Heavy rain such as a cloudburst can occasionally be a problem for air traffic because there is a risk of aircraft hydroplaning on the runway.

To keep this from happening, runways at Copenhagen Airport are designed to slope away from the middle out towards the sides so rainwater automatically runs off the runways and ends up in one of three collection basins, each of which is the size of a football pitch. 

If necessary, the airport has a special vehicle than can go out and test the friction value, i.e. the frictional resistance, on the runways and provide precise and continually updated information on conditions. The computer equipment in the vehicle then automatically sends this information to pilots, air traffic control and the airport's operational centre.
If the rain is accompanied by thunder and lightning, the situation is different. The thunder is not a problem by itself, but lightning can ruin the electrical installations, although it rarely does so. For this reason, both the airport and all aircraft have lightning rods to prevent lightning strikes.