The first step is an Abduction that abstracts a Case from the consideration of a Fact and a Rule.
Fact: C → A, In the Current situation the Air is cool.
Rule: B → A, Just Before it rains, the Air is cool.
Case: C → B, The Current situation is just Before it rains.
The final step is a Deduction that admits this Case to another Rule and so arrives at a novel Fact.
Case: C → B, The Current situation is just Before it rains.
Rule: B → D, Just Before it rains, a Dark cloud will appear. so:
Fact: C → D, In the Current situation, a Dark cloud will appear.