How comparisons work

The comparison chart on every place page sets a main year against a comparison year and the 1991–2020 normal. A comparison is only fair if both sides are measured the same way, so the rules are strict. This guide lists them.

Calendar dates, not weekdays

Days are compared by calendar date: 14 March against 14 March. The horizontal axis has 366 places, from 1 January to 31 December including 29 February. A year that is not a leap year simply has no 29 February. That is not a gap: the line goes straight from 28 February to 1 March, and nothing is shifted.

Between two years, 29 February counts only when both years have it. Against the normal it counts whenever the main year has it.

The period

You can compare a whole year, one month or a custom period. A custom period may cross the turn of the year, for example November to February; it then belongs to the year in which it starts. The comparison year always gets the same calendar window, shifted by the difference in years.

Days after the latest observation have no values, and no day after yesterday is ever included, judged in Swedish time. When the current year is involved, as the main year or as the comparison year, the window is cut at the last observed day for both years, so a half-finished year is never compared with a whole one.

The three key figures

• Mean temperature: the average of the main year's valid daily means in the window.
• Against the comparison year: the average of the difference, main minus comparison, over the days where both years have a value. Its coverage is those matched days divided by the days in the window.
• Against the normal: the average of the difference between observation and the daily reference over the days where both exist.

Each figure is judged by coverage on its own. 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. The colour of a difference only shows its sign, warm or cold.

Daily, weekly and monthly views

The chart can show daily means, a 7-day mean or monthly means. The 7-day mean looks back over the day and the 6 days before it and needs 5 valid days. A monthly point is placed in the middle of the month, and for the current month it is marked "so far". Monthly deviations for whole months use SMHI's official monthly normal; for an unfinished month the deviation is the average daily difference from the daily reference.

One station, never spliced

Both years and the normal come from the same station. Years in which that station lacks enough data are disabled in the year picker rather than filled in from another station.

A comparison describes two years at one station. It says nothing on its own about a trend, and it is never a forecast.