I came across this phrase in school and I became quite confused with whether it should be seen as a personification or not. It may be a collocation but does that mean it can't be a personification? 'Gathering' to me usually entails the getting together of people/ animals (animate) so if it is placed next to something that is inanimate (Storm clouds), do we automatically classify it as a personification?
Gather can be used in multiple ways:
- [ no obj. ] come together; assemble or accumulate: a crowd gathered in the square.
[ with obj. ] bring together and take in from scattered places or sources: we have gathered the information.
- pick up from the ground or a surface: they gathered up the dirty plates and cups.
- draw together or toward oneself: she gathered the child in her arms.
You seem to be thinking of the one with an object, which is not necessarily a personification ("somebody gathered storm clouds" and "storm clouds were gathered by someone" are not personifications). In your sentence, however, there is no object, which makes it a personification (since the clouds gathered themselves).