Heat/cold degree days and Chill hours: Difference between revisions

Heat/cold degree days and Chill hours (view source)

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→‎CAUTION: Think carefully, about what threshold to use.: Explaining how calculated

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 ===CAUTION: Think carefully, about what threshold to use.===
-If you later change the threshold, only subsequent calculations use the new threshold. All past calculations were based on the old threshold, so values shown on existing NOAA reports for a month or a year with a threshold change will be rubbish (the new threshold is shown on the report, but it has not been applied retrospectively), and need to be manually recalculated making any consequent edits directly on past reports.
+If you later change the threshold, you will have inconsistencies on your reports:
+* The report will quote the new threshold, but only ''after the time'' you changed the threshold, will the minute by minute interrogations of the weather station allow Cumulus to calculate the degree day counts based on the new threshold.
+* For the period from ''when you installed'' Cumulus version 1.9.2 (was available as default version from 5th October 2011, or as beta Build 1004 from 21 July 2011) until day before changing threshold, the integrated figures already stored in [[dayfile.txt]] at the end of the meteorological day are quoted in the report.   These counts were integrated on a minute-by-minute basis using the thresholds that existed on each day, and the totals stored after rollover in the daily log.   It is therefore not possible to affect these totals restrospectively by changing the thresholds.
+* For data which was logged before you installed version 1.9.2 (or a later build if you missed out that version), there are no degree day figures in dayfile.txt. The values for the NOAA report are calculated simply by comparing the average daily temperature with the current threshold settings, so in this case any change in the threshold does apply retrospectively for those days.
-The View menu in Cumulus allows you to generate new NOAA reports for a month (or year).  The new report will use the latest thresholds, and there is an option to save the new report (overwriting any previous one if you use the same name).
+One option is to manually recalculate an approximate value for each day simply by comparing the average daily temperature with the current threshold settings, making any consequent edits directly on past reports.   For each degree (° C or ° F) below/above the threshold of the average daily temperature it is accounted as 1 degree-day.
+The preferred option uses the View menu in Cumulus and the selection to generate new NOAA reports for a month (or year).  The new report will not be consistent with the latest thresholds unless you have ''deleted all the 'pre-threshold change' values'' stored on 'dayfile.txt', but there is an option to save the new report (overwriting any previous one if you use the same name).  Obviously if you have deleted values, your report will not have integrated totals, but simply an approximation based on the average daily temperature.
 == Configuring Chill Hours ==