12 - Observing hours and time standards

Summary

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. By convention, operational weather measurements throughout the world are made to a common time standard, using Coordinated Universal Time (UTC). The World Meteorological Organization (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. This chapter outlines common observing routines, with examples based upon current practice given to 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.