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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.

A cup anemometer infers wind speed from the rotation rate of cups mounted around a vertical axis. The cup geometry produces different aerodynamic forces on the two sides of the rotor, giving a net driving torque.

A calibration relates rotation frequency to flow speed over the useful range. Bearing friction affects low speeds, and rotor inertia limits response to rapid fluctuations. The cup speed is not simply equal to wind speed. Comparing the output with a reference flow establishes the transfer relation needed for quantitative observations.

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.