Guide
From millimetres to flooding
The same amount of rain can be a welcome watering or a basement full of water. The difference lies in time.
One millimetre of rain is one litre of water per square metre. Thirty millimetres in a day is a solid rainy day that the ground can usually handle without a problem. Thirty millimetres in an hour is a cloudburst that overwhelms the stormwater system. The amount is identical – it's the intensity that decides.
What the ground and the drains can handle
Dry, well-drained sandy soil can absorb up to 20–30 mm an hour. Clay soil manages perhaps 2–5 mm an hour, and once it's saturated after several rainy days, that figure drops towards zero. Asphalt and roofs let nothing through at all – all the water goes straight to the drain. Swedish stormwater systems in older areas are often built for rain that statistically recurs every ten years, which in practice means roughly 20–30 mm an hour. Above that level, water stands in the street no matter how well everything else works.
| Intensity | Equivalent to | Effect in urban areas |
|---|---|---|
| 1–2 mm/h | Drizzle | Roads get wet, no significant puddles. |
| 3–7 mm/h | Moderate rain | Puddles form in dips, drains cope. |
| 8–15 mm/h | Heavy rain | Running water along kerbs, poor visibility. |
| 16–30 mm/h | Very heavy | Drains overloaded, water stands in the street. |
| 30 mm/h + | Cloudburst | Tunnels and basements flood. |
Why cities are hit hardest
In a forest, tree canopies catch some of the rain, the leaf litter slows the rest, and the ground absorbs the water over a long time. In a city, most of the surface is hard, so nearly all the rain becomes runoff within minutes. That means the flow peak arrives quickly and reaches a high level. The same cloudburst that barely registers in a forest can close a underpass in a built-up area.
What you can do at home
Clear gutters and downpipe drains – most basement floods start with leaves in a drain. Make sure the ground slopes away from the house foundation by at least a centimetre or so per metre for the first few metres. Direct downpipes onto the lawn or into a water butt instead of straight into the stormwater system where that's allowed. If you have a floor drain in the basement, a backwater valve can be worth the money in low-lying areas.
When a cloudburst warning is issued: move valuables up off the basement floor, don't park in low points or under underpasses, and don't drive into water whose depth you can't judge. As little as thirty centimetres of flowing water can lift a car.
See the intensity instead of guessing
Forecasts often state millimetres per hour. In the visualiser you can set exactly that value and see how densely the drops fall and how quickly puddles grow, in both urban and forest settings. More on the units is in the rain guide.
Visualise rain