In order to calculate the cases of extreme precipitation in a uniform and comparable way, the precipitation is first considered over two days (48 hours) and then a value for the 24-hour period with the greatest precipitation value is extracted.
The analyses are based only on observations from SMHI’s official stations so that the data is comparable with time. This means that there will have been cases of extreme precipitation that have occurred between measurement stations, but which are not included in the climate indicator.
One such case occurred on 30-31 August 1997 in Fulufjället. A special study of this case gave a value of 172 mm, which is higher than any of the values included in the official series – the highest value there for that period was 154 mm.
How much precipitation feels like an extreme amount?
How much does it need to rain for it to be considered extreme? To simplify, a precipitation volume of 40 mm per day is often described as torrential (even if this is not SMHI’s definition of torrential rain). When as much as 90 mm falls during 24 hours it leads to high water levels in rivers and streams with a risk of landslides and flooding in the affected area.