"Hace mucho viento hoy."
Translation:It's very windy today.
Weather expressions in Spanish are often (though not always) idiomatic.
Hace mucho viento hoy = It makes a lot of (or much) wind today. Because they are using the noun "wind" (or "viento"), it necesitates the use of "mucho" rather than "muy."
If you were literally saying "very windy" in Spanish, it would be: "muy ventoso." But the common way to same it's very windy is to use the idiom that DuoLingo teaches here.
Why is Duolingo using hacer instead of estar or ser in this section? Isn't using hacer something of a stretch?