How much does it have to rain to be described as extreme? SMHI’s definition of torrential rain is at least 50 mm during an hour or at least 1 mm during one minute. But extreme conditions depend also on how the precipitation develops over time and how large an area is affected.
Individual cases of extreme precipitation can therefore look very different. Observations are used to provide an idea of the variation in extreme precipitation in order to present statistics over a long time period. However the method used is significant.
Most of the precipitation measurements are carried out once daily. In practice this involves emptying a can containing all precipitation collected since the last time the can was emptied. The daily precipitation from 7 until 7 the next morning has been processed for a selection of stations that have been digitised since 1900.
An important requirement for the selection is that the stations have remained at the same location or almost the same location for the whole period.
The following processes were applied to the selected stations as well as to all of SMHI’s digitised measurement stations for the periods 1900-2014 and 1960-2014:
• Average of the greatest daily precipitation for each year
• Number of observations with a daily precipitation of at least 40 mm
• Greatest daily precipitation in Sweden (only for all stations)
The following figure shows the average of the greatest daily precipitation based on about 60 of SMHI’s weather stations, that have been operational during the period 1900-2014. The 60 stations are fairly evenly distributed over the country. The number of stations in operation each year has varied between 55 and 60, with an average of 59.
Average annual greatest daily precipitation
The red curve shows a moving average based on the green bars. The black dotted curve is a moving average based on data from all of SMHI’s weather stations with digitised data since 1961. On average, around 740 stations have been available each year.
The black dotted curve is therefore based on more than 10 times as many stations as the red curve. The curves show approximately the same pattern, but with slightly higher values when all stations are included.
Since the curves show a similar pattern, we can assume that the curve with fewer stations captures the variation for the entire period.
The difference between the red and black curves could be due to having more stations, which increases the chance of capturing days with high precipitation, but the most likely explanation is difference in geographical distribution. The stations that included in the larger group (black dotted curve) include a relatively large proportion of inland stations, which generally measure greater precipitation than coastal stations.
Cases with at least 40 mm
The following figure shows the number of cases per year where the average daily precipitation was at least 40 mm for one of the 60 selected stations.
The data has been normalised so that the number of cases with at least 40 mm during the year has been divided by the number of stations that were in operation during the year and then multiplied by 100.
Greatest annual daily precipitation
The following figure shows the greatest annual daily precipitation measured at one of SMHI’s stations.
The three most recent highest values come from Hinshult in Småland, which recorded 163 mm on 7 July 2012, from Råda in Värmland (188,6 mm on 4 August 2004) and from Fagerheden in Norrbotten (198 mm on 28 July 1997). The last value is the greatest value SMHI has ever measured for 24 hours integration time.
Precipitation in the future
Precipitation in Sweden is expected to increase. By the end of the century the average annual precipitation will be 20-60% more than for the period 1961-1990, depending on which scenario is used.
Precipitation is expected to increase during all seasons, but mostly in northern Sweden and during the winter. During summertime the increase in southern Sweden is small. About half of all the climate model simulations show an increase in precipitation while the other half show a reduction, which is an indication that the change will be close to zero.
Precipitation can vary a lot from season to season. This means that there is a wide range of possible developments in the future. For each season there are climate model simulations that show little or no increase in precipitation by the end of the century.