Climate indicator - Extreme precipitation

If large amounts of precipitation fall in a short period of time, it can cause floods, traffic disruptions and landslides, among other things. Regarding the indicators Maximum daily precipitation and Absolute largest daily precipitation, an increasing trend can be seen for the year's value and the winter value. Other than that, it is difficult to see any clear development and variations from year to year are large. The largest 24-hour rainfall so far measured at an SMHI station is 198.0 millimeters, which was measured in Fagerheden in Norrbotten on 28 July 1997.

Comment on the results

Looking at the maximum daily precipitation seen over the entire Swedish station network, one can see fluctuations between periods of a number of years, with large daily amounts interspersed with periods when large daily amounts were rarer. However, a clear long-term trend is more difficult to see. There is a trend that maximum daily precipitation in winter is increasing in all parts of the country. This is most obvious for northern Norrland.

Other than that, it is difficult to see any clear trends and the variations from year to year are large.

How is extreme precipitation defined?

Precipitation can be extreme in various ways. It can be about very large amounts of precipitation during a short period of time, or more persistent and long-term precipitation with very large amounts of precipitation in total. It can also be about precipitation which is extreme for a certain place, but which would not have been extreme for a place with more precipitation.

For the climate indicator extreme precipitation, three different measures are chosen:

  • Maximum 24-hour precipitation – average of the year’s or season’s greatest 24-hour precipitation for 60 stations across Sweden.
  • Days with at least 40 millimeters – average of the number of observations per year with a daily perception of at least 40 millimeters for 60 stations across Sweden.
  •  The absolute maximum 24-hour precipitation – the year’s greatest 24-hour precipitation in Sweden (based on all stations)

When it comes to numbers per year the daily precipitation has been at least 40 millimeters, a normalization has been made. It has been conducted in a way so that the number of cases with at least 40 millimeters during the year has been divided with the number of stations in operation during the year.

Why is this indicator important?

Large amounts of precipitation in a short period of time can cause major problems in society in the form of floods, erosion and landslides, obstacles in traffic and more. This applies even if the precipitation itself is welcome for, for example, vegetation or cultivation.

Extreme or intense short-term rainfall with floods as a result, is the type of rain that creates the most problems in cities. There are very hard surfaces in cities that prevent the water from penetrating into the ground, and various solutions need to be found to take care of this.

How has the indicator been calculated?

To be able to present statistics over a long period of time and get a picture of how the extreme precipitation has varied, observations are used. An important requirement for the selection of stations is that the measurements must have been carried out at the same place or almost the same place during the entire period.

The measurements must also be relatively evenly distributed across the country. 60 stations that meet these requirements have been selected. The number of stations that have been in operation each year out of these 60 has varied between 54 and 60 with an average of 59.

Regarding the number of occasions per year when the 24-hour rainfall was at least 40 millimeters at the 60 selected stations, a normalization has been made. It has been carried out in such a way that the number of falls with at least 40 millimeters during the year was divided by the number of stations that were in operation during the year and then multiplied by 100.

Extreme precipitation in the future

Today, research can clearly link extreme weather to climate change. The probability of heavy precipitation has already increased. In a future warmer climate, the number of days with extreme precipitation is expected to increase throughout the country over the course of the year, but there are differences between different counties and seasons.