Daily mean temperature

The daily mean temperature is the single number that best describes how warm a day was. Almost everything on Temperaturkollen is built from it: the curves in the comparison chart, the monthly and annual means and the deviations from the normal.

How SMHI calculates a daily mean

Ideally a daily mean is the average of 24 evenly spaced readings, and SMHI notes that readings every three hours also give a very good mean. Many stations have only been read a few times a day, so SMHI uses the Ekholm–Modén formula for them: a weighted combination of the readings at 07, 13 and 19 with the day's highest and lowest temperature. The weights depend on the month and, more weakly, on longitude. Many countries simply average the maximum and the minimum; SMHI's method lets stations with different reading routines be compared.

Which day does a value belong to?

SMHI publishes the daily mean for the calendar day 00–24 UTC, which is 01–01 Swedish winter time and 02–02 summer time. The daily minimum and maximum are measured over a different window, 18–18 UTC, and are filed under the day on which that window ends. Because the windows differ, a daily mean can now and then be lower than that day's minimum or higher than its maximum. That is not an error, and we never mix the three series.

Today's daily mean does not exist until tomorrow. The comparison chart therefore always ends yesterday at the latest, judged in Swedish time.

Which values count

SMHI marks each value with a quality code. We use both codes that appear in the daily series: G (checked and approved values) and Y (suspect or aggregated values, roughly checked archive data and unchecked real-time data), as SMHI defines them in its data files. On import we discard any value outside −55 °C to +40 °C, and if a day's minimum is higher than its maximum both are discarded. The daily mean is not checked against the minimum and maximum, because they cover different windows.

From days to weeks, months and years

Temperatures are always averaged, never added up. These are our rules:

• 7-day mean: the average of the day and the 6 calendar days before it. It needs at least 5 days with a value, otherwise no value is shown.
• Monthly mean: the average of the month's daily means, judged by coverage. With at least 90% of the days measured, a value is complete. Between 50% and 90% it is shown as partial, with "X of Y days". Below 50% no value is shown.
• For the current month only the days up to the latest observation count, and the value is marked "so far".
• Annual mean: shown only when all 12 months are complete.

SMHI calculates its own monthly means as the average of the unrounded daily means, and its annual mean as the average of the twelve monthly means rather than of all the days. Our annual mean is the average of all valid daily means in a year whose months are all complete, so it can differ slightly from SMHI's figure.

Values are shown to one decimal. A small difference against a figure published by SMHI can come from rounding, or from SMHI having corrected a value after we imported it; the reviewed archive replaces earlier values when it is published.