Ah, I see your point. Indeed, 青空なのに、雨が降るはずがない can mean both "it can't rain, so it doesn't rain" and "it can't rain, nevertheless it is raining". The key is the sentence final particle よ. This is for the assertion of the speaker's opinion. So, he/she said "It's impossible to rain, don't warry" or something along the line. (If this is used in a situation where it's seemingly raining, e.g., the sound of the rain is heard, the ground is wet, etc, he/she means "It can't be the rain. This must be something other than rain". It would be a rare situation, though.)