They are same in hiragana, but different in Kanji. In conversation, context will help you to know which one it is. cloud=kumo＝くも＝雲 spider＝kumo＝くも＝蜘蛛

It's because of the kanji. Each one can mean different things and be pronounced different ways depending on the context. So even though kumo means cloud and spider, you wouldn't look up at the sky and say, "Look at those big fluffy white spiders in the sky!" and written down, the kanji is different.