What is a cloudburst?
DMI calls it a cloudburst (skybrud) when more than 15 millimetres of rain falls locally in half an hour. That is 15 litres of water on every square metre – more than gutters, drains and sewers in many places can carry away in such a short time. So water quickly collects in basements, underpasses and low-lying areas.
A cloudburst usually comes from a heavy shower or thunderstorm in summer and hits a small area. Even when DMI has warned of cloudbursts for a whole region, most people there won't get one. That makes them hard to forecast precisely – and is why it's wise to prepare when the warning comes.
Cloudburst or heavy rain?
DMI has two kinds of rain warning:
| Warning | Category 1 (yellow) | Category 2 (orange) | Category 3 (red) |
|---|---|---|---|
| Cloudburst – in 30 minutes | 15–25 mm | 25–35 mm | over 35 mm |
| Heavy rain – in 6 hours | 25–35 mm | 35–50 mm | over 50 mm |
| Heavy rain – in 24 hours | 30–50 mm | 50–80 mm | over 80 mm |
Heavy rain falls over a longer time and a larger area, and it can make rivers and lakes burst their banks. Since 1 September 2025 DMI has also issued separate warnings for flooding from rivers and lakes, municipality by municipality. On 21 July 2025 DMI issued a category 3 rain warning for the first time.
How you are warned
DMI can issue an early warning up to five days ahead, but the warning itself comes at most 36 hours before the rain. Warnings appear on dmi.dk and in DMI's app. We also show the warnings themselves on DMI weather warnings today a few minutes after DMI issues them – early warnings and risk notices are only on DMI's own channels. If the sirens sound, go indoors and follow DR or TV 2.
Before the cloudburst
- Clear gutters, downpipes and outdoor drains so the water can run away.
- Close windows and doors – especially roof windows.
- Lift things off the basement floor, and move anything that can't take water.
- Keep heavy plastic and sandbags ready to seal floor drains and toilets if sewage water pushes up.
- Consider, well in advance, fitting a backflow valve so sewage can't run back into the house.
- Charge your phone, and keep a power bank ready.
During the cloudburst
- Drive carefully, and don't drive through deep water. Underpasses and dips can fill within minutes.
- If a cloudburst catches you in the car, pull over and park where water doesn't collect.
- Keep away from rising sewage water. Manhole covers can be pushed up by the water.
- Only call 112 in an emergency. Help may take longer than usual when many people need it.
After the cloudburst
- Wear rubber boots and gloves. The water may contain sewage.
- Get the water out as quickly as possible, and air the rooms well to avoid mould.
- Photograph the damage before you clear up, and contact your insurer.
- Check on neighbours who may need help, if it is safe.
The great cloudburst of 2011
On 2 July 2011 a cloudburst hit Copenhagen and the surrounding area. DMI calls it the most damaging cloudburst in Danish weather history: 135 millimetres of rain fell in the Botanical Garden, several places got more than 50 millimetres in half an hour, and the damage was later put at more than 3 billion kroner.
Sources (in Danish)
- DMI: Varsler om farligt vejr i Danmark (criteria and timing)
- DMI: new warning for flooding from rivers and lakes
- DMI: one year since the Copenhagen cloudburst
- Police: extreme weather
- Danish Emergency Management Agency: extreme rain and cloudbursts
- Klimatilpasning.dk: preparedness for citizens
Advarselslisten is not an authority. Always follow the authorities' instructions, and call 112 if life is at risk.