Learn Swedish: Olyckligtvis blåser det mycket, vilket är farligt. - Unfortunately it is blowing a lot, which is dangerous.

It is formed from the adjective olycklig (unhappy or unfortunate). To create a sentence adverb—an adverb that comments on the entire sentence—you add a -t followed by the suffix -vis. This turns olycklig into olyckligtvis (unfortunately).

Why is the verb blåser placed before the subject det?

This is the Swedish V2 (verb-second) rule in action. Because the sentence starts with the adverb olyckligtvis in the first position, the verb blåser must immediately follow it in the second position, pushing the subject det to the third position.

Instead of blåser det mycket, could I use an adjective to say it is windy?

You could say det är mycket blåsigt (it is very windy), but using the verb att blåsa (to blow) is the most natural and common way Swedes talk about windy weather. They literally say that it blows a lot.

Why do we use vilket for which here? Could we use som?

You need to use vilket because it refers back to the entire previous situation (the fact that it is blowing a lot). The word som is used when pointing back to a specific noun, but when commenting on a whole clause, you use vilket.