"The wind is not very strong."
Because it's in the negative form... Think of i-adjectives as adjectives that are conjugated...very much like verbs... In this case, 強い is the present positive, 強くない is the present negative, 強かった is the past positive and 強くなかった is the past negative. Note that all of them, in formal speech, require the verb です unconjugated at the end of the sentence.