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 As a result of the UN Minamata Convention, traditional mercury-based thermometers have been progressively withdrawn from observation networks, and this chapter considers both their replacement by electronic sensors and impacts of that changeover.

In order to provide representative measurements of wind speed and direction, measuring devices must be deployed in suitable locations or sites and the instruments themselves exposed to the wind they are intended to measure in a standardised manner. This chapter sets out what those standardised conditions of site and exposure are for measurements of wind speed and direction, including the treatment of wind gusts and construction of the vector mean wind, following the guidelines laid down by the World Meteorological Organization in the so-called CIMO guide (Commission for Instruments and Methods of Observation). Well-exposed sites can be difficult to find, and optimal exposure conditions for wind instruments differ considerably from most other meteorological instruments. Instruments to measure both wind speed (cup and sonic anemometers) and wind direction (wind vanes and sonic anemometers) are covered, with their limitations, together with a description of the Beaufort wind scale, which is useful if suitable instruments (or a suitable site for them) are not available.

In order to provide representative measurements of solar radiation and sunshine duration, measuring devices must be deployed in suitable locations or sites and the instruments themselves exposed to the sky in a standardised manner. This chapter sets out what those standardised conditions of site and exposure are for measurements of solar radiation (direct, global and diffuse solar radiation) and sunshine duration, following guidelines laid down by the World Meteorological Organization in the so-called CIMO guide (Commission for Instruments and Methods of Observation). Suitably exposed sites can be difficult to find, and unlike other meteorological instruments a mast or rooftop exposure may be ideal. Various instruments have been developed to measure sunshine over the past two centuries, including a variety of recent electronic sensors, but exact agreement between different sensors with varying methods of operation has proved problematic, and the implications for long-period sunshine records are discussed.

In order to provide representative measurements of air temperature, thermometers must be deployed in suitable locations or sites and the sensors themselves exposed to the weather conditions they are intended to measure in a standardised manner. This chapter sets out what those standardised conditions of site and exposure are for measurements of air temperature, 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. Alternative methods to measuring air temperature in the traditional Stevenson screen or Cotton Region Shelter, particularly aspirated temperature measurements, are also covered in detail.

In order to provide representative measurements of precipitation (rainfall, snow and hail, drizzle, sleet and so on), measuring devices must be deployed in suitable locations or sites and the instruments themselves exposed to the weather conditions they are intended to measure in a standardised manner. This chapter sets out what those standardised conditions of site and exposure are for measurements of precipitation, following the guidelines laid down by the World Meteorological Organization in the so-called CIMO guide (Commission for Instruments and Methods of Observation). Both manual and automated (recording) raingauge measurements are covered in detail, including tipping bucket, ground flush or pit gauges and weighing gauges, together with methods to decrease losses due to wind. Snowfall measurement methods are also covered.