Synonyms For How

Are these sentences acceptable by native speakers? If yes, Are they equal in their meaning?

A rainbow sometimes comes after it has rained. How is that so?
A rainbow sometimes comes after it has rained. How so?

A rainbow sometimes comes after it has rained. How can that be?

A rainbow sometimes comes after it has rained. Why is it like that?

(Am I allowed to use "How" lonely like this one?):
A rainbow sometimes comes after it has rained. How?