Full Idea
Both the question 'what is thunder?', and the question 'why does thunder occur?', for Aristotle, are answered simultaneously, once it has been discovered what the essence of thunder it, i.e. what it is to be thunder.
Gist of Idea
Discovering the Aristotelian essence of thunder will tell us why thunder occurs
Source
Kathrin Koslicki (Essence, Necessity and Explanation [2012], 13.3.1 n10)
Book Reference
'Contemporary Aristotelian Metaphysics', ed/tr. Tahko,Tuomas [CUP 2012], p.198
A Reaction
I take this idea to be pretty much the whole story about essences.