Instrument calibrations are both one of the most important, and yet sometimes one of the most neglected, areas of weather measurement. This chapter describes straightforward methods to check and adjust calibrations for the most common meteorological instruments – precipitation (rainfall), temperature, humidity and air pressure sensors. To reduce uncertainty in the measurements themselves, meteorological instruments need to be accurately calibrated, or at least regularly compared against instruments of known calibration to quantify and adjust for any differences, or error. Calibrations can and do drift over time, and therefore instrumental calibrations should be checked regularly, and adjusted if necessary.

The term ‘humidity’ refers to the amount of water vapour in the air. The physics of water vapour is one of the main components of the atmospheric heat engine which produces ‘weather’ and as a result, humidity measurements are an essential requirement for operational meteorological analysis and forecasting, for climate studies, hydrology, agriculture and many other areas of human activity and comfort. In the meteorological context, the terms relative humidity (RH) and dew point (Td) are most often used in specifying atmospheric water vapour content. This chapter sets out how humidity measurements are made, following guidelines laid down by the World Meteorological Organization in the so-called CIMO guide (Commission for Instruments and Methods of Observation), including those from chilled mirror hygrometers, a dry and wet bulb psychrometer and electronic humidity sensors. Limitations of existing methods in some circumstances, such as air temperatures below freezing point, are covered.

In order to provide representative measurements of grass, soil or earth temperatures, thermometers must be deployed in suitable locations or sites and the sensors themselves exposed in a standardised manner. This chapter sets out what those standardised conditions of site and exposure are for measurements of grass, soil or earth temperatures, following the guidelines laid down by the World Meteorological Organization in the so-called CIMO guide (Commission for Instruments and Methods of Observation). As a result of the UN Minamata Convention, traditional mercury-based thermometers have been progressively withdrawn from observation networks, and this chapter considers both their replacement by electronic sensors and impacts of that changeover.

Metadata is literally ‘data about data’. In the context of meteorological records, metadata covers the description of the observation site and its surroundings, the instruments in use and their units together with any significant changes over time, and any other relevant information, such as where the site’s records are archived. Its importance lies in providing essential information for any other user of the records to understand more about the location and characteristics of the data, and therefore enables more informed use of the data. Metadata are especially important for elements which are particularly sensitive to exposure, such as precipitation, wind and temperature, and for professional sites, especially so for long-period records. This chapter sets out what should be recorded, following guidelines laid down by the World Meteorological Organization in the so-called CIMO guide (Commission for Instruments and Methods of Observation).

There are enormous differences in functionality and capability between basic and advanced weather stations. This chapter outlines typical system specifications within broad capability and budget boundaries. When used with the prioritized assessments of functionality from the previous chapter, it will provide clearer guidance regarding the main brands, products and suppliers within the automatic weather station sectors.

Instrumental readings are of course vital when making weather observations, but non-instrumental ‘eye observations’ (such as cloud amounts and types) and brief notes (such as short weather diary entries) help to help build a more complete picture. This chapter sets out how to include these types of record, along with documentation regarding the occurrence of fog, snowfall, thunderstorms and other elements, in a practical and useful series.