The Role of Climate Metadata
If we measure rainfall, in order for the data to be useful for future users, we also need to document where and how the measurements were made. Station documentation is information about the data or data about the data: metadata. The word metadata is made up by the addition of the Greek “meta” (beyond) and the Latin “datum” (a given fact). Metadata should reflect how, where, when, and by whom information was collected. Ideally, complete metadata should register all the changes a station has undergone during its lifetime, composing what is called the station history.
Supplementary information about the observations, such as type of instrument or exposure, can provide additional insights into interpreting the observed quantities. Sometimes when the instruments change, the observations will show an artificial increase or decrease. Such a jump in the measured amount is an example of inhomogeneity and adjustments to these data are often applied to account for the effects of the inhomogeneity. If a long-term time series is homogeneous, then all variability and change are due to the behavior of the atmosphere.
Every user and provider of climate data has to deal with metadata and homogeneity to some extent. Many climate researchers throughout the world have developed effective approaches for dealing with the many aspects of metadata and homogeneity. The following document is based on their collective experience and should and is intended to offer guidance to the NMHSs (National Meteorological and Hydrological Services) on these matters.
Good metadata are needed to ensure that the final data user has no doubt about the conditions in which data have been recorded, gathered, and transmitted, in order to extract accurate conclusions from their analysis. The knowledge of the exact date and time when a thermometer was replaced and the technical characteristics of the new and the old instrument, will surely help to remove the non-climatic fingerprint of this change in that particular temperature record. High quality and homogeneous long-term datasets are needed to assess climate-related issues.
Metadata has a key role in the process of creating such datasets, as the knowledge of the station history provides increased confidence in the statistical techniques employed to ensure that the only variations that remain in a climate time series are due to actual climate variability and change. Meteorological data users other than the climatological community, working in fields like agrometeorology, engineering or aeronautics, also benefit from good metadata. These professionals also need to extract the maximum accuracy from the observations and often compare data taken in different places or times. Complete knowledge of the measuring conditions will help them to achieve this goal.
