The consequences of extreme rainfall have to be taken into consideration when planning and designing storm water systems. In urban areas runoff often occurs from small areas with a high proportion of impervious surfaces, and this process can be rapid. When planning and constructing new buildings, it is important to account for large amounts of snow that could overload roofs, especially large flat roofs.
Heavy rain can also cause major problems for the agriculture sector where crops might rot if the fields get flooded.
Heavy or extreme precipitation refers to instances during which the amount of rain or snow significantly exceeds normal levels, for example, in a month or a day or an hour. Extreme rainfall can lead to high water flows, however this depends on how much water is trapped in the soil, rivers and lakes before the rain falls. A large part of the flooding that affects Sweden arises when several rain events have already passed, one after the other, although each individually would not have given any extreme amounts.
Basically, a rainfall of 40 millimetres per day can be considered as torrential rain.
Extensive rain with negative effects arises in connection with slow fronts, which separate very warm, moist air from the cooler and drier air. Rainfall areas are simultaneously formed along the fronts with gradually increasing winds. The rainfall areas also increase in size, while rain intensity often begins to decline.
Intense and local thunderstorms can also bring very large amounts of rain. This can cause problems in cities where storm water systems cannot handle large amounts of rainfall.
Precipitation in the future
Climate scenarios indicate that torrential rain in Sweden is becoming more common in a warmer climate. We can expect more frequent cloudbursts and an increase in intensity. As always, there are large regional and local differences in our country.
During summertime the intensity of heavy rainfall is generally estimated to increase by 10-15 % in Sweden by the end of the century. However, the range between different scenarios varies (from unchanged intensity to an increase of more than 40 %).
The rain intensity of a 10 year rain, which on average will return every ten years, with a duration of 10 minutes, 1 hour and 1 day is expected to rise by about 10 %.
In line with this the expected return period of a 20 year rain will reduce to 6-10 years during the summer and down to 2-4 years for the winter, in Sweden. This is based on comparisons between the periods 1961-1990 and 2071-2100.