The 1991–2020 normal

The current normal period is 1991–2020, the latest completed 30-year period. Temperaturkollen uses it in two forms: the official monthly and annual normals that SMHI publishes, and a daily reference that we calculate ourselves from SMHI's daily means. They are kept apart and labelled differently.

How SMHI calculates the official normals

According to SMHI, a normal is a 30-year mean given for each month and for the year. A monthly temperature is the mean of all daily values in the month, and the annual value is the mean of the twelve monthly values. Before a normal can be calculated for a station, every month of the period must be complete.

Many stations have gaps or started measuring after 1991. For them SMHI estimates the missing months from the relation between the station and PTHBV, a gridded analysis of daily temperature across Sweden from 1961 onwards. SMHI states that the method needs about five years of monthly temperatures to be reliable, and calculates temperature normals for all its stations that have at least that much. A normal for a short series is therefore partly estimated, but it is still SMHI's official normal.

Sweden in 1991–2020

SMHI publishes the 1991–2020 temperature normals as a file with monthly and annual values for about 700 stations. Comparing the normal periods, SMHI finds that the annual mean in southern Sweden went from between 6 and 8 °C in 1961–1990 to between 7 and 9 °C in 1991–2020, and in northern Sweden from between −3 and 0 °C to between −2 and +1 °C. The annual mean has risen in every overlapping 30-year period since 1961, and SMHI notes that the warming in Sweden is about twice as large as the global warming.

Official monthly normals on this site

The official normals are used for whole months, in the month table and in the deviation bars. If a station has no official normal, we use the monthly mean of our daily reference instead and label it "calculated". If neither exists, a dash is shown.

Our daily reference

The comparison chart needs a value for every calendar day, and SMHI does not publish daily normals. We never stretch the official monthly normals into daily values. Instead we calculate a daily reference from the station's own daily means:

• For each calendar day we take the mean of all valid daily means from 1991–2020 that fall within 7 calendar days either side of it. The value is used only if it rests on at least 300 daily values from at least 24 different years. If fewer than 360 of the 366 calendar days meet that requirement, the station gets no normal curve at all.
• The window wraps around the turn of the year, so the reference for 3 January also uses late December. 29 February is a calendar day of its own.
• The reference is always labelled "calculated reference 1991–2020 (SMHI data)".

A station that started measuring after the mid-1990s may have an official monthly normal from SMHI but no daily reference curve here, because our reference needs real measurements from most of the thirty years.