Timmy Greene, age 11, of Gastonia, North Carolina, for his question:

What causes a flash flood?

Flash floods and so called cloudbursts, thank goodness, are rare events. They are related weather events and sometimes they are one and the same. They occur in regions of mountains and sloping hills or in valleys and flat areas where higher ground can be seen in the distance. The event begins with a spell of severe rain, either from a sudden thunderstorm or from a general downpour. The ground in the neighborhood is deluged, but the local rainfall soon begins to drain away. The sun may come out    but showery clouds still hover above the hills drenching the distant slopes. Then a new deluge arrives, apparently from nowhere, and the ground is swamped with a rushing flash flood.

The sudden flood waters came from some distance away. Most likely it is the run off rainfall draining down from the distant slopes. It fills the streams and creeks to overflowing and often floods over into the streets. But when all the rain¬fall from the surrounding countryside is drained away by its natural routes, the flood subsides.