Reliability of the Data

A significant proportion of the climate history sources were created by institutions (e.g. hospitals, ecclesiastical, military or civil authorities). These documented their activities, e.g. in the form of bookkeeping, which was often closely related to agriculture. Climatic-historical information can usually be obtained from the climate- and weather-related timing of such activities. Relevant data in institutional sources accumulate regularly throughout the lifetime of an institution or the maintenance of the corresponding documentation, often over several centuries. They can be easily quantified and statistically linked to instrumental measurements (see Evaluation). However, it is often only possible to obtain estimated values for a period of several months from the corresponding data.

Observations by individuals are sometimes very detailed in terms of time, sometimes even by day (see weather diaries). They include all weather elements that appear necessary to describe an event and may refer to climate impacts, assumed causes and (counter)measures taken by authorities and households. On the other hand, they are subjective in their selection of events and sometimes also in their assessment. They contain gaps and end at the latest with the death of a chronicler. Original records within the lifetime of a chronicler are generally reliable. Transcripts of documents and compilations, i.e. the chronological listing of descriptions of extreme climate anomalies and (natural) disasters from different sources, on the other hand, often contain dating errors. Euro-Climhist therefore makes a strict distinction between contemporary and non-contemporary data.

In most cases, historical documentary data from individuals and institutions complement and support each other. For example, the Schaffhausen chronicler Hans Stockar wrote of the year 1522: "In April, May and June it rained heavily and was cold. In order not to freeze to death from the cold, I had to put on a fur skirt on the holy day of Pentecost [8 June], and some people heated their parlours". This individual source thus contains clear statements about the weather in the months of April, May and June. Whether it was cold or warm, dry or wet in these months also had a significant influence on the harvest dates for grapes and winter cereals. If we combine Stockar's statement with the results on grape and cereal harvest dates, which were compiled on the basis of economic history sources written by institutions (e.g. Wetter, Pfister 2011; Wetter et al. 2013), we arrive at a homogeneous finding: the three months described by Stockar as very cold did indeed considerably delay the cereal harvest and grape harvest in 1522.

Euro-Climhist: Query result for the year 1522
1522-April / very cold / Schaffhausen (SH) / S: Stockar, Chronicle
1522-May / very cold / Schaffhausen (SH) / S: Stockar, Chronicle
1522-June / very cold / Schaffhausen (SH) / S: Stockar, Chronicle
1522-June / fog, variable / Schaffhausen (SH) / S: Stockar, Chronicle
1522-July 28 / Rye harvest begins (209 days after New Year): late / Swiss Plateau / S: Weather, Pfister 2011
1522-October 21 / Grape harvest begins (294 days after New Year): late / Swiss Plateau / S: Wetter et al. 2013

Correct dating poses an additional challenge for climate history research. In 1582, Pope Gregory XIII introduced the Gregorian calendar named after him, which replaced the Julian calendar that had been in use since Roman times. As a solar year is not exactly 365 days and six hours, but a little more than eleven minutes less, there was a shift of around one day per century between the calendar position and the actual position of the sun. Gregory XIII therefore decreed that the days between 4 and 15 October 1582 would be cancelled (and in the future, individual leap years in  1700, 1800, 1900, 2100, etc. would also be cancelled for correction). Due to the then prevailing dispute between the Catholic Church and the Reformed Churches, the Reformed countries, as well as some Catholic and Orthodox countries, did not implement this reform for some time. In the Swiss Confederation, the Gregorian calendar was gradually introduced, first in most Catholic territories and then in most Protestant territories in 1700.

For long phenological series in particular, such as grape harvest, grain harvest or icing dates, all information must therefore be converted to Gregorian dating, including dates before the calendar reform of 1582. However, this has not always been done in research due to a lack of knowledge.

In addition, it is necessary to convert the dates according to saints' feasts, which were common up to the 16th century, into the form of day and month dates that is common today. To do this, knowledge of the methods developed within the framework of historical auxiliary sciences is essential.