During the review of the HadISD paper (see documents on Climate of the Past Discussions) we were asked to quantify how many stations report for how long in the ISD (Integrated Surface Dataset). Our comment that "many stations report only rarely" could have been misleading. We therefore did a quick analysis of the stations in the ISD in July 2012. I've just re-run the code to update to the current status of the ISD, and thought the results might be of wide enough interest to not remain buried on the discussion paper.
Of the 29,678 unique station IDs present in the database (on 20 June 2013), 14,159 report in fewer than 10 years (almost half), almost 18,921 for less than 20 and almost 21,962 for less than 30 years. One station reports in 81 years. The mean length is 18.2 years, but the median is only 11. The distribution is shown in Fig. 1.
|Fig. 1 Number of stations against the number of years they report for. The spike at 40 years is the result of a sudden increase in the number of stations in 1973.|
|Fig. 2 Years with records in the ISD against total file size over all years for each station ID. Created on 20/7/2013|
The file size is not a perfect proxy to use when trying to assess the completeness of a record, but if combined with the number of years in which a station reports, many stations which only report for a few years or containing very little data can easily be excluded from any station selection made.