There are two instruments used widely in the synoptic surface network for the measurement of present weather; one is essentially a forward scattering visibility sensor and the other a sensor for detecting the presence of recent precipitation. Output from these two instruments together with measurements of air temperature, relative humidity and rainfall rate are analysed to derive the present weather code that best fits the data.

Linked documentation:
The Met Office Fact-sheet #17 – Weather Observations on Land (see linked documentation) describes the instrumentation further and includes diagrams of the apparatus set-up. For more information on which instruments are used to collect measurements within specific station networks, the Met Office Surface Data Users Guide, describes the meteorological surface data and how it is obtained in the Met Office Database - MIDAS.

The World Meteorological Organisation have a list of the internationally agreed present weather codes.