Why climate normals matter

A temperature on its own says little. Twelve degrees in October is mild in Kiruna and quite ordinary in Malmö. To know whether a day, a month or a year was warm or cold, you compare it with what is normal for that place: the climate normal.

What a normal is

According to SMHI, normal values describe the average climate at a place over a set period, called the normal period. The World Meteorological Organization (WMO) recommends 30 years. Standard normals are calculated for the 30-year periods 1961–1990, 1971–2000, 1981–2010, 1991–2020 and so on.

WMO also uses two wider terms. Period averages cover at least ten years, starting on 1 January of a year ending in 1. Normal values in general are averages over a uniform, fairly long period containing at least three ten-year periods. SMHI has said it will use these WMO definitions to avoid confusion with its older terms.

A normal is often a monthly mean temperature, but it can also be a count, such as the number of days with a maximum above 25 °C, or an extreme, such as the highest or lowest temperature of a month.

What normals are used for

SMHI lists several uses:

• Describing the climate of today, using the latest completed 30-year period, currently 1991–2020.
• Describing an earlier climate, using an earlier normal period.
• Showing climate change, by comparing one normal period with another.
• Serving as a "climatological forecast" when no forecast model is available. SMHI points out that such a forecast is much less accurate than a model forecast for the next few days.

Why normals are updated

The idea of a 30-year normal goes back to 1935, when the International Meteorological Committee in Warsaw recommended it, with 1901–1930 as the first international normal period. New normals used to be calculated every 30 years. Because of ongoing global warming they are now updated every ten years, and SMHI states that the next normal period will be 2001–2030.

Regular updates keep the averages representative when the climate changes or a measurement site moves, and let averages from different countries be compared. For studies of climate change, WMO keeps 1961–1990 as a fixed reference period.

Normal is not the same as usual

A normal is an average over thirty years, not a promise about any single year. A month is rarely exactly normal: deviations in either direction are part of ordinary weather, and a single cold spell can pull a month below the normal.

On Temperaturkollen a deviation is shown with a warm or a cold colour. The colour only shows the sign of the difference. It is not a judgement, and one warm month or year says nothing on its own about a trend.

How the 1991–2020 normal is used on this site, including the daily reference we calculate ourselves, is explained in the guide on the 1991–2020 normal.