup anemometers and sonic anemometers measure local flow and why exposure and averaging conventions matter.^[ref] Its descriptions of the North Atlantic Oscillation also distinguish a large-scale circulation index from a deterministic forecast of weather at one site.

A sonic anemometer measures wind from acoustic travel times between transducers. For path length $L$ aligned with velocity component $u$, the idealized transit times are $t_{+}=L/(c+u)$ and $t_{-}=L/(c-u)$, givingThe reciprocal-time difference removes the common sound speed in this model. Several paths recover a velocity vector. Transducer wakes, path averaging and precipitation can affect the measurement, so geometry and calibration matter when using it for turbulent fluxes.