Clouds that grow all the way up to the tropopause at ten kilometres height or more can cause extreme summer storms. The atmospheric processes that lie at the basis of these so-called cumulonimbus can now be studied in more detail thanks to the PhD research of Steven Böing of CEG faculty, which he defended on January 27. He improved a programme called the Dutch Atmospheric Large Eddy Simulation software. Earlier versions of this programme calculated clouds to the height of some kilometres. Böing was the first to lift the cloud ceiling to twenty kilometres, which is basically the weather’s limit.