For weather measurements to be comparable between different locations, the time (or times) at which observations are made, and the period covered by the measurements, should be common as far as possible. WMO provides guidance on observation times for the main international synoptic observing networks, while ‘climatological’ observing practice tends to be defined at a country or regional level. It is outside the scope of this book to provide detailed guidance on all aspects of standard climatological observing practice for every country in the world, so this chapter outlines common observing routines, based around a once-daily morning observation. Examples based upon current practice within the United States, UK and Ireland are given where these illustrate generally applicable principles. The importance of common time standards and common time period/s for once-daily values, such as maximum temperature or total rainfall, are stated, and the meaning, relevance and importance of ‘terminal hours’ is introduced.

Country-specific details on observing practices, including standard observing hours and ‘terminal hours’, can be found in the websites or publications of the world’s state meteorological services listed on the WMO website [1].