Think of southerly as an arrow that has south written on it. The arrow by itself does not say who is moving; it only says that south is the end of the line that matters. For a road, a route or a place, the arrow points away from you, towards the south. For a wind, the same arrow has to be read backwards: the wind comes from the south. So the question to ask whenever you meet the word is a simple one - is this about where something is going, or about where it came from?
From the word’s history
Wiktionary gives the word as souther + -ly, the same -erly ending found in northerly, easterly and westerly - a family of words that name a direction.
字典把 southerly 拆成 souther ＋ -ly，也就是 south 再加上講方向的 -erly 字尾。northerly、easterly、westerly 是同一家，所以它們的規矩也一樣：講風的時候指來處，講路線的時候指去處。
Wiktionary entries for southerly, packet etymology_text: "From souther + -ly."
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