Learn Polish: Wiało tak mocno, że nie było nic widać. - It blew so hard that nothing could be seen.

Tak mocno, że means "so hard that," emphasizing the extreme intensity of the wind. You could grammatically use na tyle mocno, że ("hard enough that"), but it subtly changes the focus. Tak highlights the extreme degree, while na tyle just states that the wind reached the threshold required to block visibility.

Wind (wiatr) is a masculine noun in Polish. Why is the verb wiało in the neuter form?

This is an impersonal weather construction. Just like in English when we say "it blew" without specifying what "it" is, Polish uses the third-person singular neuter form (wiało) when the natural force isn't explicitly named as the subject.

How does the phrase było widać work to mean "could be seen"?

Widać is a special impersonal word meaning "is visible" or "can be seen." Because it never changes its form to show tense, we have to add the past tense neuter helper verb było to put the sentence into the past tense.

Is nie było nic widać a double negative?

Yes, and it is mandatory here. Polish grammar requires double negation, meaning if you use a negative pronoun like nic (nothing), you must also negate the main verb phrase with nie. Literally, it says "it was not nothing visible."