You are right.
Winds drop, fall, decrease or lessen
- On the south coast winds will pick up during the afternoon, becoming strong by the evening.
I think the verb in bold means "to start becoming stronger", am I right?
Now, is it pick down or calm down the opposite of this verb? I mean, how to say that winds are becoming weaker?
You are right.
Winds drop, fall, decrease or lessen
No, a storm blows over when it's done.
Winds pick up, winds drop off. (Or decrease or lessen, as stated above, but "drop off" is the most common one I hear.)
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
COCA gives 19 examples of 'wind dropped', and none of 'wind dropped off'.
I saw that three of the 'wind dropped' examples were not relevant - 'The wind dropped the handkerchief', for example - but that still leaves 16.
I would say that winds "die down".